P0405 — Exhaust Gas Recirculation Sensor A Circuit Low — LLY / LBZ / LMM Duramax
Diagnostic reference for LLY/LBZ/LMM Duramax 2004.5–2010. EGR valve position sensor signal voltage is below valid range.
What it means
Generic definition: EGR valve position sensor signal voltage is below its valid range. Platform note: LLY/LBZ/LMM Duramax 2004.5–2010 have EGR valve + cooler; LMM adds DPF, no SCR/DEF on any of these engines.123
Most likely causes
- EGR valve position sensor internal failure causing low signal voltage — Sensor element or internal circuitry fault pulls signal low.1
- Open or shorted signal/reference/ground wire between ECM and EGR valve position sensor — Wiring damage, chafing, or corrosion creates low voltage on the sensor circuit.1
- EGR valve electrical connector loose, corroded, or contaminated — Poor terminal contact or moisture ingress at the EGR valve connector biases sensor voltage low.1
- Sooted/stuck EGR valve affecting sensor operation — Heavy carbon buildup on the EGR valve can accompany sensor circuit low readings on this platform.1
How to diagnose
- Verify code P0405 is present and check for related EGR codes; record freeze-frame data.
- Inspect EGR valve electrical connector for corrosion, moisture, and terminal fit; check harness for chafing near EGR and along valve cover area.
- Check EGR valve position sensor reference voltage and ground at the connector with key on; compare to expected values and look for short to ground.
- Inspect EGR valve and passages for excessive soot accumulation that may affect sensor operation.
- If wiring and connector are intact, evaluate EGR valve position sensor performance; replace valve assembly if sensor is integrated and fault persists.
Common mistake to avoid
Replacing the EGR valve without testing it. If the valve still moves freely, a carbon cleaning often fixes the flow code; and if it opens on command but the code stays, the blockage is in the passages or cooler, not the valve. Source: FleetRabbit — EGR valve problems and repair.
Parts and tools you may need
- Digital multimeter
- Scan tool with live data
- Terminal cleaning supplies
- Inspection light
Tools you’ll need
- digital multimeter — Measures reference voltage, ground, and signal voltage at the EGR valve position sensor connector to detect a short to ground or out-of-range signal causing P0405.
- OBD-II scan tool — Reads P0405, related EGR codes, and freeze-frame data to confirm the fault and monitor EGR position sensor feedback during diagnosis.
- inspection mirror — Allows visual inspection of the EGR valve connector, harness routing, and soot buildup in tight engine bay areas on LLY/LBZ/LMM Duramax.
- terminal cleaning kit — Cleans corrosion and moisture from the EGR valve electrical connector terminals to restore proper sensor signal and reference connections.
Rough cost and difficulty
| Likely repair | Parts | Shop labor | Who usually does it |
|---|---|---|---|
| Wiring or connector repair | $10–$100 | 1–4 hrs | DIY with tools and a manual |
| EGR valve/passage carbon cleaning | $20–$100 | 1–4 hrs | DIY with tools and a manual |
| EGR position/temperature/differential pressure sensor | $60–$300 | 0.5–1.5 hrs | DIY with tools and a manual |
| EGR valve | $150–$700 | 1–3 hrs | DIY with tools and a manual |
Typical US independent-shop figures; labor runs about $120–$180 per hour (dealers charge more). Confirm the diagnosis before buying parts.
When to see a pro
If wiring faults are suspected in high-pressure fuel systems, or if diagnosis requires internal EGR valve service, have a qualified diesel technician perform the work.
Check for recalls
Recalls are tied to your truck’s VIN, so check yours directly: enter the VIN at NHTSA’s recall lookup. Open safety recalls are repaired free at a dealer.
Sources
Spotted an error, or something missing? Tell us
Run a LLY / LBZ / LMM Duramax? This goes straight to the editor — it is not published.