What is the P0099 Code?
The P0099 error code is a generic OBD-II code related to the powertrain. It indicates that the Powertrain Control Module (PCM) has detected an intermittent or erratic signal from the Intake Air Temperature (IAT) sensor circuit #2. This issue can affect various brands (Ford, Mazda, Mercedes-Benz, etc.), although specific diagnostic procedures may vary.
Role of the IAT Sensor and Symptoms
The IAT (Intake Air Temperature) sensor is crucial for the proper functioning of the engine. The PCM uses its data, together with that from the Mass Air Flow (MAF) sensor, to calculate the optimal air-fuel ratio (typically 14.7:1) and ignition timing. A faulty signal disrupts these calculations, leading to:
- Decreased engine performance
- Increased fuel consumption
- Jerking or hesitation during acceleration
- Irregular idle
- Possible illumination of the Malfunction Indicator Lamp (MIL)
Due to its impact on the air-fuel mixture, a P0099 code should be considered serious and diagnosed promptly.
How the IAT Sensor Works
The IAT sensor is a thermistor whose electrical resistance varies with the temperature of the intake air. It is often located in the intake duct, the air filter housing, or integrated into the MAF sensor. The PCM sends it a reference voltage (usually 5V) and measures the returned voltage, which it then converts into temperature. An intermittent signal is interpreted as a fault, triggering the P0099 code.
Possible Causes of the P0099 Code
- Electrical Problems: Frayed wires, loose, oxidized, or corroded connector on the IAT sensor #2.
- Faulty Sensor: The IAT sensor #2 itself is malfunctioning.
- Air Intake Problems: A clogged air filter or a cracked air duct can distort readings.
- Problem with the MAF Sensor: A faulty MAF sensor can sometimes be related.
- Faulty PCM: More rarely, the Powertrain Control Module may be the cause.
How to Diagnose and Repair a P0099 Code?
⚠️ Safety Tip: Before starting, check your manufacturer’s Technical Service Bulletins (TSBs) to see if this issue is subject to a recall or specific procedure.
1. Visual Inspection
- Check that the IAT sensor #2 connector is properly plugged in. This is a very common cause after work on the air filter.
- Inspect the wiring and connector for signs of corrosion, damage, or loose wires.
- Check the condition of the air filter and intake duct for possible cracks or obstructions.
2. Testing with an OBD2 Scanner and Multimeter
- Using an OBD2 scanner, read the live data from the IAT sensor. The value should be plausible and change slowly when the engine is cold and then warms up.
- Use a multimeter to measure the sensor’s resistance (out of circuit) and compare it to the reference values provided by the manufacturer at different temperatures.
- Check for the presence of the reference voltage (5V) and ground at the sensor connector.
3. Corrective Actions
- Clean or tighten the electrical connector if necessary.
- Repair or replace any damaged wiring.
- If the sensor is out of specifications, replace the IAT sensor #2.
- Replace the air filter if it is dirty and repair any damaged air duct.
💡 Pro Tip
If all the wiring and the sensor are in perfect condition, the problem may come from a faulty PCM, but this is extremely rare. Check the simple causes first.
Related Codes
Codes related to temperature and air sensors: P0095, P0096, P0097, P0098, P0110, P0111, P0112, P0113.
Conclusion
The P0099 code indicates an intermittent problem with the intake air temperature sensor circuit #2. Although often simple to resolve (an unplugged connector), it should not be ignored as it affects fuel consumption and performance. A thorough visual inspection is the first step towards a successful repair.
Not comfortable with these procedures? Entrust the diagnosis to a professional mechanic for a quick and guaranteed resolution.