P0550 — Power Steering Pressure Sensor Circuit Malfunction

Causes

• Open or short in the sensor signal, power or ground circuit
• Corroded, bent or disconnected sensor connector/pins
• Failed power steering pressure sensor
• Low or dirty power steering fluid / internal hydraulic fault causing unexpected sensor output
• Intermittent wiring fault from chafing or water intrusion
• Poor ground at sensor or ECU

Symptoms

• Steering warning lamp or message on dash (e.g., Power Steering Fault)
• Reduced or variable power steering assist; heavier steering at low speed
• Intermittent or constant fault code stored (P0550)
• Possible vehicle enters reduced-assist or limp-steer mode
• Noisy steering pump or unusual steering feel if hydraulic issue present

What to check

• Scan vehicle and record freeze-frame and related codes; clear codes and re-test
• Visually inspect steering pressure sensor, connector and harness for damage, corrosion or loose pins
• Check power and ground at sensor connector with chassis grounded and key ON (refer to wiring diagram for pin locations)
• Back-probe sensor signal wire and record voltage with key ON and while turning steering lock-to-lock
• Wiggle test harness while monitoring live data for intermittent changes
• Inspect power steering fluid level and condition; check for leaks

Signal parameters

• Typical sensor type: 3‑wire (reference, signal, ground) or 2‑wire variable resistor depending on model
• Common reference voltage: 5 V (verify with factory wiring diagram)
• Expected signal range: approximately 0.5–4.5 V (low pressure ≈0.5–1.5 V, higher pressure ≈2.5–4.5 V) — verify Mercedes-Benz spec
• Response: signal voltage should rise smoothly with increased steering load/pressure (turning from center to lock)
• No continuity (open) or short to battery voltage/ground is a fault

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