P0405 on GMC: How to Diagnose & Fix
Exhaust Gas Recirculation Sensor 'A' Circuit Low
1 GMC model · 1 documented issue · 1 critical · $900-$2,800 repair
P0405 on GMC by model: symptoms, diagnosis and fix
The exhaust gas recirculation cooler on 6.6L Duramax LMM and LML engines is prone to internal cracking, allowing coolant to enter the EGR system and combustion chamber. Owners experience white steam from the exhaust, unexplained coolant loss, and in severe cases hydrolocking. This is a well-documented failure mode covered under extended warranty campaigns and referenced in NHTSA complaint database for 2008–2016 model years.
Common Symptoms
- White or grey smoke from exhaust at idle or under load
- Unexplained coolant loss without visible external leak
- Coolant smell from exhaust or air intake
- Engine overheating or coolant temperature fluctuations
- Milky residue in coolant reservoir or degas bottle
How to Fix
Replace EGR cooler assembly (GM part 98010538 for LML, or ACDelco 12642878 for LMM) and thoroughly flush cooling system. Inspect EGR valve for carbon buildup and replace if necessary. Upgraded aftermarket EGR coolers (Fleece Performance, BorgWarner) offer improved durability. Labor is 4–8 hours; coolant flush and fill required.
Sources for this issue (1)
P0405 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0405 mean on a GMC?
P0405 stands for "Exhaust Gas Recirculation Sensor 'A' Circuit Low." P0405 is a generic OBD-II code meaning the engine computer detected an abnormally low voltage signal from the Exhaust Gas Recirculation (EGR) sensor 'A' circuit. This sensor reports the EGR valve's position or flow, which the computer uses to control exhaust gas recirculation for lowering emissions and combustion temperatures. A low signal indicates the reading is below the expected range, often due to a wiring or sensor fault. It mainly affects emissions and may cause light drivability issues but is not an immediate safety concern. On GMC it is documented across 1 model: Sierra 3500HD.
What are the most common causes of P0405?
The most common causes of P0405 are: Faulty EGR position sensor (or DPFE sensor), Open, shorted, or corroded EGR sensor wiring/connector, EGR valve stuck or carbon-clogged, Loss of reference voltage or ground to the sensor, Poor connector contact / pin corrosion, Damaged EGR valve assembly, PCM fault (rare). The specific cause varies by vehicle.
How much does it cost to fix P0405 on a GMC?
Published repair estimates on GMC range from $900 to $2,800 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
Looking for P0405 on a different make?
View P0405 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.