What causes fault code P055D?

• Most likely: Faulty NOx sensor (internal heater control unit burnt or shorted) • Likely: Wiring issues (corrosion in connectors or chafed harness to CAN-bus/power supply) • Possible: Defective component supply relay or blown fuse for the aftertreatment system • Rare: ECU software issues or extreme exhaust leaks upstream of the sensor

How do you fix fault code P055D?

STEP 1: Check fuses for the aftertreatment system (SCR/NOx). Inspect the NOx sensor connector for signs of moisture or corrosion. STEP 2: Measure supply voltage to the sensor. It should have battery voltage (approx. 12.6-14.4V) and a solid ground connection (less than 0.3 Ohm to chassis). Check CAN-High and CAN-Low signals (approx. 2.5V each with ignition on). STEP 3: Use a diagnostic tool to monitor 'NOx Sensor Heater Status'. If the sensor remains in 'Pre-heating' status despite a warm engine, the internal heater control unit is faulty. STEP 4: Replace the NOx sensor. NOTE: Replacement often requires a 'Reset of Adaptations' or coding via the diagnostic tool for the ECU to recognize the new component.

Powertrain (Engine & Gearbox)Standard EOBD

P055D - Nitrogen Oxide (NOx) Sensor Heater Control Unit Performance

Q: What are the symptoms of fault code P055D?

• Check Engine Light (MIL) illuminated • Service Exhaust System / AdBlue fault message on dash • Vehicle may enter 'Limp Mode' (reduced power) • Potential no-start countdown (inducement strategy) • Increased fuel consumption due to lack of SCR optimization

Last updated:1 February 2026

Related:NOx Sensor (Bank 1 Sensor 1 / Bank 1 Sensor 2), SCR Control Module, Engine Control Unit (ECU), AdBlue Doser, CAN-bus wiring

System

Powertrain (Engine & Gearbox)

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Technical Description

P055D is stored when the Engine Control Unit (ECU) detects that the NOx sensor integrated control module is unable to correctly regulate the heater element. The NOx sensor requires a high operating temperature (approx. 700-800°C) to allow the ceramic sensing elements to function. The ECU monitors current draw and the time taken to reach operational temperature via PWM (Pulse Width Modulation). If the heater's thermal response or current consumption deviates from the mapped logic, P055D is triggered. This fault directly impacts the SCR system's efficiency and AdBlue dosing accuracy.

Possible Causes

Faulty NOx sensor (internal heater control unit burnt or shorted)

Wiring issues (corrosion in connectors or chafed harness to CAN-bus/power supply)

Defective component supply relay or blown fuse for the aftertreatment system

ECU software issues or extreme exhaust leaks upstream of the sensor

Symptoms

Check Engine Light (MIL) illuminated

Service Exhaust System / AdBlue fault message on dash

Vehicle may enter 'Limp Mode' (reduced power)

Potential no-start countdown (inducement strategy)

Increased fuel consumption due to lack of SCR optimization

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P055D - Nitrogen Oxide (NOx) Sensor Heater Control Unit Performance

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P055D - Nitrogen Oxide (NOx) Sensor Heater Control Unit Performance

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