P0099 - Intake Air Temperature Sensor (IAT) Sensor '2' Circuit Intermittent/Erratic (Bank 1)
System
Powertrain (Engine & Gearbox)
Technical Description
The control module has detected a fault in the Intake Air Temperature Sensor (IAT) Sensor '2' Circuit Intermittent/Erratic (Bank 1) (P0099). The fault is sporadic — usually a loose terminal, corrosion, or a chafed wire breaking contact under vibration or temperature change. Pertains to downstream oxygen sensor (post-catalyst diagnostic sensor) evaluating catalytic converter conversion efficiency.
Multimeter Test Points & Nominal Values
| Test Point / Component | Measurement | Nominal Target Range | Test Condition |
|---|---|---|---|
| Intake Air Temperature Sensor (IAT) Sensor '2' Circuit Intermittent/Erratic (Bank 1) – Resistance at 20 °C | Resistance (kΩ) | 2.0 - 3.0 kΩ | Sensor disconnected, cold engine |
| Intake Air Temperature Sensor (IAT) Sensor '2' Circuit Intermittent/Erratic (Bank 1) – Resistance at 80 °C | Resistance (Ω) | 250 - 400 Ω | Sensor disconnected, at operating temp |
| Intake Air Temperature Sensor (IAT) Sensor '2' Circuit Intermittent/Erratic (Bank 1) – Signal Voltage | Voltage (V) | 0.3 - 4.5 V (falls as temperature rises) | Ignition ON, sensor connected |
| 5 V Reference / Signal Ground | Voltage (V) | 4.95 - 5.05 V / < 0.05 V | At sensor connector |
Sensor disconnected, cold engine
Sensor disconnected, at operating temp
Ignition ON, sensor connected
At sensor connector
Oscilloscope Waveform Analysis
30 s/div, 1 V/divPossible Causes
Loose terminal pin, vibration fretting, or moisture corrosion inside Intake Air Temperature Sensor (IAT) Sensor '2' Circuit Intermittent/Erratic (Bank 1) connector
Bank 1 specific:Inspect wiring harness to cylinder bank 1 (firewall/right bank, exposed to exhaust manifold heat).
Sensor 2 (Downstream post-cat):Inspect underbody wiring for mechanical road-debris damage or water intrusion inside the subframe connector.
Chafed or pinched wiring harness along the signal path to Intake Air Temperature Sensor (IAT) Sensor '2' Circuit Intermittent/Erratic (Bank 1)
Poor ground connection or excessive voltage drop (> 0.1 V) at the ground point for Intake Air Temperature Sensor (IAT) Sensor '2' Circuit Intermittent/Erratic (Bank 1)
Defective internal output driver or reference circuit in the control module for Intake Air Temperature Sensor (IAT) Sensor '2' Circuit Intermittent/Erratic (Bank 1)
Symptoms
5
Recommended Solution
Monitor downstream sensor voltage at steady cruise (should hold steady between 0.55 and 0.75 V; active switching flags depleted catalyst).
Cross-swap the component on Bank 1 with Bank 2 to rule out mechanical oil passages and internal timing wear.
Perform a loaded harness wiggle test while monitoring signal trace on a fast scan-tool graph or oscilloscope.
Verify loaded supply voltage and harness terminal pins at Intake Air Temperature Sensor (IAT) Sensor '2' Circuit Intermittent/Erratic (Bank 1).
Check circuit continuity and ground-side voltage drop (< 0.05 V) for Intake Air Temperature Sensor (IAT) Sensor '2' Circuit Intermittent/Erratic (Bank 1).
Replace the sensor if resistance is off-curve, otherwise repair wiring/connector.
Mechanic's Tip
Jumper the sensor connector and watch the scan tool: the value must peg to max temp instantly. Short it to ground and it must peg to min. If the value sweeps freely the sensor is at fault — otherwise the wiring or module is.