P0402 on the 2007.5-2010 LMM Duramax means the EGR system is flowing more exhaust than the ECM commanded. Excess EGR flow can make the engine run rough and cause elevated soot, so it is worth diagnosing carefully rather than clearing and hoping.
What P0402 means
P0402 is defined as Exhaust Gas Recirculation Flow Excessive. The LMM routes exhaust through an EGR cooler and valve into the intake. When the ECM sees more flow than requested, or the valve is not closing when commanded, it sets P0402. A valve stuck partially open or a false high-flow reading are typical triggers.
Common causes
- EGR valve stuck partially open due to carbon buildup
- Failed or sticking EGR valve actuator
- Faulty EGR position sensor reporting incorrect data
- Wiring or connector fault on the EGR circuit
- Related intake or temperature sensor errors skewing the flow estimate
Symptoms
- Check engine light with stored P0402
- Rough idle, especially when cold
- Hesitation or reduced power
- Increased soot or occasional failed regen
- Possible limp mode in some cases
How to diagnose it
Read stored and pending codes for companion faults. Use a scan tool to command the EGR valve closed and verify position feedback shows it fully seating. Inspect the valve for carbon holding it off its seat, a common problem on soot-heavy Duramax EGR systems. Check the position sensor signal and the wiring for damage. Confirm the flow estimate is not being thrown off by a faulty intake air temperature or MAP reading before replacing the valve.
How to fix it
Clean the EGR valve if carbon is preventing it from closing, or replace it if the valve or actuator is worn. Replace a faulty position sensor and repair any damaged wiring. Correct related sensor faults that distort the flow calculation. After repair, clear the code, run a full drive cycle, and confirm the valve seats on command and P0402 does not return.
Frequently asked questions
Is P0402 the opposite of P0401?
Yes. P0402 is excessive EGR flow while P0401 is insufficient flow. Both often come down to a carboned valve, just stuck in different positions.
Can carbon cause P0402?
Absolutely. Soot can hold the EGR valve slightly open so it flows when it should be closed, which the ECM reads as excessive flow.
Do I need a new EGR cooler?
Not necessarily for P0402. The valve and its actuator are the usual fix, though a heavily fouled system may warrant cleaning the cooler too.
Need parts or a second opinion? The team at Core Diesel Parts can help you match the right components to your engine. Email sales@coredieselparts.com with your VIN and symptoms and we will point you to the correct parts.
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