What is the P0099 Code?
P0099 is a generic OBD-II powertrain-related error code. It indicates that the Engine Control Module (PCM) has detected an intermittent or erratic signal from the Intake Air Temperature (IAT) sensor circuit 2. While specific diagnostic procedures may vary, this issue can potentially affect various brands (such as Ford, Mazda, Mercedes-Benz, etc.).
Role of the IAT Sensor and Symptoms
The Intake Air Temperature (IAT) sensor is crucial for the engine’s proper operation. The PCM uses its data, along with data 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:
- Reduced engine performance
- Increased fuel consumption
- Stalling or hesitation during acceleration
- Irregular idling
- Potential illumination of the Malfunction Indicator Lamp (MIL)
Due to its impact on the air-fuel ratio, the P0099 code is considered serious and requires prompt diagnosis.
How the IAT Sensor Works
The IAT sensor is a thermistor whose electrical resistance changes with the temperature of the incoming air. It is often located in the intake duct, inside the air filter housing, or integrated into the MAF sensor. The PCM sends a reference voltage (typically 5V), measures the returning voltage, and then converts it into a temperature reading. An intermittent signal is interpreted as a fault, triggering the P0099 code.
Possible Causes of the P0099 Code
- Electrical Issues: Frayed wiring, or loose, rusty, or corroded connectors on the IAT sensor circuit 2.
- Faulty Sensor: A malfunction of the IAT sensor circuit 2 itself.
- Intake Issues: A clogged air filter or a cracked air duct can distort measurements.
- MAF Sensor Problems: A faulty MAF sensor can sometimes be related.
- PCM Malfunction: More rarely, the Engine Control Module itself could be the cause.
Diagnosing and Repairing the P0099 Code
⚠️ Safety Advice: Before starting, consult the manufacturer’s Technical Service Bulletins (TSBs) to check if this issue is subject to a recall or specific procedure.
1. Visual Inspection
- Ensure the IAT sensor circuit 2 connector is properly connected. This is a very common cause, especially after work on the air filter.
- Inspect the wiring and connectors for signs of corrosion, damage, or loose wires.
- Check the condition of the air filter and intake ducts for cracks or blockages.
2. Testing with an OBD2 Scanner and Multimeter
- Use an OBD2 scanner to read the IAT sensor’s live data. The values should be plausible and change gradually as the engine warms up from cold.
- Use a multimeter to measure the sensor’s resistance (out of circuit) and compare it with the manufacturer’s reference values at different temperatures.
- Verify the presence of the reference voltage (5V) and ground at the sensor connector.
3. Corrective Actions
- Clean or tighten electrical connectors as necessary.
- Repair or replace any damaged wiring.
- If the sensor is out of specification, replace the IAT sensor circuit 2.
- Replace a dirty air filter and repair any damaged air ducts.
💡 Pro Tip
If all wiring and sensors are in perfect condition, the issue might stem from a faulty PCM, although this is very rare. Always check the simpler causes first.
Related Codes
Temperature and air sensor-related codes: P0095, P0096, P0097, P0098, P0110, P0111, P0112, P0113.
Conclusion
The P0099 code indicates an intermittent issue in the Intake Air Temperature sensor circuit 2. While often easily resolved (e.g., a disconnected connector), it should not be ignored due to its impact on fuel economy and performance. A careful visual inspection is the first step towards a successful repair.
Not confident performing these operations? Entrust the diagnosis to a professional mechanic for a quick and guaranteed solution.