Ambient Air Temperature Sensor Mercedes S-Class W220 SL R230 SLK R171 2001-2012

The intake air temperature (IAT) sensor measures the temperature of air entering the engine and sends this data to the ECU. Cold dense air contains more oxygen than warm thin air and requires more fuel for the same combustion result. The ECU uses IAT data alongside manifold pressure or airflow data to calculate the correct fuel injection quantity and ignition timing for the current air temperature conditions. On many vehicles the IAT sensor is integrated into the mass airflow (MAF) sensor housing.

Common symptoms include poor fuel economy as the ECU miscalculates fuel delivery based on incorrect temperature data, rough idle or hesitation under acceleration, black smoke from the exhaust due to over-fuelling, hard starting particularly from cold, and a Check Engine light with IAT circuit fault codes P0110 to P0114. Some vehicles may also experience difficulty starting because the ECU cannot correctly calculate the cold start enrichment required.

The IAT sensor is a thermistor that changes resistance with temperature. With a cold engine (ambient temperature) the resistance is high - typically 2,000 to 3,000 ohms at 20 degrees C. As temperature rises resistance falls. You can test resistance with a multimeter at the sensor connector and compare against the temperature/resistance chart in your vehicle service manual. An OBD2 scanner showing an IAT reading that does not match ambient temperature when the engine is cold is also a strong indicator of failure.

No - they are different sensors, although on many modern vehicles the IAT sensor is built into the MAF sensor housing as a combined unit. The MAF sensor measures the mass of air entering the engine using a hot wire element. The IAT sensor measures the temperature of that air using a thermistor. If you have a combined MAF/IAT unit that fails, you typically need to replace the complete assembly. Use the Vehicle Part Finder to confirm which type is fitted to your vehicle.

Yes. An incorrect IAT reading causes the ECU to miscalculate fuel delivery, typically resulting in a rich running condition with higher CO and hydrocarbon emissions. An active fault code from the IAT sensor will also cause most roadworthy and emissions inspection systems to flag the vehicle as failing the on-board diagnostics check before exhaust emissions are even measured. Resolve any IAT sensor faults before presenting for an emissions test.

No - the IAT sensor is a passive thermistor requiring no calibration or programming. It is a direct plug-in replacement. Clear any stored fault codes with an OBD2 scanner after fitting the new sensor. The ECU will immediately begin reading the new sensor data and adjust fuelling accordingly. If the vehicle has been running rich due to the faulty sensor, fuel trim values will self-correct over several drive cycles.