P0400
Fix within a few daysYour engine's Exhaust Gas Recirculation (EGR) system has a flow malfunction.
Code P0400 means your engine's Exhaust Gas Recirculation (EGR) system has a flow malfunction. Common causes: Clogged EGR passages (40%), faulty EGR valve (35%), faulty EGR solenoid (15%). Drivability: Yes, for short trips only. Repair cost: $50-$250 DIY, $150-$600 professional.
Can I Drive With P0400?
Yes, for short trips only
Essential trips under 10 miles, avoid highways and heavy acceleration. Fix within a few days.
Stop driving immediately if: Flashing Check Engine Light, loud engine knocking, significant power loss, or a strong burning smell.
Symptoms of P0400
- 1Check Engine Light is on
- 2Rough or unstable idle
- 3Engine hesitation or stumbling during acceleration
- 4Engine knocking or pinging (detonation) sounds
- 5Reduced fuel economy
- 6Engine stalling at low speeds
Common Causes of P0400
Parts most likely causing this code, ranked by probability
Carbon buildup restricts the flow of exhaust gas, preventing the EGR system from functioning correctly.
40% of P0400 cases are fixed by cleaning clogged EGR passages
The EGR valve can stick open or closed due to carbon buildup or internal electrical failure, disrupting exhaust gas flow.
35% of P0400 cases are fixed by replacing the EGR valve
This component controls the vacuum or electrical signal to the EGR valve, and its failure prevents proper valve operation.
15% of P0400 cases are fixed by replacing the EGR solenoid or transducer
A leak in the vacuum lines supplying the EGR valve can prevent it from opening or closing as commanded.
5% of P0400 cases are fixed by repairing vacuum leaks
Damaged wiring or a poor electrical connection to the EGR valve or solenoid can prevent it from receiving proper commands.
5% of P0400 cases are fixed by repairing wiring or electrical issues
Check These First (Before Buying Parts)
- Visually inspect the EGR valve and all connected vacuum lines or electrical connectors for any obvious damage, cracks, or disconnections.
- Listen carefully for any hissing sounds around the EGR valve or intake manifold, which could indicate a vacuum leak.
How to Diagnose P0400
- 1Visually inspect the EGR valve, its electrical connector, and any vacuum lines for damage, disconnections, or cracks.
- 2With the engine running, listen for any hissing sounds indicating a vacuum leak around the EGR system or intake manifold.
- 3If your EGR valve is vacuum-operated, use a vacuum pump to manually actuate the valve and check for engine RPM changes. If electronically controlled, use a scan tool to command the valve open and closed while monitoring live data.
- 4Remove the EGR valve and inspect the valve itself and the EGR passages in the intake manifold for heavy carbon buildup. Clean any restricted passages thoroughly.
- 5Test the EGR solenoid or transducer for proper operation and resistance if it's a separate component from the valve.
P0400 on Popular Vehicles
Honda Accord
2003-2012P0400 on these models is very commonly linked to clogged EGR passages in the intake manifold, requiring thorough cleaning in addition to checking the EGR valve itself.
60% fixed by cleaning EGR passages
Ford F-150
2004-2014EGR valve failures, either due to carbon buildup causing it to stick or internal electrical issues, are a frequent cause of P0400 on these trucks. The EGR solenoid can also be a culprit.
55% fixed by replacing EGR valve/solenoid
Chevrolet Silverado
2007-2013Similar to Ford trucks, P0400 on Silverado models often points to a failing EGR valve or its associated solenoid. Carbon buildup can also affect the valve's operation.
50% fixed by replacing EGR valve/solenoid
Frequently Asked Questions
What does P0400 mean?
P0400 indicates that your vehicle's engine computer has detected an issue with the flow of exhaust gas through the Exhaust Gas Recirculation (EGR) system. This means the system is not moving the correct amount of exhaust gas back into the engine.
Can I drive with P0400?
Yes, for short trips only. You can drive your vehicle for essential, short trips, but you should fix this code within a few days. Prolonged driving with P0400 can lead to rough engine operation, reduced fuel economy, and potential engine knocking.
Will P0400 clear itself?
No, P0400 will not clear itself. The Check Engine Light will remain on until the underlying issue with the EGR system is diagnosed and repaired. After the repair, you will need to clear the code with a scan tool or drive several drive cycles for it to clear automatically.
