# Power steering
**Power steering** is any system that reduces the physical effort a driver needs to turn a vehicle's steered wheels by adding assist from an external power source. Three technology families exist: hydraulic power steering (HPS), which uses an engine-driven pump; electro-hydraulic power steering (EHPS), which drives the same kind of hydraulic circuit with an electric pump; and electric power steering (EPS), which assists with an electric motor and no hydraulic fluid at all. A fourth architecture, steer-by-wire, removes the mechanical connection between steering wheel and wheels entirely[1](https://ieomsociety.org/proceedings/bangladesh2024/77.pdf) • [2](https://cecas.clemson.edu/cvel/auto/systems/ep_steering.html).
| Key fact | Detail |
|---|---|
| Architectures | HPS: engine-driven vane pump and open-center rotary valve, typically no ECU; EHPS: motor-driven pump with ECU; EPS: no pump, no rotary valve, no hydraulic circuit[1](https://ieomsociety.org/proceedings/bangladesh2024/77.pdf) |
| Energy cost of HPS | A constant-flow hydraulic system accounts for about 3% of a car's total energy consumption; over 70% of the fuel it consumes is avoidable[3](https://www.jstage.jst.go.jp/article/isfp1989/2008/7-1/2008_7-1_133/_pdf) • [4](https://www.mordorintelligence.com/industry-reports/north-america-electro-hydraulic-power-steering-market) |
| EPS energy example | An EPS motor delivering 5.23 Nm at 45 rpm needs 48.06 W of shaft power, with no hydraulic circulation losses[1](https://ieomsociety.org/proceedings/bangladesh2024/77.pdf) |
| Market share | A Frost & Sullivan study reports 98.2% EPS penetration over its 2022–2031 study period in the North American passenger-vehicle steering market; most current Japanese-market models also use EPS[5](https://store.frost.com/north-american-passenger-vehicle-power-steering-systems-growth-opportunities.html) • [6](https://www.marklines.com/en/report/wsw0201_202508) |
| Safety rating | The main EPS hazard, "unwanted steering", is treated as an ASIL D hazard in the cited EPS safety guidance and must be detected within a fault-tolerant time interval on the order of milliseconds[7](https://documentation.infineon.com/aurixtc3xx/docs/iat1745576007982) |
| Steer-by-wire | First commercialized on the 2014 Infiniti Q50; Tesla Cybertruck, Lexus RZ 450e, GMC Hummer EV, Rolls-Royce Spectre, and Lotus Eletre have since shipped with it[2](https://cecas.clemson.edu/cvel/auto/systems/ep_steering.html) • [8](https://www.automotive-iq.com/chassis-systems/articles/what-is-steer-by-wire-how-it-works-why-it-matters-and-whats-next) |
| Replacement cost | EPS system replacement typically runs USD 800 to 1,500[9](https://www.mordorintelligence.com/industry-reports/automotive-power-steering-motor-market) |
## History
Hydraulic power steering reached production in the early 1950s[13](https://mdpi-res.com/d_attachment/materials/materials-15-02406/article_deploy/materials-15-02406-v2.pdf?version=1648197381). An EOLSS technical chapter credits [General Motors](https://www.edgepedia.org/general-motors) in the USA and Bishop in Australia with developing and introducing the first hydraulically assisted system in the 1950s, using recirculating-ball steering with a belt-driven rotary vane pump[10](https://www.eolss.net/Sample-Chapters/C05/E6-39A-05-09.pdf). Chrysler's account differs: it offered the 'Hydraguide' system on its Imperials in 1951, and specialist histories treat that as the first commercially available passenger-car power steering[11](https://www.pistonheads.com/news/ph-origins/ph-origins-hydraulic-power-steering/38931). Ford filed a British patent application on October 17, 1952 (published 1953 as No. 28818/53) for a hydraulic mechanism built around a power piston whose hollow rod pivots to the steering linkage[12](https://www.freepatentsonline.com/2711797.html). Hydraulic assist was fitted to most passenger cars produced from the early 1990s until the late 2000s[13](https://mdpi-res.com/d_attachment/materials/materials-15-02406/article_deploy/materials-15-02406-v2.pdf?version=1648197381).
