Fix P0400: Mechanic’s solutions that WORKED

The code P0400: EGR (Exhaust Gas Recirculation) System Performance indicates a malfunction in the EGR system, and it is most likely caused by flow restrictions, faulty EGR components, or electrical issues.

P0400 quick overview

Meaning	P0400: EGR System Performance
Is it serious?	Moderately serious. While it might not cause immediate breakdowns, it can lead to reduced engine performance, increased emissions, and potential long-term damage if not addressed.
Possible causes	– Restricted flow in the EGR valve or tubes due to carbon buildup. – Faulty EGR valve or solenoid. – Electrical issues, such as damaged wiring or poor connections. – Malfunctioning backpressure transducer. – Vacuum leaks. – Faulty MAP sensor. – Blocked or damaged EGR passages.
How to diagnose?	1. Use a diagnostic tool to record and retrieve any relevant fault codes. 2. Visually inspect the EGR valve, its tubes, and associated components for any signs of damage or carbon deposits. 3. Employ the scanner or do it manually to activate the EGR valve and see its operation. If the engine shuts down when the EGR is triggered, it confirms the valve’s functionality. 4. Search for vacuum leaks around the components of the EGR system, either by utilizing a smoke machine or by being attentive to hissing sounds. 5. Review the solenoids, electrical connections, and wiring for any signs of damage, disconnections, or corrosion. 6. Use relevant equipment to monitor the vacuum or backpressure within the system during different driving conditions. 7. If there are suspicions of blockages, extract and cleanse the EGR tubes and passages, particularly emphasizing the flow pipe and the passage in the intake plenum. 8. Evaluate related sensors, like the MAP sensor, for proper functionality. 9. If a specific component is believed to be faulty after diagnosis, consider its replacement and re-assess the system.
What is EGR?	The EGR system redirects a portion of the engine’s exhaust gases back into the engine’s combustion chambers. Introducing these exhaust gases into the combustion chamber reduces peak combustion temperatures, which in turn reduces the formation of NOx.

How to fix P0400 – learn from mechanics

I looked into P0400 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Mazda

Vehicle Information:
2002 Mazda Millenia 2.5L

Issue + Repair History:

• The vehicle displayed a malfunction indicator lamp (MIL) with code P0400 (EGR system flow malfunction).
• EGR Valve and EGR Boost Solenoid Pack were replaced, but the code persisted.

Suggestions from the Community:

• Use a scan tool to check if the PCM is requesting EGR; the issue could be with the PCM or wiring to the solenoid.
• The engine’s EGR passages in the intake manifold near the throttle body might be plugged; recommended cleaning with carb cleaner.
• It’s likely that the passages in the intake manifold are partially clogged.
• Confirm that the “snake fang” holes at the front might appear open but could still be partially clogged, triggering the code.
• The vehicle has a dual-plane intake manifold with specific holes known as the EGR vacuum supply, which might be plugged.

Final Fix:
The EGR valve intake manifold passages were found to be restricted. An Induction System Cleaner was used with the EGR port attachment to inject a chemical into the intake manifold. Both the intake and exhaust ports for the EGR valve were treated, resolving the issue.

Case #2: Subaru

Vehicle Information:
1999 Subaru Outback 2.5L

Issue + Repair History:

• After a few days, the car would set code P0400 (EGR system flow malfunction).
• Checked vacuum hoses, EGR backpressure transducer, EGR solenoid, and EGR valve – all seemed fine.
• EGR passage and valve were checked and found to be okay.
• Dynamic testing of the EGR valve showed it was functioning correctly, but the code P0400 still set.
• A new EGR solenoid was installed previously.

Suggestions from the Community:

• Replace the transducer and ensure all passages are clean.
• Consider trying a pressure sensor that detects the manifold pressure difference when the EGR opens, located on the passenger’s side strut tower.
• Check the backpressure transducer by removing the bottom hose; if it allows air to flow through, it’s likely defective and needs replacement.
• Inspect metal tubes connected to vacuum hoses for rust and potential clogging.
• Continue searching for potential restrictions or leaks.
• Test the vacuum nipple going to the EGR with a vacuum tester; if it leaks, it could set off the P0400 code. Also, check lines to the manifold for clogging due to condensation from a ruptured diaphragm.

