P0099 explained: IAT sensor 2 is usually mounted in the charge-air or post-intercooler pipe on turbo and supercharged engines. An intermittent signal usually means a loose connector or damaged wire - rarely the sensor itself.
🎯 Top Causes & Probability
55%
#1 - Most Likely
Damaged Connector or Loose Wire
Vibration and engine heat cycle the connector. Pins back out or wires break inside the insulation. This is the most common cause.
🔨 Part
$5-$40
👨🔧 Labor
$40-$140
⚡ DIY
Easy
30%
#2 - Check First
Failed IAT Sensor 2
The thermistor inside the sensor becomes unstable. Resistance jumps around even at steady temperature. Replacing the sensor solves this.
🔨 Part
$15-$80
👨🔧 Labor
$30-$80
⚡ DIY
Easy
10%
#3 - Less Common
PCM Reference Voltage Issue
Rare. The 5V reference circuit from the PCM is intermittent due to a shorted sensor elsewhere on the same circuit. Unplug other sensors to isolate.
🔨 Part
$0-$300
👨🔧 Labor
$80-$220
⚡ DIY
Hard
🚗 Most Affected Vehicles
🔧 Step-by-Step Diagnosis
- Locate IAT sensor 2. On a turbo engine it is downstream of the intercooler. On a normally aspirated engine with two IAT sensors, it is in the intake manifold.
- Wiggle the harness while watching live data on a scan tool. A jumping IAT 2 reading confirms a wiring issue.
- Inspect the connector for green corrosion, melted plastic, or backed-out pins.
- Unplug and measure sensor resistance. At 70F it should read roughly 2,500 ohms. Compare to a known-good spec.
- Clear the code and drive 15-20 minutes through varying temperatures. If P0099 returns, replace the sensor.
❓ Frequently Asked Questions
Can I drive with P0099?
Yes. The PCM falls back to a default IAT value and runs slightly richer. Driving for weeks is fine but you will lose 1-2 MPG.
Will P0099 fail emissions?
Yes in OBD-II readiness states. Any unresolved powertrain code fails the test.
Why is there a second IAT sensor?
Turbo and supercharged engines compress and heat the intake air. The intercooler then cools it. A second IAT after the intercooler tells the PCM the actual charge-air temperature for accurate fuel and timing.
Can a boost leak trigger P0099?
No, a boost leak triggers different codes (P0299, P0234). But on turbo engines a boost leak might cause unusual IAT swings if hot exhaust air gets in through a cracked intercooler pipe.
How do I test the IAT sensor?
Measure resistance at room temperature - should be 2,000-3,000 ohms. Heat it gently with a hair dryer and resistance should drop smoothly. Jumpy or no change means a bad sensor.