Diesel Trouble Codes

P0402 — Exhaust Gas Recirculation Flow Excessive (LLY / LBZ / LMM Duramax)

Diagnostic reference for LLY/LBZ/LMM Duramax 2004.5–2010. Diagnose, do not give step-by-step repair of safety-critical systems.

Updated September 2, 2026

What it means

P0402 means the ECM has detected measured EGR flow higher than commanded on this platform. The code applies to LLY/LBZ/LMM Duramax 2004.5–2010 which are EGR equipped; LMM adds DPF, no SCR/DEF on any of these engines.1

Most likely causes

  1. EGR valve stuck open or mechanically binding, causing flow higher than commanded — On LLY/LBZ/LMM the EGR valve + cooler is present. A valve that remains open or sticks open will raise measured EGR flow above the commanded value. This is consistent with the generic definition of P0402 as measured EGR flow higher than commanded.1
  2. EGR position feedback / control circuit performance issue leading to over-positioning — P0404 describes EGR control circuit range/performance on this platform. A control or feedback fault can cause the ECM to command a position but the actual valve position results in excessive flow.2
  3. Incorrect airflow modeling due to MAF sensor signal out of expected range — The platform uses MAF + MAP and MAF feeds EGR/airflow modeling per platform facts. P0101 is MAF circuit range/performance on LLY/LBZ/LMM. An erroneous MAF signal can skew the ECM’s EGR flow calculation, contributing to a P0402 set condition.3
  4. PCM calibration / software-related EGR control strategy anomaly — NHTSA reports for a 2007 LBZ show P0402 occurrences resolved by PCM calibration updates, with power limitation symptoms. This supports a calibration-related cause on this platform.4

How to diagnose

  1. Verify code and freeze frame; confirm P0402 is present on LLY/LBZ/LMM and note sub-engine.
  2. Inspect for related codes: P0404 EGR control range/performance, P0101 MAF range/performance, and any VGT or fuel-system codes that could affect load/airflow modeling.
  3. Review PCM calibration history; NHTSA cases show P0402 on LBZ addressed by calibration updates. Confirm current calibration versus latest GM release for the specific sub-engine.
  4. Assess EGR system integrity via data: commanded vs actual EGR position, EGR error, MAF vs MAP correlation at idle and cruise. Compare to expected values for LLY/LBZ/LMM with EGR + cooler; LMM also has DPF.
  5. Check for conditions that alter airflow/EGR modeling: intake leaks, dirty MAF, or restrictions that change measured airflow. Platform uses MAF for EGR/airflow modeling.

Parts and tools you may need

Rough cost and difficulty

Costs vary by sub-engine, labor rates, and whether calibration update, sensor, or EGR component service is required. LMM includes DPF; LLY/LBZ do not. No SCR/DEF on any of these engines.

When to see a pro

Refer to a qualified diesel technician if EGR control diagnostics require internal inspection, or if PCM reprogramming is needed. Do not attempt step-by-step repair of high-pressure fuel or safety-critical systems. NHTSA reports show dealer PCM updates for P0402 on this platform.

Sources

  1. Curated DTC seed — LLY / LBZ / LMM Duramax (curated) ↩︎ ↩︎

  2. Curated DTC seed — LLY / LBZ / LMM Duramax (curated) ↩︎

  3. Curated DTC seed — LLY / LBZ / LMM Duramax (curated) ↩︎

  4. NHTSA owner report 10302196 ↩︎

Watch a repair

Third-party videos, not ours — for illustration. Confirm anything against the factory service information for your truck.

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