Final Fix:
A new EGR backpressure transducer was installed, which resolved the P0400 code.

Case #3: Jaguar

Vehicle Information:
1996 Jaguar XJS 2+2 4.0L

Issue + Repair History:

• Customer initially sent to the dealer for EGR code and transmission limp-home codes P0727 & P1775.
• Dealer performed open recalls and a “tune-up” as per the invoice.
• The vehicle now shows codes P0121 (throttle position sensor issue) and P0400 (EGR flow malfunction).
• The EGR valve was later confirmed to have been replaced by the dealership.

Suggestions from the Community:

• The throttle potentiometer, related to code P0121, frequently fails and might be affected by sludge from the throttle body. Recommended action: Replace it, set to the correct voltage, and clean the throttle body.
• The EGR sensor, related to code P0400, can become carbon-coated. Suggested fix: Remove and clean the EGR sensor.
• A common issue with the EGR system is that the tube leading to the exhaust manifold might get plugged.
• The two codes (P0121 and P0400) are not interrelated. For the EGR flow malfunction, check the flow sensor and the passage for restrictions.
• Check the coolant sensor and associated wiring, and inspect the thermotime switch and its wiring due to potential heat-related corrosion.
• Unhooking or swapping O2 sensors might help reset adaptations and prevent the EGR valve from becoming clogged.
• Ensure that the MAP sensor vacuum hose is properly attached, as its signal might influence the EGR code.

Final Fix:
The EGR valve was replaced by the dealership. The technician cleaned the heavily carbon-coated EGR temperature sensor and the ports associated with the EGR valve, resolving the issue.

Case #4: Mercedes

Vehicle Information:
1995 Mercedes-Benz C220 2.2L

Issue + Repair History:

• Check engine light is on with codes P1700 and P0400 (EGR system flow malfunction).
• Six months prior, the vehicle had the same issues, and the EGR solenoid valve and a broken shift down cable were replaced.

Suggestions from the Community:

• Check and possibly replace the upshift delay switch-over valve; inspect for a plugged EGR pipe.
• Ensure the EGR tube at the throttle controller isn’t restricted; clean out any blockage, especially at the throttle body.
• Inspect the shift delay vacuum valve connected to the kickdown cable.
• The EGR tube leading into the intake under the throttle housing might be partially clogged; consider replacing or cleaning the tube.
• Test the vacuum box on the side of the transmission for potential issues.
• Ensure the EGR tube is clean, and inspect if the transmission upshift delay valve is holding vacuum.

Final Fix:
The EGR tube was found to be clogged and was replaced. The Shift Delay Vacuum Box was found to be defective and was subsequently replaced. After these repairs, the customer experienced no further problems for at least ten days.

Case #5: Chevrolet

Vehicle Information:
1998 Chevrolet Tracker 1.6L

Issue + Repair History:

• MIL (Malfunction Indicator Light) lamp turns on after 30-50 miles of driving.
• The vehicle initially displayed code P1510 (Backup power supply). After replacing the PCM (customer sourced a used one), the code changed to P0400 (EGR system performance).
• Despite changes made to the vehicle’s system, the P0400 code still appeared intermittently.

Suggestions from the Community:

• Ensure proper functionality of the EGR bypass solenoid, the backpressure transducer, and check connections at the MAP sensor and bypass solenoid.
• Observe that if the EGR valve is opened, the engine should stall, indicating the system is working as designed.
• The EGR system might have a restriction in its flow that needs addressing.
• The intake plenum might be causing the restriction; specifically, the port in the intake can be half-plugged.
• Clean out all ports, even if the engine stalls when the EGR is activated.
• Use a long screwdriver to open up and clean the port going into the intake.
• Ensure the MAP port is wide open, as it might cause a slow response during EGR tests.
• The EGR tube feeding into the intake might be partially plugged. This hole should be 10 mm in diameter.

Final Fix:
The flow pipe from the EGR valve that connects to the intake plenum had a short passage through the plenum casting that was slightly restricted. The restriction was cleared using a long shank screwdriver and a valve guide brush, along with cleaner and compressed air, resolving the issue.