What causes a power steering assist fault?
Power steering assist faults are usually caused by issues in the electric power steering system (EPS) or hydraulic power steering system, including faulty sensors or modules, low or dirty fluid, leaks, worn pumps or belts, or electrical problems.
Understanding the power steering systems
Most modern cars use either hydraulic power steering (HPS), electric power steering (EPS), or a hybrid arrangement that blends both. In EPS-equipped vehicles, an electric motor provides assist under the control of a steering-angle sensor, torque sensor, and the vehicle’s computer. In hydraulic systems, a pump driven by the engine circulates hydraulic fluid to create assist, regulated by valves and pressure feedback. When a fault occurs, the car may display a warning light and the steering feel can change dramatically—from effortful to nearly effortless, or from stable to erratic.
Key distinctions
EPS faults are often electrical or software-related and can trigger warning lights even if hydraulic fluid is fine. Hydraulic faults typically present as stiffness, noise, or leaks and may not always illuminate EPS-related warnings.
Common causes of a steering assist fault
Below are the main culprits that can trigger a steering assist fault, organized by the system involved. This helps technicians narrow down the root cause and decide on the right repair path.
- Electric power steering (EPS) related causes:
- Faulty EPS motor or actuator, worn torque sensor, or damaged wiring to the motor.
- Faulty steering angle sensor or control module/ECU software issue.
- Circuit or CAN bus communication errors causing the ECU to lose track of steering input.
- Low or fluctuating battery voltage or poor grounding affecting the EPS electronics.
- Overheating or thermal shutdown of the EPS unit.
- Hydraulic power steering (HPS) related causes:
- Low fluid level, dirty or contaminated fluid, or air in the system after leaks or changes.
- Leaking hoses, seals, or the steering rack; pump inlet leaks reduce assist.
- Worn or slipping power steering pump or worn drive belt; belt tension issues reduce pump output.
- Reservoir cap issues or clogged return lines causing pressure imbalances.
- Other contributing factors:
- Faulty fuses or relays feeding the EPS or hydraulic pump.
- Damaged or corroded connectors, grounds, or wiring near the steering column.
- Steering column play or mechanical binding within the rack and pinion.
- Recent repairs that disturbed wiring or connectors.
- Software updates or manufacturer service bulletins addressing known EPS faults.
Identifying the exact cause often requires diagnostic codes read from the vehicle’s computer, testing of electrical circuits, and sometimes specialized equipment to measure steering torque, motor current, and hydraulic pressure under load.
When a fault is present, warning lights such as EPS, Power Steering, or ESP/BAS may illuminate, and the steering feel can become heavy, inconsistent, or suddenly go limp. If you notice any of these signs, plan a safe stop and seek professional diagnosis promptly.
Diagnostic path and immediate safety steps
To guide owners and technicians through a basic initial assessment, consider these steps. This helps determine whether the issue is likely electrical, hydraulic, or environmental (e.g., battery/charging system) and whether immediate action is warranted.
- Check for warning lights and retrieve codes with an OBD-II scanner. EPS-related codes (if present) point to motor, sensor, or communication issues; hydraulic faults may not always produce EPS codes but can appear as steering-related codes or hydraulic system warnings.
- Inspect the hydraulic system (if your car uses HPS): verify fluid level and condition, look for leaks around the pump, hoses, reservoir, and steering rack, and check for air intrusion.
- Inspect the drive belt and tensioner (for hydraulic systems): a worn or slipping belt can reduce pump output and steering assist.
- Test the electrical health of the vehicle: ensure the 12V battery is healthy, connections are clean and tight, and grounds are solid. Low voltage can disrupt EPS performance.
- Inspect wiring and connectors to the EPS motor and related sensors for corrosion, fraying, or loose pins.
- Consider software/firmware updates or service bulletins from the manufacturer, as some faults are resolved with updates.
If any fault codes are retrieved, or if the symptoms persist, arrange a professional diagnostic with a qualified technician who has access to OEM diagnostic tools and the vehicle’s service literature.
What to do if you experience a steering assist fault
Safety is paramount. If you encounter a steering assist fault while driving, gradually slow down and pull over to a safe location if possible. Do not continue driving aggressively or at highway speeds with uncertain steering.
- Check for obvious issues such as fluid leaks (hydraulic systems) or loose/damaged wiring near the steering rack or column.
- If equipped, note any warning lights and attempt a vehicle restart to see if the EPS warning resets, but do not rely on this as a fix.
- Have the vehicle towed or driven only with extreme caution to a service facility for a formal diagnosis.
- Request a full EPS/HPS diagnostic, including fault-code readout, sensor and motor tests, and an inspection of the electrical system and software.
Repair approaches vary by root cause. EPS faults may require motor replacement, torque/angle sensor repair, or ECU/software updates, while hydraulic faults often involve fluid service, pump or hose replacement, and possible rack repair. In some cases, safety controls may limit steering assist to protect the driver, resulting in a stubbornly heavier steering feel until repairs are completed.
Summary
Power steering assist faults typically arise from electrical issues in EPS or from hydraulic problems in HPS. Common culprits include a faulty motor or sensor, wiring/connectivity problems, low or contaminated hydraulic fluid, leaks, a worn pump or belt, and software or ECU faults. Accurate diagnosis hinges on fault codes, electrical testing, and sometimes live-system measurements, and driving with a suspected fault should be avoided until a professional evaluation is completed.
