What does it mean?
P1191 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Inlet Air Temperature Sensor Rational/Performance .
Symptoms
- Check engine light on
- Hard starting when cold or when hot, depending on which direction the reading is wrong
- Rough idle and hesitation during acceleration
- Higher fuel consumption and black smoke if the module adds too much fuel
- Reduced power or limited boost on turbocharged and supercharged engines
- Cooling fan behaviour may change
Likely causes
- Sensor element drifted or coated with oil film and dust, so it responds slowly and reads wrong
- Sensor located where heat soak from the engine distorts the reading, worsened by a missing air duct or heat shield
- High resistance in the sensor wiring or corroded connector pins shifting the calculated temperature
- Incorrect replacement sensor with a different resistance curve
- Charge air cooler blocked or its airflow obstructed on boosted engines, giving genuinely high inlet temperatures
- Intake leak drawing hot engine bay air instead of cool filtered air
- Rarely, a control module input or calibration fault
How to fix it
- After the vehicle has stood overnight, compare intake air temperature with coolant temperature in live data; both should be close to ambient
- Measure the actual air temperature at the sensor with a thermometer and compare it with the scan tool value
- Remove the sensor and inspect the element for oil film and dirt; clean carefully or replace it
- Measure sensor resistance and compare with the temperature-resistance table
- Check the wiring for high resistance with a voltage drop test and inspect the connector for corrosion
- Confirm the air ducting, heat shields and charge air cooler are intact and unobstructed
- Replace the sensor if it is out of specification, then verify a plausible reading during warm-up and driving