How do I fix P0400 myself?
Start by visually inspecting the EGR valve and vacuum lines for damage. If they look good, remove the EGR valve and check for carbon buildup in the valve and the passages leading to the intake manifold. Clean any carbon with a specialized EGR or carburetor cleaner. If cleaning doesn't work, you may need to replace the EGR valve or its controlling solenoid.
How much does it cost to fix P0400?
DIY repair costs for P0400 range from $50 to $250, primarily for cleaning supplies or a new EGR valve/solenoid. Professional repair costs typically range from $150 to $600, covering diagnosis, parts, and labor.
P0400 keeps coming back - why?
If P0400 keeps returning, it often means the initial repair didn't fully address the problem. Common reasons include incomplete cleaning of EGR passages, a faulty EGR valve that wasn't replaced, a hidden vacuum leak, or an issue with the EGR solenoid or wiring that was overlooked.
Will P0400 fail emissions/smog test?
Yes, P0400 will cause your vehicle to fail an emissions or smog test. The Check Engine Light being on due to this code indicates an emissions system malfunction, which is an automatic failure in states with such inspections.
What's the difference between P0400 and P0401?
P0400 is a general code indicating an EGR system flow malfunction, meaning the computer detected an incorrect flow. P0401 is more specific, indicating 'EGR Flow Insufficient Detected,' meaning the computer detected that not enough exhaust gas was flowing through the system.
P0400 on Honda Accord?
On many Honda Accord models, especially those with higher mileage, P0400 is frequently caused by excessive carbon buildup in the EGR passages, particularly in the intake manifold. Cleaning these passages and the EGR valve often resolves the issue.
Questions to Ask Your Mechanic
Did you check for carbon buildup in the EGR passages, not just the valve?
Good answer: Yes, I removed the EGR valve and inspected the intake manifold passages. They were partially clogged, and I cleaned them thoroughly. I also verified the valve's operation.
What were the live data readings for the EGR valve position sensor or commanded EGR percentage?
Good answer: I monitored the EGR position sensor and commanded EGR percentage with the scan tool. The actual position was not matching the commanded position, indicating a flow issue or a stuck valve.
Repair Cost Estimate
DIY Repair
Parts only
$50-$250
Shop Repair
Parts + labor
$150-$600
Save $100-$350 by doing it yourself
Difficulty
intermediate
Time
1-3 hours
Intermediate - requires some mechanical aptitude and possibly a vacuum pump or scan tool for proper diagnosis and testing of the EGR system.
Cost of Waiting
Ignoring P0400 can lead to continued rough idling, engine hesitation, and reduced fuel economy. More critically, it can cause engine knocking or pinging, which can lead to long-term engine damage if left unaddressed. A $50 cleaning now prevents potential engine damage later.
While You're In There...
Always replace the gasket when removing or replacing the EGR valve to ensure a proper seal and prevent vacuum leaks.
If you're already accessing the intake area to clean EGR passages, it's a good time to clean the throttle body for smoother idle and better throttle response.
If the EGR passages are severely clogged and require removing the intake manifold for cleaning, replace the intake manifold gasket to prevent future vacuum leaks.
Tools You'll Need
- Basic hand tools (sockets, wrenches)
- EGR or carburetor cleaner spray
- Gasket scraper
- Vacuum pump (for testing vacuum-operated EGR valves)
- Scan tool (to monitor EGR position and clear codes)
- Shop rags or paper towels
P0400 is a common emissions code, and most repairs involve cleaning carbon buildup or replacing a component. Your engine will run smoother and more efficiently once this issue is resolved.
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Your Next Steps
Step 1: Visually inspect the EGR valve, its electrical connector, and any vacuum lines for damage, cracks, or disconnections. Ensure all connections are secure.
Step 2: If the visual inspection doesn't reveal an obvious issue, remove the EGR valve. Inspect the valve itself for carbon buildup and thoroughly clean the EGR passages in the intake manifold using an EGR or carburetor cleaner. Replace the EGR valve gasket.
Step 3: If cleaning doesn't resolve the code, test the EGR valve's operation (manually with a vacuum pump or electronically with a scan tool). If it's not functioning correctly, replace the EGR valve. If the valve is good, test the EGR solenoid/transducer if it's a separate component.