P0408: EGR Sensor B Circuit High
The P0408 code indicates that the Engine Control Module (ECM) has detected a high voltage signal from the Exhaust Gas Recirculation (EGR) sensor B circuit. This sensor monitors the position of the EGR valve or the flow of exhaust gases. A high voltage reading typically means the sensor is sending a signal above the expected range, which can be caused by a faulty sensor, wiring issues, or a malfunctioning EGR valve. This can lead to improper EGR flow, affecting engine performance and emissions.
1. Most Common Causes
- Short circuit in the EGR sensor B signal wire to reference voltage or battery positive (B+)
- Faulty EGR position or flow sensor (Sensor B) with altered internal resistance
- Corroded, damaged, or moisture-contaminated sensor connector or terminal causing a pin-to-pin short
- Harness wiring rubbing against engine components or excessive heat causing insulation damage
- Excessive resistance or poor ground circuit for the sensor
- Abnormally high reference voltage (VREF) from the PCM (rare case)
2. Symptoms You Will Notice
- Check Engine Light illuminated
- Rough idle or hesitation
- Poor fuel economy
- Failed emissions test
- Engine knocking or pinging
3. Step-by-Step Diagnosis
Step 1: Use an OBD2 scanner to confirm the code and check for other codes.
Step 2: Visually inspect the EGR sensor B wiring and connector for damage, corrosion, or loose connections.
Step 3: Test the sensor voltage with a multimeter; compare to manufacturer specifications.
Step 4: Check for a short to voltage by disconnecting the sensor and checking the signal wire voltage.
Step 5: If wiring and sensor are okay, test the EGR valve operation and replace if stuck or faulty.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
It is generally safe for short trips, but prolonged driving may cause reduced fuel economy, rough running, and potential damage to the EGR system or catalytic converter. It's best to diagnose and repair promptly.
How much does it cost to fix?▼
The cost typically ranges from $50 to $150 for parts and labor, depending on the cause. A new EGR sensor can cost $20-$60, and labor is usually 0.5-1 hour.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.