Electrification came in stages. EHPS systems appeared in the mid-1980s from suppliers such as TRW, decoupling the hydraulic pump from the engine by driving it with an electric motor[14](https://web.archive.org/web/20190716120558/https:/www.embedded.com/design/real-world-applications/4006457/Electric-power-steering-one-good-turn-deserves-another). JTEKT developed and mass-produced the world's first column-type electric power steering (C-EPS) for small vehicles in 1988[15](https://www.jtekt.co.jp/e/engineering-journal/assets/1021/1021e_11.pdf), and Honda introduced all-electric power steering on the Acura NSX earlier than 1993 that was not the first worldwide, using a brush [DC motor](https://www.edgepedia.org/dc-motor) concentric around the rack[14](https://web.archive.org/web/20190716120558/https:/www.embedded.com/design/real-world-applications/4006457/Electric-power-steering-one-good-turn-deserves-another). The 2000 [Fiat Punto](https://www.edgepedia.org/fiat-punto) used Delphi's E-STEER in the first high-volume EPS application, and Volkswagen's 2001 Lupo 3L TDI offered E-STEER with a 1.3 mpg fuel-economy improvement[10](https://www.eolss.net/Sample-Chapters/C05/E6-39A-05-09.pdf). In 2001 hydraulic systems still accounted for the majority of power-steered vehicles; EPS production rose rapidly from 2010 as the technology expanded into large vehicles[16](http://eb-cat.ds-navi.co.jp/enu/jtekt/tech/ej/img/no1013e/1013e_03.pdf).
## How hydraulic power steering works
A hydraulic system has three parts: an engine-driven pump, a control valve integrated into the rack or gearbox, and a hydraulic actuator cylinder. The pump runs continuously, maintaining pressure whether or not the driver is turning[17](https://technav.ieee.org/topic/power-steering/).
The rotary valve is the key element. A torsion bar sits between the steering input shaft and the pinion; when the driver rotates the wheel, the bar twists slightly, opening a port that directs pressurized fluid to one side of the actuator piston. The magnitude of assist is proportional to the pressure differential across the piston[17](https://technav.ieee.org/topic/power-steering/). Inside the rack, a sealed piston divides the gear's internal cavity into two chambers; pressurized fluid applied to one side while fluid returns from the other pushes the rack, and a valve on the pinion shaft controls the flow[18](https://www.caranddriver.com/features/a15122019/electric-vs-hydraulic-power-steering/).
Because the pump runs whenever the engine does, the system pays a continuous parasitic loss. One estimate puts it at nearly 15% of engine power for small automobiles and about 10% for larger ones[10](https://www.eolss.net/Sample-Chapters/C05/E6-39A-05-09.pdf); a Japanese fluid-power study measured constant-flow hydraulic steering at about 3% of a car's total energy consumption[3](https://www.jstage.jst.go.jp/article/isfp1989/2008/7-1/2008_7-1_133/_pdf). The two figures measure different things, engine power share versus total vehicle energy. Speed-sensitive hydraulic variants electronically modulate a flow-control valve to reduce assist at highway speeds[17](https://technav.ieee.org/topic/power-steering/).
## Electric and electro-hydraulic power steering
**EPS replaces the hydraulic circuit with a motor.** The ECU calculates the assisting power needed from the driver's steering torque, steering wheel position, and vehicle speed, then commands an electric motor[2](https://cecas.clemson.edu/cvel/auto/systems/ep_steering.html). In the column-type layout, the torque sensor detects steering torque, the ECU adds motor current based on the torque signal and vehicle speed, and a reduction gear multiplies the motor torque before it reaches the pinion or rack[19](https://jtekt.com.my/adm/files/products/Electric_Power_Steerings.pdf). Bosch's system works the same way, with variants covering passenger cars through light commercial vehicles[20](https://www.bosch-mobility.com/en/solutions/steering/electric-power-steering-systems).
