P011B: Engine Coolant Temperature/Intake Air Temperature Correlation
What to do
Say whether the temperature gauge reaches its normal position and how long the heater takes to get warm. A car that stays cool and heats slowly has a thermostat stuck open — the sensors are telling the truth and the fix is a cheap part rather than an electrical diagnosis.
Leaving the car overnight helps diagnosis considerably. These faults are found from stone cold, when all the temperature sensors should agree; a warm engine tells the garage much less, so expect to leave it rather than wait.
Mention any recent cooling system work. Air trapped in the system after a coolant change can sit around the sensor and produce a false reading.
Not urgent, but do not ignore it. An engine running cold uses more fuel, wears faster, and will struggle to complete particulate filter regeneration on a diesel.
What this code means
The coolant temperature and the intake air temperature disagree by more than the module allows.
This is a correlation code, so it names a relationship rather than a component — and the test behind it is one of the neatest in engine management. After a vehicle has stood overnight, every temperature sensor on it must read close to ambient and close to each other. Coolant, intake air, oil, fuel: all of them have had hours to reach the same temperature as the air around the car.
So the module does not need a road test to catch a drifted sensor. It compares them at the next cold start, and the one that disagrees at that moment has declared itself.
Which sensor is wrong is the question the code does not answer, and there are two ways to settle it. Compare both against a third temperature — ambient air, oil, or fuel — or simply against a thermometer in the workshop.
There is also a genuinely mechanical cause worth eliminating first, because it is common and cheap. A thermostat stuck open keeps the engine running far below its proper temperature. The coolant sensor is then reporting the truth, the disagreement is real, and no sensor needs replacing at all.
A car that never reaches full temperature on the gauge, takes a long time to give heat, and returns poor economy is describing a stuck thermostat, and this code alongside those symptoms is the confirmation rather than a separate fault.
Likely causes
Start here
- Thermostat stuck open · moderate
- Faulty coolant temperature sensor · easy
- Faulty intake air temperature sensor · easy
Less common
- Air trapped in the coolant circuit · moderate
- High resistance in sensor wiring · moderate
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
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