OBD-II Code · Emissions
P0408
EGR Sensor B Circuit High
Severity
low
Lowest on our scale. Fix it when convenient.
Can you drive it
Only if running normally
A short trip to a shop may be reasonable if the light is steady and the car runs normally. Stop safely for a flashing light, severe shaking, overheating, loss of power, or another urgent warning. The code alone cannot confirm safety.
Indicative cost (USD)
$100–$400
Varies by vehicle, diagnosis, parts, and local labor. This is not a repair quote.
Secondary EGR position high.
How a shop works this
Work from evidence to part, not the other way round. A code names a circuit or a symptom; it does not name the part that failed.
Note every code that is set, not just this one.
Codes that appear together usually narrow the cause faster than any one of them read alone.
Read the freeze frame — the conditions the fault was recorded under.
Cold or warm, idle or load, road speed and engine speed. A fault that only appears cold is a different fault from one that only appears under load, even with the same code on the scanner.
Confirm the fault is current rather than history.
Clear the code and see whether it returns under those same conditions. A code that does not come back may have been logged during a repair, a flat battery, or a bad connection that has since been disturbed.
Then work the likely-causes list in the order shown.
That order is what a shop checks first for this code — wiring and connections before the component they run to, because a connector costs nothing to inspect and is a common answer.
If the code comes back after a repair, the underlying fault is still there. That is the point to bring in a scanner with live data and freeze-frame, or a shop that has one.
Common symptoms
- Check engine light
Likely causes
- Failed sensorMost common
- Wiring short
Read the full diagnostic procedureP0408 is the high-side counterpart to P0407 — EGR position sensor B reads above the maximum voltage threshold, typically above 4.8V.
P0408 is the high-side counterpart to P0407 — EGR position sensor B reads above the maximum voltage threshold, typically above 4.8V.
Same fault topology as P0406
internal sensor short to 5V reference, external harness short, or mechanical pintle jammed at full-open driving an honest-high reading. Diagnostic ladder: backprobe the B-signal pin at rest, unplug the connector to see if the short clears on the sensor side or stays on the harness side, and verify the supply/ground integrity at the harness. On dual-sensor EGRs, compare A-signal to B-signal on a scan tool in live data — if A reads sensible (0.7V closed, sweeping to 4.2V open with command) and B reads pegged-high, the B sensor is failed independently and the valve assembly typically has to be replaced because B isn't a serviceable sub-component on most platforms. On diesel EGRs specifically (Cummins 6.7L, Powerstroke 6.7L, VW/Audi TDI, Sprinter OM642), soot accumulation between the sensor PCB and the actuator housing can short B-signal to the supply rail through conductive soot films — same diagnosis path, same replacement-only outcome. Verify mechanical pintle freedom before electrical replacement on every P040X position code: a stuck-open valve gives an honest sensor reading that looks like an electrical fault and gets sensors replaced unnecessarily.
Vehicle-specific patterns2011-2016 Ford 6.7L Powerstroke throws P0408 from the same soot-contaminated dual-sensor EGR assembly that produces P0407 — replace the assembly, don…
- 2011-2016Ford 6.7L Powerstroke throws P0408 from the same soot-contaminated dual-sensor EGR assembly that produces P0407 — replace the assembly, don't bother cleaning.
- 2007-2014Cummins 6.7L throws P0408 from internal sensor short on the secondary feedback channel; full valve replacement, ~$500-$900 part.
- 2010-2014VW/Audi 2.0L TDI throws P0408 alongside DPF and EGR-cooler codes when the cooler fails internally and routes hot soot toward the sensor — the cooler-and-valve pair replaces together as a job.
- 2009-2014Ford 6.4L Powerstroke (earlier generation) throws P0408 from the well-documented EGR cooler failure that floods the intake with coolant and shorts the position sensor — diagnose the underlying cooler before installing a new valve or you'll cook the new one in 500 miles.
Estimated repair$200 (light-duty) to $2,800+ (6.4L Powerstroke EGR cooler + valve + intake cleaning).
Look up another code
P0408 — frequently asked
What does a P0408 code mean?
Secondary EGR position high.
What causes a P0408 code?
Common causes: Failed sensor; Wiring short.
What are the symptoms of a P0408 code?
Typical symptoms: Check engine light.
Is it safe to drive with a P0408 code?
Only if running normally. A short trip to a shop may be reasonable if the light is steady and the car runs normally. Stop safely for a flashing light, severe shaking, overheating, loss of power, or another urgent warning. The code alone cannot confirm safety. Estimated repair: $100-$400.
How do I diagnose a P0408 code?
Work from evidence to part, not the other way round. A code names a circuit or a symptom; it does not name the part that failed. 1. Note every code that is set, not just this one. 2. Read the freeze frame — the conditions the fault was recorded under. 3. Confirm the fault is current rather than history. 4. Then work the likely-causes list in the order shown.
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