P0111 and P0112: intake air temperature sensor faults

Cold air is denser than warm air, so the same volume contains more oxygen and needs more fuel. The intake air temperature sensor is how the ECU knows which it is dealing with.

It is a thermistor, it costs very little, and on many engines it is built into the mass airflow sensor rather than standing alone.

Code	Meaning
P0111	IAT sensor range/performance
P0112	Circuit low input
P0113	Circuit high input

Related: P0113 to P0118 coolant and intake temp sensors.

P0111 is the interesting one

P0112 and P0113 are electrical — a short or an open, and the reading will be pinned at an extreme.

P0111 means the circuit is intact but the value is implausible. The ECU cross-checks intake air temperature against coolant temperature and ambient temperature, particularly after a long soak, and flags a sensor that disagrees with the others.

A reading of exactly −40 °C or the top of the scale is not a measurement. Those are the default values when a thermistor circuit opens or shorts.

The overnight test

The single most useful diagnostic, and it needs no tools beyond an adapter.

After the car has stood overnight, everything under the bonnet is at the same temperature. Before starting:

1. Read intake air temperature.
2. Read coolant temperature.
3. Read ambient air temperature if the vehicle has that sensor.

All three should agree within a few degrees. Whichever disagrees is the faulty one.

Result	Verdict
All within ~3 °C	Sensors healthy
IAT alone is off	IAT sensor at fault
IAT pinned at −40	Open circuit
All three off together	Suspect the reference or the scan data, not three sensors

What is normal that people mistake for a fault

Intake air temperature is not outside air temperature, and the difference is by design.

• After a long idle in traffic, IAT rises well above ambient — the sensor is measuring air heated by the engine bay. On a hot day it can read 20–30 °C above outside air. That is correct.
• On a turbocharged engine, post-intercooler IAT rises under sustained boost. Also correct.
• In cold weather at motorway speed, IAT should track close to ambient because airflow is high.

The reading that indicates a problem is one that does not respond to conditions at all. A sensor stuck at a plausible-looking 20 °C in every situation is worse than one stuck at −40, because nothing flags it.

What goes wrong

Contamination

Where the sensor sits in the intake tract, oil mist from the crankcase ventilation system coats it. A coated thermistor responds slowly, which is exactly what P0111 describes.

On engines where the sensor lives inside the MAF housing, cleaning the MAF often cleans this too: MAF/MAP sensor diagnostics.

Oil in the intake is itself a finding — heavy contamination points at crankcase ventilation, which is worth checking: air filter and engine breathing.

Connector and wiring

Two wires, one connector, usually accessible. Corrosion and chafing are the failures.

The sensor

Thermistors drift with age. A sensor that reads 10 °C high consistently is drifting, not broken, and it will not set a code until the drift exceeds the plausibility window.

What it costs you while it is wrong

Nothing dramatic, which is why people live with it:

• Slightly wrong fuelling, so slightly worse economy: fuel consumption increased causes.
• Ignition timing corrected for the wrong air density, so a little less power.
• Poor cold-start behaviour if the sensor reads warm when the engine is cold.
• Fuel trims drifting to compensate, which can eventually set mixture codes.

That last one is the reason to fix it. A wrong IAT pushes long-term fuel trim, and a pushed fuel trim eventually reads as P0171: fuel trim STFT and LTFT explained.

Practical sequence

1. Do the overnight comparison — it identifies the faulty sensor immediately.
2. Inspect the sensor for oil contamination and clean it if it is dirty.
3. Check the connector.
4. Watch IAT respond to a drive: it should rise in traffic and fall at speed.
5. Replace if it does not track conditions.
6. Clear long-term fuel trim after the repair so the ECU relearns from a correct reading.

Try it on the free tier of the app — pair an ELM327 adapter and compare intake, coolant and ambient temperature before your first start of the day. That one comparison is the whole diagnosis.