Exhaust Gas Recirculation Sensor B Circuit High
The P0408 code means the vehicle's onboard diagnostic system has detected: Exhaust Gas Recirculation Sensor B Circuit High. This is a moderate severity code.
- Keep driving?
- Short distances only
- DIY difficulty
- hard
- Estimated cost
- $120 - $450 (parts and labor)
Best-selling code reader on Amazon. Reads and clears check engine codes on any 1996+ vehicle. Under $30.
Check Price on AmazonAffiliate link -- we earn a commission at no extra cost to you.
Essential for diagnosing sensor codes, checking voltage, and testing circuits. Under $15 and pays for itself on the first job.
Check Price on AmazonAffiliate link -- we earn a commission at no extra cost to you.
Common Symptoms
- Check Engine Light illuminated
- Rough idle or engine surging
- Decreased fuel economy
- Higher exhaust emissions
- Failed emissions test
Probable Causes (Ranked by Likelihood)
- Faulty secondary EGR position sensor reading high Very High
- Open ground circuit to EGR sensor B High
- Signal wire shorted to 5V reference Moderate
- ECM sensor B input circuit fault Low
Step-by-Step Diagnostic Procedure
-
Use service information to identify sensor B location and wiring for your vehicle
-
Check sensor B voltage on a scan tool -- voltage above 4.5V with valve closed indicates high circuit
-
Verify ground wire continuity from sensor B connector to ECM
-
Disconnect the sensor and check if signal voltage drops on the ECM side
-
Replace the sensor if wiring and connectors are confirmed good
Frequently Asked Questions
Can I drive with code P0408?
You can drive short distances. The ECM receives an incorrectly high signal from the secondary EGR sensor, which may result in improper EGR operation and rough running.
What is the difference between P0407 and P0408?
P0407 indicates sensor B voltage is below the expected range (too low), while P0408 indicates it is above the expected range (too high). Both relate to the secondary EGR position sensor.
Do I need to replace the EGR valve for P0408?
Not necessarily. If sensor B is a separate component, only the sensor needs replacement. If integrated into the EGR assembly, the entire unit may need replacement. Check service information for your vehicle.
Sources
This page is built from documented references. Verify against your own service info before repair work.
P0408 on Diesel, EV & Equipment
The same code ID appears across other engines and platforms. The diagnostic flow varies by manufacturer — these are the platform-specific breakdowns:
Explore More
Was this helpful?
Share carcodefinder.com with a friend who needs to decode their check engine light.