Cause
Faulty coolant temperature sensor
The sensor telling the module how warm the engine is, drifted or failed. It drives more decisions than almost any other single input — cold-start enrichment, when closed-loop fuelling begins, ignition timing, cooling fan operation, the cold-start catalyst warm-up strategy, and on a diesel whether particulate filter regeneration is permitted at all.
Which is why its faults present as something else entirely. A sensor reading warm on a cold engine silently disables the whole warm-up strategy: the module believes the engine is up to temperature, so it skips the enrichment and the retarded ignition that light the catalyst off. Nothing feels wrong and the emissions are far worse. A sensor reading cold does the opposite — permanent enrichment, poor economy, and a catalyst fed a rich mixture.
Many vehicles carry two sensors, one for the module and one for the gauge, so a normal-looking temperature gauge does not rule this out.
The cold soak is the decisive test: after standing overnight it must read close to ambient and close to every other temperature sensor. Check the wiring too — added resistance shifts the reading consistently and looks exactly like a drifted sensor.
Typically an easy fix. Often worth checking before paying anyone to look.
Codes this can cause (7)
Codes where this is a likely explanation first, rather than merely a possible one.
- P00B2 Radiator Coolant Temperature Sensor Circuit Range/Performance Likely for this code Moderate
- P011B Engine Coolant Temperature/Intake Air Temperature Correlation Likely for this code Moderate
- P050A Cold Start Idle Control System Performance Likely for this code Moderate
- P050B Cold Start Ignition Timing Performance Likely for this code Moderate
- P015E Excessive Time to Enter Closed Loop Fuel Timing Control Moderate
- P050E Cold Start Engine Exhaust Temperature Too Low Moderate
- P245B Exhaust Gas Recirculation Cooler Bypass Control Circuit Range/Performance Bank 1 Moderate
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