P0402
- Powertrain
- Generic (SAE)
- Emissions
Diagnose soon
Not a safety system and no immediate mechanical damage risk. It becomes more serious when the excess flow causes stalling, because an engine that dies at a junction or while turning across traffic is a real hazard. It will also fail an emissions inspection.
- Safe to drive
- Generally driveable, but if it stalls at idle or when coming to a stop, get it fixed before driving in traffic.
Exhaust Gas Recirculation Flow Excessive Detected
The computer measured more exhaust gas flowing back into the engine than it asked for - usually flow when the EGR valve was supposed to be fully shut. That extra exhaust dilutes the intake charge and typically shows up as a rough or stalling idle.
The PCM commands the EGR valve closed at idle and during cold operation and expects zero flow. Flow present under those conditions, or flow exceeding the commanded amount, sets P0402. The measurement method varies by system: differential pressure across a metering orifice on DPFE designs, manifold pressure change on MAP-based designs, mass air flow deviation on some others, and direct lift measurement on valves with a position sensor. A false high reading from the measuring device itself can set this code with the valve behaving perfectly.
How the car detected itDuring a monitored window - commonly closed throttle at idle or deceleration - the PCM commands EGR fully closed and looks at its flow indicator. On MAP-based systems it may briefly command the valve open and closed and compare the manifold pressure delta against expected values. Flow indicated when none is commanded, sustained across the monitor window, sets the code.
A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.
- Safe to drive
- Generally driveable, but if it stalls at idle or when coming to a stop, get it fixed before driving in traffic.
- Urgency
- Diagnose soon
- Will it pass emissions
- No
- If ignored
- Persistent rough idle and stalling. Carbon continues to accumulate in the EGR passages and intake, making the eventual cleaning job larger. Emissions inspection failure. Sustained charge dilution can cause misfire and slowly load the catalyst with unburned fuel.
- Check engine light
- Solid
- Clearing the light
- A completed drive cycle is normally required before the monitor re-runs and the light can clear.
These are the causes this code can have. For the order to work through them in, see what to check first.
| # | Cause | How often | Component |
|---|---|---|---|
| 1 | EGR valve stuck open | Very common | — |
| 2 | Valve seat eroded — does not seal | Common | — |
Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.
In order. Each of these is cheaper or faster than what follows it, and each one can make the rest unnecessary.
Idle quality when fully warm and stopped in gear
A stuck-open EGR valve is worst at idle, when the smallest amount of exhaust dilution has the biggest proportional effect. If idle is smooth, suspect the sensor or its plumbing rather than the valve.
EGR position or flow reading in live data with the engine idling
Commanded should be zero at warm idle. If the sensor reports flow while the valve is visibly shut, the fault is in the measuring circuit, not the valve.
The condition of the EGR pressure hoses on DPFE-style systems
These small silicone hoses harden and crack near the hot exhaust. A cracked hose skews the pressure reading and is a very common and very cheap cause.
Whether an EGR valve or gasket was recently replaced
A gasket left off, fitted the wrong way round, or a valve installed with a blocked or unblocked orifice mismatch produces this code immediately after service.
Vacuum lines and the EGR vacuum solenoid on vacuum-operated systems
A solenoid stuck passing vacuum, or a line routed to constant manifold vacuum, holds the valve open regardless of what the PCM commands.
Read codes and confirm the EGR system type on this vehicle
Tools Scan tool, service information
Establish whether the vehicle uses a vacuum-operated valve with a DPFE sensor, a vacuum valve with a position sensor, or a fully electric EGR motor. The diagnostic path differs completely between them. Look for companion codes such as P0401, P0405, or P0406, which tell you whether the feedback device is already suspect.
ExpectedA clear picture of the system architecture and any companion EGR codes.
Observe EGR feedback in live data at warm idle
Tools Scan tool with live data
With the engine fully warm and idling in park, watch EGR position or DPFE voltage. Then snap the throttle and watch the reading respond. Compare the idle reading against the value stored in freeze frame.
ExpectedA stable closed-position reading at idle. On DPFE systems this is a low, steady voltage. Exact values are vehicle-specific.