Four EPS forms exist, distinguished by assist-motor position: column assist (C-EPS), pinion assist (P-EPS), direct drive (D-EPS), and rack assist (R-EPS). Rack-assist types suit mid- to full-sized vehicles, which is why EPS adoption followed that path as it moved up from small cars[2](https://cecas.clemson.edu/cvel/auto/systems/ep_steering.html) • [16](http://eb-cat.ds-navi.co.jp/enu/jtekt/tech/ej/img/no1013e/1013e_03.pdf).
**EHPS keeps hydraulics but electrifies the pump.** JTEKT describes an energy-saving hydraulic system equipped with an independent electric pump, cited for flexible installation, EV suitability, and improved fuel efficiency[19](https://jtekt.com.my/adm/files/products/Electric_Power_Steerings.pdf). EHPS systems have a significantly higher rack load capability than HPS systems[4](https://www.mordorintelligence.com/industry-reports/north-america-electro-hydraulic-power-steering-market).
## Steer-by-wire
Steer-by-wire removes the mechanical linkage between the steering wheel and the steered wheels. The driver's input is measured, transmitted electronically, and executed by a road-wheel actuator, while steering feel is artificially generated by an electric feedback motor providing resistance to the driver's input[21](https://journals.sagepub.com/doi/10.1177/0954407012468413). The 2014 [Infiniti Q50](https://www.edgepedia.org/infiniti-q50) was the first commercial vehicle to implement it, retaining a mechanical back-up clutch[2](https://cecas.clemson.edu/cvel/auto/systems/ep_steering.html).
Current production deployments include the [Tesla Cybertruck](https://www.edgepedia.org/tesla-cybertruck), Lexus RZ 450e, [GMC Hummer EV](https://www.edgepedia.org/gmc-hummer-ev), Rolls-Royce Spectre, and Lotus Eletre[8](https://www.automotive-iq.com/chassis-systems/articles/what-is-steer-by-wire-how-it-works-why-it-matters-and-whats-next). Availability is not uniform: the US-spec 2026 Lexus RZ ships with a conventional wheel and rack because US steering regulations, including crash-behavior requirements, differ from those in Europe and Japan, and Toyota's testing team judged the system too "peaky" at low speed[22](https://carbuzz.com/why-lexus-steeering-yoke-isnt-sold-in-the-us/).
## By the numbers
The energy argument for EPS is direct. In a worked example, an EPS motor delivering 5.23 Nm at 45 rpm required 48.06 W of shaft power to drive the steering gear, with no hydraulic circulation losses; in EHPS, fluid returning to the reservoir when the rotary valve opens is a circulation loss that EPS eliminates[1](https://ieomsociety.org/proceedings/bangladesh2024/77.pdf). A measured EHPS electric-to-hydraulic conversion efficiency was 19.72%, meaning most of the electrical energy spent driving the pump never reaches the wheels[1](https://ieomsociety.org/proceedings/bangladesh2024/77.pdf). [Market analysis](https://www.edgepedia.org/market-analysis) puts over 70% of the fuel consumed by a conventional HPS as unnecessary and avoidable[4](https://www.mordorintelligence.com/industry-reports/north-america-electro-hydraulic-power-steering-market).
Adoption is now near-universal in passenger cars. A Frost & Sullivan study reports 98.2% EPS penetration over its 2022–2031 study period in the North American passenger-vehicle steering market, with 100% power-steering adoption overall[5](https://store.frost.com/north-american-passenger-vehicle-power-steering-systems-growth-opportunities.html), and most models in the current Japanese market are equipped with EPS rather than hydraulic systems[6](https://www.marklines.com/en/report/wsw0201_202508). When an EPS system does fail, replacement costs range from USD 800 to 1,500[9](https://www.mordorintelligence.com/industry-reports/automotive-power-steering-motor-market).
