P0099 - Intake Air Temperature Sensor 2 Circuit Intermittent/Erratic
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System
Powertrain (Engine & Gearbox)
Related Fault Codes
Technical Description
P0099 is set when the engine control module (ECM) detects an erratic or intermittent signal voltage from the Intake Air Temperature (IAT) sensor number 2. The IAT sensor is a negative temperature coefficient (NTC) thermistor. The ECM supplies a 5-volt reference circuit and monitors the voltage drop across the sensor. This specific code refers to 'Sensor 2', typically located post-intercooler in forced induction systems to monitor charge air cooler efficiency. The fault is triggered when the signal fluctuates rapidly outside of predefined logical parameters, often caused by poor connections or internal sensor failure.
Possible Causes
Poor connection or corrosion in the IAT sensor 2 connector
Faulty IAT sensor 2 (internal intermittent open circuit)
Damaged wiring harness (chafing against chassis or engine block)
Faulty ECU software or internal ECM failure
Symptoms
Recommended Solution
- Visual inspection: Check IAT sensor 2 connector for corrosion or backed-out pins. Clean with contact cleaner.
- Measurement: Verify 5V reference and ground with ignition on. At 20°C (68°F), sensor resistance should typically be 2000-3000 Ohms.
- Advanced diagnosis: Monitor live data for IAT sensor 2 while performing a 'wiggle test' on the wiring harness to identify intermittent opens or shorts.
- Component replacement: If wiring and reference voltages are correct but data remains erratic, replace the IAT sensor 2.
Mechanic's Tip
Before replacing expensive components, always double-check wiring and connections. 80% of all sensor faults are actually due to corroded contacts or broken wires.
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