L5P Duramax Glow Plug Codes – What You Need to Know
If you’re seeing glow plug-related codes on your L5P Duramax, it doesn’t always mean the plugs themselves are bad. In many cases, it could be a simple software issue.
Common Causes
Glow Plug Not Relearned: If a glow plug was replaced but the “glow plug learn” procedure wasn’t completed afterward, the system may throw codes.
EEPROM or Data Corruption: The Glow Plug Control Module (GPCM) can experience internal memory errors, which may cause incorrect readings or faults.
How to Fix It
• Make sure your GPCM has the latest software update.
• Perform a Glow Plug Learn procedure using a proper scan tool.
Check Resistance Values for Each Glow Plug
• Less than 1.8 ohms (Ω) = Good
• 1.8 Ω or higher = Replace that glow plug
Glow Plug Resistance Testing Tip
Use a multimeter to measure resistance across each glow plug terminal. This quick test can save you from unnecessary parts swaps.
Don’t Skip This Step: Use a Reaming Tool
When installing new glow plugs, always clean out the carbon from the glow plug bore using a Glow Plug Reaming Tool. Carbon buildup can cause improper seating and premature failure of the new plugs. This step is critical for ensuring solid contact and long-term reliability.
DmaxStore Glow Plug Reaming Tool (2017–2025 L5P/LM2/LZ0)
Common Glow Plug Trouble Codes (DTCs)
Low/High Voltage Faults:
P066A – Cylinder 1 Low Voltage
P066B – Cylinder 1 High Voltage
P066C – Cylinder 2 Low Voltage
P066D – Cylinder 2 High Voltage
P066E – Cylinder 3 Low Voltage
P066F – Cylinder 3 High Voltage
General Circuit Faults:
P0671 to P0678 – Cylinders 1–8 Glow Plug Circuit
Offset Learning Limit Codes:
P1338 to P133F – Glow Plug Offset Exceeded on various cylinders
Need New Glow Plugs?
Grab genuine GM glow plugs from DmaxStore:
Single Glow Plug
Set of 8 Glow Plugs
Don’t guess—test. And always follow up replacement with the learn procedure to avoid false codes down the line.