Inspect the DPFE sensor and its hoses
Tools Multimeter, backprobe leads
On DPFE-equipped systems do this before touching the valve - it is the cheapest and fastest check on the system. Remove both pressure hoses and inspect for hardening, cracking, collapse, or carbon blockage. Check the sensor's output voltage with both hoses removed - it should sit at its zero-flow baseline. Replace hardened hoses on principle; they are inexpensive.
ExpectedBoth hoses supple and clear. Sensor at its baseline voltage with no differential pressure applied. Baseline values are vehicle-specific.
Verify mechanically that the valve is closed at idle
Tools Hand vacuum pump, inspection light
With the engine idling, look at or feel the EGR valve pintle or diaphragm. On a vacuum valve you can normally see the diaphragm plate. Applying vacuum to the valve at idle should make the engine stumble or stall - if it does not, the valve was already open or the passage is blocked.
ExpectedValve fully seated at idle. Engine stumbles noticeably when vacuum is applied, confirming the valve does move and the passage is clear.
Test the vacuum control side on vacuum-operated systems
Tools Vacuum gauge, hand vacuum pump
Disconnect the vacuum line at the EGR valve with the engine idling and check for vacuum present. There should be none at warm idle. If vacuum is present, the fault is the control solenoid or its wiring, not the valve. Also check that the valve holds applied vacuum - a leaking diaphragm will not hold.
ExpectedNo vacuum at the valve at warm idle. The valve holds applied vacuum without bleeding down.
Remove the valve and inspect the seat and passages
Tools Socket set, gasket scraper, inspection light
Take the valve off and look at the pintle and the seating surface. Carbon flakes trapped on the seat are the classic cause. Check whether the seat is pitted or eroded, which cleaning will not fix. Probe the EGR passages in the manifold - heavy carbon there affects flow measurement too.
ExpectedA clean, unpitted seat and a pintle that seats fully. Passages open.
Command the valve with a scan tool and watch the response
Tools Bidirectional scan tool
On systems that support bidirectional control, command the valve open in steps at idle and watch RPM drop and the feedback reading rise in proportion. A valve that shows flow at zero command, or feedback that does not track command, tells you which half of the loop is broken.
ExpectedRPM falls smoothly as commanded EGR increases, and returns to normal at zero command.
Clear codes and run the drive cycle to confirm the fix
Tools Scan tool with readiness monitor status
The EGR monitor needs specific conditions to run and will not complete on a short trip. Confirm the monitor has actually run and passed rather than assuming the absence of a light means success.
ExpectedEGR monitor showing complete and no return of the code.
| Repair | Only after you confirm | Typical cost | Who |
|---|---|---|---|
| Clean carbon from the EGR valve and passages | Valve removed and found held off its seat by carbon, with the seating surface itself still intact. | $100–$300 | DIY |
| Replace the EGR valve | Valve seat pitted or eroded, diaphragm not holding vacuum, or the valve fails to seat after cleaning. | $200–$700 | DIY |
| Replace the EGR differential pressure or position sensor | Sensor reports flow at its baseline condition with hoses removed and the valve verified mechanically closed. | $110–$320 | DIY |
| Replace cracked or blocked EGR pressure hoses | Hoses found hardened, split, or carbon-blocked on inspection. | $70–$200 | DIY |
| Replace the EGR vacuum control solenoid | Vacuum present at the EGR valve at warm idle with the valve commanded closed. | $130–$400 | DIY |
| Replace EGR valve gasket or base plate | Leak or misfit found at the valve mounting face, particularly after recent EGR service. | $80–$250 | DIY |
Cost turns mostly on access. An EGR valve bolted to the front of the manifold is a short job. One buried behind the intake or under the throttle body on a transverse V6 can double the labor. Cleaning is often worth trying first if the seat is intact.
- Replacing the EGR valve when the DPFE sensor or its cracked hoses were producing a false flow reading.
- Cleaning a valve whose seat is pitted and eroded - cleaning cannot restore a sealing surface that has been eaten away.
- Ignoring the vacuum control solenoid on vacuum systems and going straight to the valve.
- Cleaning the valve but leaving the manifold EGR passages packed with carbon.
- Blocking the EGR passage as a fix, which sets other codes, raises combustion temperature, and increases knock and NOx.
- Assuming the repair worked because the light stayed off during a short test drive, when the monitor never ran.
- Definition
- Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.
- Diagnostic guidance
- Written and reviewed in-house. See our editorial policy.
- Review status
- Reviewed
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