Fault Tree: P0401/P0402 EGR flow faults
EGR faults are usually a stuck valve, clogged ports, or a control issue (vacuum/solenoid/position feedback). Follow the flow to avoid guessing.
Quick triage & tools
- Freeze-frame matters: RPM, load, temp, trims and battery voltage when the fault set.
- Don’t skip basics: battery/charging stability, grounds, and connectors.
- Have ready: scan tool + multimeter. A smoke tester / vacuum gauge helps for many faults.
Decision flow
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Identify which direction the fault is pointing
- P0401: insufficient EGR flow (blocked/coked passages, valve not opening, vacuum/command issue).
- P0402: excessive EGR flow (valve stuck open, control fault) causing rough idle/stall.
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Quick symptom check
If it idles rough or stalls at idle, suspect EGR stuck open (P0402 bias). If it drives fine but sets code, suspect clogged passages (P0401 bias).
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Command vs response (scan tool)
- Use live data: EGR command %, EGR position feedback (if available), MAP/MAF response.
- Command open at idle/light load and watch for a MAP change / idle change (where safe).
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Vacuum-actuated EGR checks
- Hand vacuum pump to the valve (if applicable): does it move and hold vacuum?
- Check the vacuum supply, solenoid, and cracked hoses.
Related: Vacuum testing with a gauge → -
Electronic EGR checks
- Check power/ground and connector pins.
- Verify position sensor plausibility (smooth movement, no dropouts).
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Clogged ports / cooler / passages (very common)
- Inspect for carbon blockage at the intake/EGR port.
- Clean as required (follow manufacturer procedure).
Warning: Don’t “shotgun” chemicals into sensors. Remove parts and clean properly. -
After repair: verify readiness
- Clear codes, complete a drive cycle, re-check command/feedback behaviour.
Fast clues
- P0402 + stalling/rough idle is often EGR stuck open.
- P0401 on high-mileage diesels often = carbon-blocked passages or cooler restrictions.
- If EGR command changes but MAP/MAF never responds, suspect blocked flow.
Print / save checklist
Tick these off as you work. If you need to hand this to a mechanic, print it as a short job card.
- Freeze-frame captured / conditions noted
- Battery voltage checked (resting + cranking)
- Basic visual checks (hoses, connectors, grounds, fuses)
- One test at a time (don’t change multiple variables)
- Confirm fix by reproducing the original condition
What to do next
Use the links below to deepen the test you’re about to perform, cross-check related codes, or jump to a faster symptom-led flow.
Find another symptom flow
Jump to the symptom selector and filter by your exact complaint.
Open Workshop Guides
Deep dives on testing methods (wiring, fuel pressure, smoke tests, CAN, etc.).
Browse Diagnostic Codes
Look up related OBD2 codes and common patterns.
Use the AI tools
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