If you own a L5P Duramax and have any of the following codes it may just be a software anomaly. This can happen if the glow plug has been replaced and the glow plug learn wasn't performed after installation, or if the glow plug module experiences an EEPROM error or data corruption. In order to correct this issue you will have to have the Glow Plug Control Module with the latest available software. After completing the glow plug learn procedure you will need to verify the glow plug resistance, if it is less than 1.8 Ω then the glow plugs are good. IF any of the glow plugs are 1.8 Ω or greater they will need to be replaced.

You can find Genuine GM glow plugs for your Duramax here:
Single
Set of 8

P066A: Cylinder 1 Glow Plug Control Circuit Low Voltage
P066B: Cylinder 1 Glow Plug Control Circuit High Voltage
P066C: Cylinder 2 Glow Plug Control Circuit Low Voltage
P066D: Cylinder 2 Glow Plug Control Circuit High Voltage
P066E: Cylinder 3 Glow Plug Control Circuit Low Voltage
P066F: Cylinder 3 Glow Plug Control Circuit High Voltage
P0671: Cylinder 1 Glow Plug Control Circuit
P0672: Cylinder 2 Glow Plug Control Circuit
P0673: Cylinder 3 Glow Plug Control Circuit
P0674: Cylinder 4 Glow Plug Control Circuit
P0675: Cylinder 5 Glow Plug Control Circuit
P0676: Cylinder 6 Glow Plug Control Circuit
P0677: Cylinder 7 Glow Plug Control Circuit
P0678: Cylinder 8 Glow Plug Control Circuit
P1338: Cylinder 1 Glow Plug Offset Exceeded Learning Limit
P1339: Cylinder 2 Glow Plug Offset Exceeded Learning Limit
P133A: Cylinder 3 Glow Plug Offset Exceeded Learning Limit
P133C: Cylinder 5 Glow Plug Offset Exceeded Learning Limit
P133D: Cylinder 6 Glow Plug Offset Exceeded Learning Limit
P133E: Cylinder 7 Glow Plug Offset Exceeded Learning Limit
P133F: Cylinder 8 Glow Plug Offset Exceeded Learning Limit