Code
P0408
GWM
P — Powertrain
- High Sensor B of the Exhaust Gas Recirculation
Views:
UK: 15
EN: 21
RU: 17
AI status
Completed
Completed
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Causes
- Short to battery or 5V reference on EGR sensor B signal wire
- Failed/defective EGR position/feedback sensor (sensor B)
- Corroded, loose, or damaged connector or wiring (open/short)
- Poor ground or loss of reference voltage to sensor
- Carbon build-up or mechanical seizure of EGR valve causing abnormal sensor position/feedback
- PCM/ECM internal fault or software error (rare)
Symptoms
- Check Engine Light (MIL) illuminated
- Reduced engine performance or hesitation
- Rough idle or stalling in some cases
- Poor fuel economy and increased emissions
- Possible failed emissions test
- Stored EGR-related trouble codes or limp mode depending on vehicle strategy
What to check
- Read and record freeze-frame and all stored codes with a capable scan tool
- Visually inspect EGR sensor B connector and wiring for corrosion, damage, or pin pushed out
- Backprobe the sensor connector to verify reference voltage (typically ~5 V), signal voltage, and ground
- Check for continuity and resistance between sensor ground and chassis battery negative
- Command EGR operation with scan tool while observing live sensor B signal response
- Inspect and operate the EGR valve to confirm it moves freely; check for carbon deposits or blockage
Signal parameters
- Reference voltage: typically ~5.0 V supply to sensor (verify manufacturer spec)
- Signal voltage range (typical): ~0.5 V (closed) to ~4.5 V (open) — a sustained voltage above ~4.7–4.9 V is considered high/ out-of-range on many systems
- Ground continuity: near 0 ohms to chassis ground
- When commanding EGR movement, signal B should change smoothly and proportionally; sudden spikes or fixed high reading indicate fault
Diagnostic algorithm
- Connect a scan tool and verify P0408 and any related codes; record freeze-frame data and current sensor B values.
- Visually inspect the sensor, connector, and wiring harness for damage, corrosion, or contamination. Repair obvious problems.
- With key ON (engine off), backprobe the sensor connector: verify reference voltage (≈5 V), good ground, and the sensor signal voltage. If signal is high with key on, suspect a short to voltage or failed sensor.
- With engine running (or while commanding EGR with scan tool per service procedure), observe live sensor B signal. Command the EGR (if supported) and watch for proportional change. No change or fixed high indicates sensor or circuit fault.
- Check continuity and resistance of the signal wire between sensor connector and ECM connector; look for short to 12V or 5V and short to power. Repair wiring as needed.
- Perform a wiggle test on harness while monitoring signal to identify intermittent faults.
- Inspect and, if present, clean EGR valve and passages. Verify valve moves and is not mechanically stuck causing incorrect feedback.
- If wiring and valve check good but signal remains high, replace the EGR sensor (sensor B) and retest.
- If problem persists after sensor replacement, test/replace PCM only after confirming wiring, grounds, and connectors are good and checking for any available software updates from manufacturer.
- Clear codes, perform a test drive under monitored conditions, and verify the code does not return.
Likely causes
- Shorted signal wire to 5V supply or battery
- Failed EGR position/feedback sensor B
- Damaged/corroded connector at sensor or ground
- Stuck or clogged EGR valve producing incorrect feedback
- Intermittent wiring fault (pinched harness, water ingress)
Fault status
Status
P0408: EGR Sensor B circuit voltage high. The ECM has detected a higher-than-expected voltage on the EGR sensor B signal circuit indicating a possible short-to-voltage, sensor failure, wiring/connector problem, or EGR mechanical issue.
Repair difficulty: Medium
Diagnostic time: 1.0-2.5 hours
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