It appears your GMC diesel truck has detected a failure while monitoring the glow plug(s). The glow plug system is monitored whenever the glow plugs (either bank 1 or bank 2) are commanded on for more than 35 seconds and the truck's battery voltage is between 11.5-14 volts. A trouble code will set if the reading is less than 39 amps on either bank. Monitoring usually occurs at start-up, and as such, if the fault is not repaired, you'll notice the check engine light turn on soon after starting the engine, even if the trouble code has been cleared from the ECU (but the fault not repaired).
The fault is more than likely caused due to a defective glow plug or loose fitting pins in one of the glow plug connectors. You should inspect all glow plugs, glow plug wiring and connections. Any electrical fault which might cause high resistance in the glow plug circuit and not allow at least 39 amps to be measured by the Bus Bar will trigger a trouble code, and subsequently, turn on the check engine light. ny fault which causes the check engine light to illuminate will cause a smog check failure.
If you can not afford to repair the glow plug fault, please look into the State's Consumer Assistance Program. You may be eligible for up to $500.00 in repair assistance.
www.smogtips.com/consumer_assistance.cfm