## How it compares
| Attribute | HPS | EHPS | EPS | Steer-by-wire |
|---|---|---|---|---|
| Power source | Engine-driven pump | Electric pump | Electric motor | Electric actuators, no linkage |
| Hydraulic fluid | Required | Required | None | None |
| Maintenance | Routine flushes; sometimes a drive belt | Pump and fluid service | None of those procedures | None of those procedures |
| ADAS/autonomy fit | Limited | Depends on mechanical torque input | Integrates with ADAS; suitable for autonomous vehicles | |
Sources: [1](https://ieomsociety.org/proceedings/bangladesh2024/77.pdf) • [23](https://www.motor.com/2021/02/steering-away-from-hydraulics/) • [16](http://eb-cat.ds-navi.co.jp/enu/jtekt/tech/ej/img/no1013e/1013e_03.pdf)
**Steering feel is the contested point.** Early EPS systems had worse steering effort and feedback than hydraulic systems, making it hard to sense when tires were running out of grip[24](https://www.caranddriver.com/features/a27888229/power-steering/). In a later [Car and Driver](https://www.edgepedia.org/car-and-driver) staff comparison, BMW's electric system won seven of ten rating categories, with hydraulic winning only the on-center feedback comparison; the same staff had for over a decade found EPS cars lacking in feel, poorly tuned, and sometimes weird[18](https://www.caranddriver.com/features/a15122019/electric-vs-hydraulic-power-steering/). The technical basis for tuning is real: on-center steering feel can be improved by adjusting system parameters[25](https://www.inderscience.com/info/inarticle.php?artid=98941), and in steer-by-wire the feel is reconstructed from estimated rack force, with aligning torque plus friction modeled by a hyperbolic tangent function, achieving feedback equivalent to conventional EPS with speed-dependent, personalized characteristics[26](https://link.springer.com/article/10.1007/s12239-023-0094-6).
**Heavy vehicles remain hydraulic territory.** Heavier vehicles still use some form of hydraulic steering assist because of the high power density typical of hydraulic systems[13](https://mdpi-res.com/d_attachment/materials/materials-15-02406/article_deploy/materials-15-02406-v2.pdf?version=1648197381), and EHPS's higher rack load capability serves that segment[4](https://www.mordorintelligence.com/industry-reports/north-america-electro-hydraulic-power-steering-market). For high-availability applications such as automated driving and heavy vehicles, two EPS systems can operate in parallel or hot standby, with a failing channel going silent while the other takes over[7](https://documentation.infineon.com/aurixtc3xx/docs/iat1745576007982).
## Maintenance, failures, and safety
**Failure modes.** The most dangerous EPS failure is "unwanted steer": if the transistors driving the assist motor stick in the on state, the system can steer the vehicle without driver input[14](https://web.archive.org/web/20190716120558/https:/www.embedded.com/design/real-world-applications/4006457/Electric-power-steering-one-good-turn-deserves-another). Under [ISO 26262](https://www.edgepedia.org/iso-26262), effective since 2011, these hazards are categorized as ASIL-D in the cited EPS safety analyses, and the fault must be detected within a fault-tolerant time interval on the order of milliseconds[16](http://eb-cat.ds-navi.co.jp/enu/jtekt/tech/ej/img/no1013e/1013e_03.pdf) • [7](https://documentation.infineon.com/aurixtc3xx/docs/iat1745576007982). China's GB/T 45829-2025 standard requires EPS systems to meet safety metrics for unintended lateral motion, loss of lateral motion control, and heavy steering when assist is lost[27](https://www.chinesestandard.net/PDF.aspx/GBT45829-2025). A NHTSA-funded ISO 26262 assessment of a generic EPS with active steering and four-wheel steering identified four vehicle-level safety goals and 125 system safety requirements[28](https://rosap.ntl.bts.gov/view/dot/37209/dot_37209_DS1.pdf).
**Sensors and electronics.** [Torque](https://www.edgepedia.org/torque) sensors typically use diverse redundancy and communicate with the ECU via the SENT protocol; GB/T 45829-2025 requires periodic detection of short circuit, open circuit, stuck signal, and deviation faults, with redundant sensors suggested as a detection method[7](https://documentation.infineon.com/aurixtc3xx/docs/iat1745576007982) • [27](https://www.chinesestandard.net/PDF.aspx/GBT45829-2025). On the drive side, the standard requires detection of MOSFET bridge short circuits and of over- or under-voltage, with the motor-drive link shut down when voltage exceeds the normal operating range[27](https://www.chinesestandard.net/PDF.aspx/GBT45829-2025).
