"Steering assist reduced": voltage before you condemn the rack

Electric power steering replaced hydraulic systems because it uses energy only when the steering is turned. The price is that when it does work, it draws a lot of current — enough that the electrical system’s health directly determines whether it works at all.

Related: power steering warning light.

Why voltage comes first

Turning the steering at parking speeds is when the motor demands most. If supply voltage sags under that load, the module reduces or removes assistance to protect itself and to keep the rest of the vehicle running.

The message says the steering is reduced. The cause is often the supply.

Check, in order:

1. Battery terminals — corroded or loose: battery terminals corrosion.
2. Battery condition, tested under load rather than by voltage reading.
3. Ground straps, engine-to-body and body-to-battery.
4. Charging output: alternator charging diagnostics.
5. The module’s own supply and ground.

P0560 to P0563 system voltage faults.

Log voltage across a drive that includes parking manoeuvres. A sag during steering is the fault, recorded.

The overheat protection

Not a fault, and worth recognising.

The motor and its electronics have a thermal limit. Sustained heavy use — repeated full-lock manoeuvres while parking, or manoeuvring a trailer — heats them, and the system reduces assistance to protect itself.

The signature: it happens after several minutes of heavy steering use, and recovers after a few minutes of normal driving.

That is designed behaviour. If your fault fits that pattern exactly, there may be nothing wrong.

Genuine faults

The steering angle sensor

Assistance varies with speed and steering angle. If the module cannot trust the angle input, it reduces assistance.

Very often a missing calibration after alignment work: C0051 steering position sensor, U0126 lost communication with steering angle and steering angle sensor calibration.

The torque sensor

Measures how hard you are turning the wheel, and is what tells the motor how much to help. Failure produces reduced or erratic assistance.

Usually integrated into the column or rack assembly.

The motor or module

Genuine failures, and expensive. Diagnosed after supply and sensors are eliminated — not before.

Water

Column-mounted units are inside the cabin, and rack-mounted units are underneath. Both can be reached by water: a blocked scuttle drain for one, road spray for the other.

Damp carpet and electrical faults and water leak into cabin.

Network communication

Assistance depends on vehicle speed, which arrives as a message. Lose that message and the module cannot vary assistance safely.

If the speedometer also stopped, that is the connection: speedometer not working and U0073 control module bus off.

Driving with it

Steering does not fail completely — it becomes heavy, like a car without power steering. That is manageable at speed and hard work at parking speeds.

It is a safety-relevant reduction, particularly in an emergency manoeuvre. Get it resolved rather than adapting to it.

Power steering heavy diagnosis.

Hydraulic systems are different

If your vehicle has hydraulic power steering, this article does not apply. There, the causes are fluid level, the belt, the pump and leaks.

Power steering fluid and noise.

In what order

1. Note whether it happens after heavy steering use — that is thermal protection.
2. Load-test the battery and inspect the terminals.
3. Log voltage during parking manoeuvres.
4. Check whether alignment work was done without calibration.
5. Check whether other systems also lost function.
6. Read the module where accessible.

This check needs no workshop — an ELM327 adapter and the free tier of the app cover it. Log system voltage while turning the steering lock to lock at a standstill. A dip during that test is the answer, and it is a much cheaper answer than a steering rack.