A 2015 Mercedes-Benz GLA came in with a dead power steering system that miraculously cured itself after a restart. The vehicle is an X156 chassis in the UK market. The technician noted the complaint exactly as the driver experienced it: "Power steering failed, now working again. Warning light came on for power steering malfunction." We see this specific scenario often in our own data at Wrenchlane. A system shuts down completely, the driver pulls over, restarts the car, and the steering wheel feels perfectly normal. The scanner pulls a single code, C159209.
This code points to a serious malfunction, but the intermittent nature makes it tough to diagnose confidently. You cannot simply throw a steering rack at a car without proving the fault, but you also cannot give the car back to a customer with steering that might drop out at highway speeds. The diagnostic platform we build ranked the internal electric power steering actuator motor and control unit hardware failure as the highest probability here, sitting at 70 percent severity high.
Analyzing the Hardware Inside the EPS Actuator
The electromechanical steering gear on the X156 chassis is a complex piece of engineering. It incorporates a three-phase brushless DC motor, an integrated electronic power steering control unit known as N68, and a rotor position sensor sealed directly into the steering rack housing. Prolonged highway driving creates a harsh environment for this assembly. The steering system experiences sustained thermal soak from the engine compartment and the underbody exhaust. Because the driver makes minimal steering movements on a straight highway, the system gets very little dynamic cooling.
Code C159209 specifically denotes an internal hardware failure of this steering actuator motor assembly. Inside the integrated assembly, laser-welded busbar pins, phase winding terminations, or surface-mount motor bridge driver components face constant thermal expansion. They can develop micro-cracks or high electrical resistance. When the internal control unit senses phase current asymmetry or resolver tracking errors, it immediately initiates a fail-safe shutdown. This prevents uncommanded steering intervention, which is exactly why the assist vanished. Once the ignition is cycled and the assembly cools down, thermal contraction temporarily restores electrical continuity. That explains why the power assistance returned for the technician.
To prove this is happening, the suggested tests involve monitoring live scan data for the EPS motor phase current, internal temperature, and rotor angle during a steering sweep. A technician should also perform a thermal and mechanical vibration tap test on the actuator motor housing while watching the steering assist status. Checking the internal motor phase winding resistance and isolation resistance at the motor terminals provides a solid confirmation before condemning the entire rack.
Chasing Voltage Drops in the Power Supply
If the internal hardware tests pass, the next most likely culprit is external power delivery. Our engine ranked EPS high-current power supply circuit resistance or pre-fuse terminal degradation at 45 percent. The electric power steering actuator motor demands massive current, often exceeding 70 to 100 amps during steering maneuvers. This power routes from the primary 12V battery through the front pre-fuse box (F32). It passes through a dedicated high-current fuse, usually an 80A or 100A unit at position F32f3, before traveling down a heavy-gauge cable to the steering gear.
Bolted eyelet connections at the pre-fuse box or intermediate cable joints can suffer from loose hardware, corrosion, or heat-induced oxidation. This drives up electrical resistance. Under extended highway driving, continuous underhood heat causes thermal expansion at any compromised terminal. The result is an acute voltage drop right when the driver requests steering torque. The EPS control unit perceives this sudden drop in supply voltage as an actuator motor driver failure. It instantly disables assistance and logs C159209.
Finding this requires a visual and torque inspection of the bolted high-current busbars and fuses in the front pre-fuse box. A loaded voltage drop test along the B+ circuit from the battery positive terminal to the EPS power connector under full steering lock is essential. Technicians should also run a charging system ripple and voltage stability test under full electrical accessory load.
Inspecting the Chassis Ground Path
The ground side is just as vulnerable. We ranked EPS chassis ground stud oxidation or high resistance at 38 percent. The steering rack relies on a dedicated chassis ground stud, W10, located on the front wheelhouse and frame rail structure. This stud must sink up to 100 amps of motor current. Because the steering gear sits low on the front subframe, the ground cable and terminal stud endure constant road salt, water spray, and thermal cycling.
Moisture intrusion beneath the ground eyelet causes galvanic oxidation, which creates an intermittent high resistance path. Under elevated electrical loads, this resistance causes a floating ground. The EPS motor control logic detects the voltage shift across its internal H-bridge MOSFETs as a hardware fault, triggering the same code and shutting down the assist. Testing this involves a voltage drop test on the EPS ground circuit between the rack casing and the negative battery post under full steering load. Physical disassembly, wire-brush cleaning, and retorquing of chassis ground stud W10 is often required to ensure a clean path.
Looking at the Wiring Harness and Connectors
The final piece of the puzzle is the wiring harness itself. EPS harness connector terminal fretting or moisture intrusion ranked at 32 percent probability. The EPS assembly connects to the vehicle wiring via a high-current power harness and a multi-pin signal harness that carries CAN bus and wake-up lines. These connectors are located low on the front axle carrier, exposing them to significant road wash, extreme temperature fluctuations, and relentless road vibration.
When the weather seals deteriorate, or when continuous highway driving induces micro-vibrations, fretting corrosion develops on the tin-plated or silver-plated terminal pins. This corrosion introduces micro-scale oxide layers. Those layers cause transient resistance spikes or signal dropouts between the harness and the internal actuator circuitry. If the EPS control unit detects momentary micro-interruptions in phase current feedback or supply voltage lines, it logs C159209 and shuts down power assist to prevent erratic behavior.
Diagnosis requires disconnection, cleaning, and microscopic terminal inspection of the EPS multi-pin and power plugs. Technicians must perform a terminal pin drag test using the appropriate female probe gauges to check retention tension. A thorough harness wiggle test while monitoring live EPS CAN communication and sensor voltages will usually reveal if fretting is the root cause.
Next Steps for the Workshop
Intermittent steering faults require a strict diagnostic process rather than immediate parts replacement. By checking the internal hardware data, verifying the high-current power and ground paths, and inspecting the connectors for fretting, a workshop can pin down exactly why a steering system shuts down and then recovers. Have you seen similar thermal expansion faults on the X156 platform in your bays?