**What happens when assist fails.** EPS retains the mechanical linkage between steering wheel and steering gear as a backup, so the driver can steer manually; the safe state cuts motor current with the motor freewheeling[7](https://documentation.infineon.com/aurixtc3xx/docs/iat1745576007982). Fail-safe mechanisms disconnect power from the motor if an ECU problem is detected[2](https://cecas.clemson.edu/cvel/auto/systems/ep_steering.html). Motorola's Seed Vector Test on MG3 EPS systems can bring the motor safely off-line in under 20 ms on a detected fault[14](https://web.archive.org/web/20190716120558/https:/www.embedded.com/design/real-world-applications/4006457/Electric-power-steering-one-good-turn-deserves-another). NHTSA treats an increase in steering effort as an acceptable warning signal to the driver when a power steering failure occurs[29](https://static.nhtsa.gov/odi/inv/2010/INRL-PE10020-42020P.pdf).
**A recent case.** NHTSA closed its engineering analysis into power steering loss affecting approximately 376,241 [Tesla Model 3](https://www.edgepedia.org/tesla-model-3) and Model Y (2023 model year) vehicles on June 27, 2026, following Tesla's over-the-air software recall in early 2025. The root cause was an overvoltage breakdown overstressing motor-drive components on the EPS assist module PCB, disabling assist; the fix was delivered entirely as a software update[30](https://lightmytesla.com/news/tesla-nhtsa-power-steering-probe-closed-376k-june-2026).
**Fluid service debates.** Conventional hydraulic systems require routine maintenance such as power steering system flushes, and in some systems a separate drive belt; EPS requires none of those procedures[23](https://www.motor.com/2021/02/steering-away-from-hydraulics/). In the service trade, fluid exchange machines are reported to remove about 85% of used fluid, and an inline filter on the low-pressure return line with service every 30,000 miles is reported to extend system life[31](https://gearsmagazine.com/magazine/power-steering-an-evolution-of-systems-fluids-and-essential-care/). EPS motor-control repairs require specialized diagnostic equipment and technician training that many independent shops lack, concentrating repairs at dealers[9](https://www.mordorintelligence.com/industry-reports/automotive-power-steering-motor-market).
## Recent developments
Steer-by-wire systems with no mechanical connection between steering wheel and wheels are in production and on the market, with the European market taking the lead[6](https://www.marklines.com/en/report/wsw0201_202508). Electric steering applied to rear wheels as well as front wheels has been commercialized[6](https://www.marklines.com/en/report/wsw0201_202508). EPS is particularly suitable for electric vehicles because it is powered by the battery rather than by an engine-driven pump[1](https://ieomsociety.org/proceedings/bangladesh2024/77.pdf), and regulatory divergence now shapes product lineups, as the Lexus RZ's different US and Japanese/European specifications show[22](https://carbuzz.com/why-lexus-steeering-yoke-isnt-sold-in-the-us/).
## References
1. [A Comparison Study of Vehicle's EHPS, EPS and HPS Power Steering System, IEOM 2024](https://ieomsociety.org/proceedings/bangladesh2024/77.pdf)
2. [Electric Power-Assisted Steering, Clemson Vehicular Electronics Laboratory](https://cecas.clemson.edu/cvel/auto/systems/ep_steering.html)
3. [Hydraulic power steering energy consumption, ISFP 2008 (J-Stage)](https://www.jstage.jst.go.jp/article/isfp1989/2008/7-1/2008_7-1_133/_pdf)
4. [North America Electro Hydraulic Power Steering Market, Mordor Intelligence](https://www.mordorintelligence.com/industry-reports/north-america-electro-hydraulic-power-steering-market)
5. [North American Passenger Vehicle Power Steering Systems, Frost & Sullivan](https://store.frost.com/north-american-passenger-vehicle-power-steering-systems-growth-opportunities.html)
