P0400: How to Diagnose & Fix — Exhaust Gas Recirculation (EGR) Flow Malfunction
5 vehicles · 5 makes · $75-$3,500 repair
How to diagnose and fix P0400: most-reported vehicles
Ranked by owner reports. Each card gives the symptoms, how to confirm the cause, the fix, parts and sources for that vehicle.
Volkswagen Jetta · 2009–2014
The 2009-2014 Jetta TDI ran the EA189 2.0 TDI (CBEA/CJAA) at the centre of the diesel emissions scandal. The court-approved Approved Emissions Modification - an ECU recalibration, plus hardware on some generations - brings the car into compliance largely by leaning on the exhaust gas recirculation system far harder and altering injection. Owners and independent shops have documented the aftereffect: heavier EGR duty cycles accelerate carbon fouling of the EGR valve and cooler, and the changed combustion produces more soot, so the diesel particulate filter loads and regenerates more often, shortening its service life. Symptoms are rough idle, power loss and EGR flow codes (P0401 and relatives), or a DPF that will no longer complete a regeneration. The pattern is strong enough that Volkswagen Group has been refunding or goodwilling EGR failures in some markets, while individual dealers have told owners the failures are unrelated to the fix.
Common Symptoms
- Rough idle and hesitation after the emissions recall was performed
- Loss of power or limp mode
- Check engine light with EGR flow codes
- Frequent DPF regeneration cycles and dropping fuel economy
- Smell of hot exhaust and elevated idle during regeneration
- DPF warning light or a regeneration that will not complete
How to Fix
First establish whether the car has had the Approved Emissions Modification applied and whether it is still inside the extended emissions warranty that accompanied the settlement - the settlement provided an extended warranty on emissions-related components for modified vehicles, and EGR and DPF claims should be pressed against it before paying. Mechanically, the repair is EGR valve and cooler replacement plus intake cleaning; a saturated DPF needs a forced regeneration, a professional clean, or replacement. Owners doing mostly short trips should ensure the car reaches full operating temperature regularly so passive and active regeneration can complete.
Sources for this issue (3)
Mazda MX-6 · 1996–1997
The KL-DE V6 EGR system is highly prone to carbon clogging. Soot accumulates inside the EGR valve diaphragm and inside the EGR passages cast into the throttle body and intake. When the valve sticks open, exhaust gas enters at idle and produces a rough, surging, or stalling idle with a CEL. Owners report that on the MX-6, P0400 (EGR flow insufficient) is the most common emissions code, and approximately 95% of the time it is a clogged valve or completely blocked EGR ports in the throttle body, not a failed component. Access is poor: the upper intake plenum and throttle body must usually come off to clean it properly.
Common Symptoms
- Rough, surging, or hunting idle
- Stalling at stops or when cold
- Check engine light with P0400-series code
- Failed emissions / smog test for high NOx or EGR flow
- Hesitation or sputter during light acceleration
How to Fix
Remove the EGR valve and clean the diaphragm and pintle with carb cleaner, then snake out the EGR passages in the throttle body and intake plenum (this is required if cleaning only the valve does not clear the code). Replace the EGR gasket on reinstall. A new OEM valve is typically only needed if the diaphragm is torn or the electrical solenoid fails.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Fel-Pro 70714 EGR valve gasketOEMFel-Pro 70714
New valve-to-manifold gasket after the EGR valve is removed for the 1996-1997 KL-DE diagnostic/cleaning branch Fitment/availability: The inspected direct page identifies Fel-Pro 70714 as a valve-to-manifold EGR gasket, showed $5.89 and A…
- CRC 05078 throttle-body and air-intake cleaner, 12 ozOEMCRC 05078
Cleaning carbon/varnish from removed compatible throttle-body and intake surfaces during the confirmed clogged-passage repair Fitment/availability: The inspected direct page identifies CRC 05078 throttle-body and air-intake cleaner, show…
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Sources for this issue (3)
Nissan Maxima · 1995–1999
Nearly every high-mileage 4th-gen Maxima eventually sets a P0400 (EGR flow insufficient) code. The root cause is almost never the EGR valve itself; instead the small EGR pipe and the port where the EGR temperature sensor sits pack with carbon, so exhaust gas can no longer flow past the sensor and the ECU reports low EGR flow. It commonly appears alongside a P0325 knock-sensor 'ghost' code. This is a classic emissions-inspection failure item on these cars.
Common Symptoms
- Check engine light
- Failed emissions/inspection
- Occasional rough idle
- P0400 with intermittent P0325
How to Fix
Remove the EGR tube/passage and the EGR temperature sensor, then clear the carbon with a pick and carburetor cleaner; clean the ports and passages rather than replacing the valve. Inspect/replace the EGR temp sensor if damaged. Full EGR valve replacement is only needed if the valve itself is stuck.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- CRC throttle body and air intake cleaner 05078
Universal solvent consumable, not a vehicle-fit part. Follow label/SDS and service procedure; keep away from sensors, coatings and friction/electrical surfaces not approved by the manufacturer.
- Genuine Nissan EGR valve gasket 14719-38U00
Direct listing covers 1995-1999 VQ30DE Maxima; confirm VIN and disassembly path.
As an Amazon Associate, we earn from qualifying purchases; other part links may also earn au7o a commission. Confirm fitment by VIN before buying.
Sources for this issue (3)
Chrysler PT Cruiser · 2001–2010
Mercedes-Benz Sprinter · 2007–2020
Find P0400 by make
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P0400 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0400 mean?
P0400 stands for "Exhaust Gas Recirculation (EGR) Flow Malfunction." P0400 is a generic powertrain code indicating the engine computer detected a problem with exhaust gas recirculation (EGR) flow — either too much, too little, or incorrect flow compared to what was commanded. The EGR system routes a small amount of exhaust back into the intake to lower combustion temperatures and reduce emissions. A fault can cause rough idle, pinging/knock, increased emissions, and failed emissions testing, though it is generally not an immediate driving hazard.
What are the most common causes of P0400?
The most common causes of P0400 are: Clogged or carbon-blocked EGR passages, Faulty EGR valve (stuck open or closed), Faulty EGR position or DPFE/flow sensor, Vacuum or electrical control fault to the EGR valve, Blocked or cracked EGR tube, Wiring/connector issues, PCM fault (rare). The specific cause varies by vehicle.
How much does it cost to fix P0400?
Repair costs for P0400 range from $75 to $3,500, depending on the vehicle and root cause.
Which vehicles are affected by P0400?
Au7o has documented P0400 across 5 vehicle models from 5 manufacturers: Chrysler, Mazda, Mercedes-Benz, Nissan, Volkswagen.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.