P1109
SubaruIntake Air Temperature (IAT) Sensor 'B' Circuit Intermittent
Medium SeverityThe engine control module (ECM) has detected an inconsistent or erratic signal from the Intake Air Temperature Sensor B. This intermittent signal prevents the ECM from accurately calculating the air density, which can lead to inefficient fuel mixture adjustments and minor engine performance issues.
⚠️ Before You Begin (Safety First)
Always ensure the engine is cool before working on any components. Disconnect the negative battery terminal before performing any electrical work to prevent accidental short circuits or injury. Wear appropriate safety gear such as gloves and eye protection.
Common Symptoms
- Check Engine Light (CEL) illuminated
- Slight decrease in fuel economy
- Rough idle or poor engine performance
- Engine hesitation or stalling, especially when cold
Common Causes
- Faulty or failing Intake Air Temperature (IAT) Sensor B
- Corroded, damaged, or frayed wiring in the IAT Sensor B circuit
- Loose or poor electrical connection at the IAT sensor or ECM
- Contamination on the IAT sensor element
- Faulty Engine Control Module (ECM) (rare)
Frequently reported on: Subaru Forester (2008-2013), Subaru Impreza (2008-2011), Subaru Legacy (2005-2009), Subaru Outback (2005-2009), Subaru WRX/STI (2008-2014)
Diagnostic Steps
- Locate the Intake Air Temperature (IAT) Sensor B, which is typically found in the air intake ducting or intake manifold.
- Carefully inspect the sensor and its electrical connector and wiring for any visible signs of damage, corrosion, or loose connections.
- Disconnect the negative terminal of the vehicle's battery.
- Disconnect the electrical connector from the IAT Sensor B.
- Remove the IAT Sensor B from its mounting location. This may involve twisting it out or removing a retaining clip/bolt.
- Install the new IAT Sensor B, ensuring it is securely seated.
- Reconnect the electrical connector to the new sensor.
- Reconnect the negative battery terminal.
- Use an OBD-II scanner to clear the P1109 code from the ECM.
- Start the engine and perform a test drive to confirm the repair and ensure the code does not return.
👨🔧 When to Call a Professional
While replacing the Intake Air Temperature Sensor B is a straightforward DIY task, an intermittent signal can sometimes be due to underlying wiring issues or even a faulty Engine Control Module (ECM). If replacing the sensor does not resolve the code, or if you suspect damaged wiring, it's best to consult a professional mechanic for complex electrical diagnostics.
Commonly Asked Questions
What does 'IAT Sensor B' refer to?
IAT Sensor B refers to a second Intake Air Temperature sensor in systems that may use multiple sensors, or it can distinguish between different banks on a V-style engine, though for Subaru it's often a specific sensor in the intake path.
Can a dirty air filter cause code P1109?
A dirty air filter itself is unlikely to directly cause an intermittent IAT sensor B signal. However, severe restriction or debris could potentially affect airflow readings or sensor function indirectly.
Is it safe to drive with an intermittent IAT Sensor B code?
While it's generally safe to drive for a short period, it's best to address the issue promptly. An inaccurate IAT reading can lead to less efficient engine operation, reduced fuel economy, and potentially increased emissions over time.
What's the difference between an IAT sensor and a MAF sensor?
The Intake Air Temperature (IAT) sensor specifically measures the temperature of the air. The Mass Air Flow (MAF) sensor measures the volume and density of air entering the engine. Sometimes, the IAT sensor is integrated into the MAF sensor unit.
Do I need to reset the ECM after replacing the IAT Sensor B?
Yes, it's recommended to clear the P1109 diagnostic trouble code using an OBD-II scanner after replacing the sensor. This will turn off the Check Engine Light and allow the ECM to relearn optimal air temperature readings.