P0408 means the EGR position sensor 'B' is sending a voltage above its normal operating range. Sensor B exists on engines with dual EGR valves (V6/V8 diesels) or dual feedback sensors. The fault is usually an open signal wire, missing ground, or a sensor that has failed open internally.
P0408 means the EGR position sensor 'B' is sending a voltage above its normal operating range. Sensor B exists on engines with dual EGR valves (V6/V8 diesels) or dual feedback sensors. The fault is usually an open signal wire, missing ground, or a sensor that has failed open internally.
P0408 is a emissions-related diagnostic trouble code classified as moderate severity. When your vehicle's ECU detects this condition, it stores P0408 and illuminates the check engine light.
If your vehicle has triggered P0408, you may notice one or more of these symptoms:
Here are the most common causes of P0408, ranked by how likely they are to be the culprit:
Generally yes, for short-term driving. Driveable. P0408 affects the secondary EGR position sensor (sensor B), reporting voltage above the expected range. The ECU disables the affected EGR bank or enters limp mode on diesels. Expect rough idle, reduced power, and a failed emissions test.
Follow these steps to pinpoint the root cause of P0408:
Repair costs for P0408 vary depending on the root cause and your vehicle. Here are typical estimates:
Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.
P0408 behaves differently depending on your vehicle. Select your car below for model-specific causes, known issues, and adjusted cost estimates:
These codes are often seen alongside P0408 or indicate related issues:
P0407 means sensor B signal is too low (short to ground or failed sensor). P0408 means sensor B signal is too high (open wire, missing ground, or sensor failed open). The diagnostic approach is similar, but the failure modes are opposite.
Short distances yes. The ECU disables EGR on the affected bank, so the engine runs without that EGR. Diesels may enter limp mode. Long-term driving accelerates intake carbon buildup and may trigger secondary DPF or SCR faults. Plan repair within a few weeks.
Because most 4-cylinder engines only have a single EGR position sensor. Sensor B exists on engines with dual EGR valves (Audi 3.0 TDI V6, Mercedes OM642 V6, BMW B57 with high/low pressure EGR) or on engines with redundant feedback sensors. On a 4-cylinder, P0408 is unusual and usually means a calibration or labeling quirk.
Not always, but it is recommended on high-mileage cars (over 150,000 km) because both valves share the same age, soot exposure, and thermal cycling. If one EGR valve has failed, the other is usually close behind. Workshops often quote both for cost efficiency, since labor for the second side is much lower once the engine is partially apart.
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