P0408
Exhaust Gas Recirculation Sensor B Circuit High
If your vehicle's onboard computer has flagged the diagnostic trouble code P0408, it refers to a detected anomaly regarding "Exhaust Gas Recirculation Sensor B Circuit High". This systemic engine fault needs a targeted check before symptoms expand.
Driver's Summary
Storing code P0408 is your car's way of telling you something is wrong with the exhaust gas recirculation sensor b circuit high. Drivers typically experience stalling at stops, surging at low speeds, check engine light when this code is active. This is a moderate-severity fault — plan a repair shop visit within the week to keep it from escalating.
Symptoms
Stalling at stops, surging at low speeds, check engine light
Common Causes
- Short to power in EGR sensor B circuit
- Defective EGR sensor
- EGR valve stuck partially open
- PCM fault
How to Fix
- 1 Fix shorted wiring
- 2 Replace EGR valve
- 3 Clean intake and EGR valve of carbon
- 4 Diagnose PCM
Technical Explanation
Detection of P0408 occurs when the ECM cross-references multiple sensor inputs and determines that the reported values are physically inconsistent or out-of-range. For EVAP system codes, the module seals the fuel vapor system and monitors the fuel tank pressure sensor for pressure decay or build-up that confirms purge flow or leak presence. The test only runs under specific ambient temperature, altitude, and fuel level conditions to avoid false positives. The fault remains stored in memory even after the MIL is cleared; it becomes a confirmed DTC after failing two consecutive drive cycles, and the PCM logs a freeze frame record of the engine's exact operating state at the moment of detection.
Is It Safe to Drive?
You can drive short distances, but the symptoms — stalling at stops, surging at low speeds, check engine light — indicate the affected system is compromised. Leaving this unresolved will lead to progressively worse fuel economy and potential damage to components beyond the original fault.
Mechanic's Pro Tip
The most common mistake with P0408 is replacing the sensor without verifying the reference voltage and ground integrity first. Use a scan tool to monitor the sensor's live output; a truly failed sensor shows a stuck, flatlined reading — a sensor that fluctuates but reads slightly off usually indicates a wiring or vacuum issue, not a dead sensor. Always spray electrical contact cleaner on the connector pins before condemning the sensor.
EGR Valve replacement: $200 - $350