6. [Analysis Report: Power Steering (Japanese Market), MarkLines, August 2025](https://www.marklines.com/en/report/wsw0201_202508)
7. [AN1100 Functional Safety in a Nutshell — Electric Power Steering, Infineon AURIX TC3xx](https://documentation.infineon.com/aurixtc3xx/docs/iat1745576007982)
8. [What Is Steer-by-Wire?, Automotive IQ](https://www.automotive-iq.com/chassis-systems/articles/what-is-steer-by-wire-how-it-works-why-it-matters-and-whats-next)
9. [Automotive Power Steering Motor Market, Mordor Intelligence](https://www.mordorintelligence.com/industry-reports/automotive-power-steering-motor-market)
10. [Electric Power Assisted Steering System for Automobiles, EOLSS](https://www.eolss.net/Sample-Chapters/C05/E6-39A-05-09.pdf)
11. [PH Origins: Hydraulic power steering, PistonHeads](https://www.pistonheads.com/news/ph-origins/ph-origins-hydraulic-power-steering/38931)
12. [Power steering device of the hydraulic type, Ford Motor Co. patent](https://www.freepatentsonline.com/2711797.html)
13. [Dynamic NVH Numerical Analysis of Power Steering, Materials (MDPI)](https://mdpi-res.com/d_attachment/materials/materials-15-02406/article_deploy/materials-15-02406-v2.pdf?version=1648197381)
14. [Electric power steering: one good turn deserves another, Embedded.com (archived)](https://web.archive.org/web/20190716120558/https:/www.embedded.com/design/real-world-applications/4006457/Electric-power-steering-one-good-turn-deserves-another)
15. [JTEKT Engineering Journal No.1021E](https://www.jtekt.co.jp/e/engineering-journal/assets/1021/1021e_11.pdf)
16. [JTEKT Engineering Journal No.1013e](http://eb-cat.ds-navi.co.jp/enu/jtekt/tech/ej/img/no1013e/1013e_03.pdf)
17. [Power steering, IEEE Technology Navigator](https://technav.ieee.org/topic/power-steering/)
18. [Electric vs. Hydraulic Steering: A Comprehensive Comparison Test, Car and Driver](https://www.caranddriver.com/features/a15122019/electric-vs-hydraulic-power-steering/)
19. [JTEKT Electric Power Steering Systems brochure](https://jtekt.com.my/adm/files/products/Electric_Power_Steerings.pdf)
20. [Bosch Electric Power Steering Systems](https://www.bosch-mobility.com/en/solutions/steering/electric-power-steering-systems)
21. [Design of full electric power steering with enhanced performance over hydraulic power-assisted steering, Proc. IMechE 2013](https://journals.sagepub.com/doi/10.1177/0954407012468413)
22. [Why Lexus' Steer-By-Wire Yoke Isn't Sold In America Yet, CarBuzz](https://carbuzz.com/why-lexus-steeering-yoke-isnt-sold-in-the-us/)
23. [Steering Away from Hydraulics, MOTOR](https://www.motor.com/2021/02/steering-away-from-hydraulics/)
24. [What Is Power Steering and How Does It Work?, Car and Driver](https://www.caranddriver.com/features/a27888229/power-steering/)
25. [Study on parameters affecting steering feel of column assist EPS, IJVD 2018](https://www.inderscience.com/info/inarticle.php?artid=98941)
26. [Personalized Steering Feel Design for Steer-by-Wire Systems, Int. J. Automotive Technology](https://link.springer.com/article/10.1007/s12239-023-0094-6)
27. [GB/T 45829-2025 Functional safety of EPS systems](https://www.chinesestandard.net/PDF.aspx/GBT45829-2025)
28. [Functional Safety Assessment of a Generic Electric Power Steering System, US DOT/NHTSA](https://rosap.ntl.bts.gov/view/dot/37209/dot_37209_DS1.pdf)
29. [NHTSA ODI investigation PE10-020](https://static.nhtsa.gov/odi/inv/2010/INRL-PE10020-42020P.pdf)
30. [NHTSA Closes Tesla Power Steering Investigation, LightMyTesla](https://lightmytesla.com/news/tesla-nhtsa-power-steering-probe-closed-376k-june-2026)
31. [Power Steering: An Evolution of Systems, Fluids, and Essential Care, Gears Magazine](https://gearsmagazine.com/magazine/power-steering-an-evolution-of-systems-fluids-and-essential-care/)
---
*Topic: Encyclopedia › Technology and the built world › Transport and spaceflight › Road transport › Automobiles › Automotive electronics and systems*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*
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