What causes fault code P050B?

• Most likely: Faulty or dirty Idle Air Control (IAC) valve or throttle body • Likely: Faulty Engine Coolant Temperature (ECT) sensor providing incorrect readings • Possible: Worn spark plugs or failing ignition coils • Possible: Vacuum leak downstream of the throttle body • Rare: Faulty Crankshaft Position (CKP) sensor or ECU software issue

How do you fix fault code P050B?

STEP 1: Inspect the throttle body and clean carbon deposits if necessary. Check vacuum lines for any signs of leaks. STEP 2: Use a scan tool to compare Engine Coolant Temperature (ECT) and Intake Air Temp (IAT) on a cold engine; they should be within 2-3 degrees Celsius of each other. ECT resistance should be approx. 2000-3000 Ohms at 20°C. STEP 3: Inspect spark plug condition. If ignition performance is suspected, test ignition coil resistance (primary/secondary). STEP 4: Perform a 'Throttle Relearn' procedure via scan tool if the throttle body was cleaned. Check for ECU software updates regarding cold start strategies.

Powertrain (Engine & Gearbox)Standard EOBD

P050B - Cold Start Ignition Timing Retard Performance

Q: What are the symptoms of fault code P050B?

• Check Engine Light (MIL) illuminated • Rough idle immediately after cold start • Engine may stall during the initial start sequence • Increased emissions during cold start (may affect emissions testing)

Last updated:1 February 2026

Related:ECT sensor, Throttle Body, Spark Plugs, Crankshaft Position Sensor, IAC Valve

System

Powertrain (Engine & Gearbox)

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Technical Description

P050B is stored when the Engine Control Unit (ECU) monitors the ignition timing during a cold start and determines that the timing retard performance is not meeting the calibrated strategy. During cold start, the ECU retards ignition timing to increase exhaust gas temperature, which accelerates catalytic converter light-off. If the crank sensor feedback indicates that the engine speed is unstable or the timing adjustment isn't achieving the desired thermal effect within the specified time window, the code is triggered.

Possible Causes

Faulty or dirty Idle Air Control (IAC) valve or throttle body

Faulty Engine Coolant Temperature (ECT) sensor providing incorrect readings

Worn spark plugs or failing ignition coils

Vacuum leak downstream of the throttle body

Faulty Crankshaft Position (CKP) sensor or ECU software issue

Symptoms

Check Engine Light (MIL) illuminated

Rough idle immediately after cold start

Engine may stall during the initial start sequence

Increased emissions during cold start (may affect emissions testing)

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P050B - Cold Start Ignition Timing Retard Performance

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P050B - Cold Start Ignition Timing Retard Performance

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