How to Troubleshoot the Ignition System in a GMC Sierraby eHow Cars Editor; Updated November 07, 2017
Items you will need
Spark plug gapping tool
How to Troubleshoot the Ignition System in a GMC Sierra. A GMC Sierra is a big truck, but it won't go far if the ignition system isn't working. You can troubleshoot the ignition system by checking the ignition coils and spark plugs. Most service should be handled by an expert, especially if the truck has a distributorless system.
Ignition Coils in a Distributor System
Turn off your Sierra's ignition switch before you begin. Tag the spark plug wires to indicate which cylinder they connect to, then disconnect them from the ignition coil.
Set a digital ohmmeter to its high scale. Probe a terminal in the coil's primary connector on its top front with the ground terminal near the bottom. Replace the coil if the reading isn't infinite.
Switch to the ohmmeter's low scale and probe both terminals in the primary connector. The reading now should be 0.1 ohms.
Go back to the high scale and probe a primary terminal with the secondary terminal (within the cylinder structure in back). The reading should be between 5,000 and 25,000 ohms.
Reconnect all wires to the coil after completion.
Ignition Coils in a Distributorless System
Connect a scan tool to the Sierra's Data Link Connector. This should be located below the dash on the driver's side.
Download the diagnostic trouble codes from the scan tool. Read all the instructions with the device and follow then properly to get these codes.
Look for a code matching an ignition coil problem. P0351 through P0358 indicate problems with a coil control circuit. P1351 means high voltage and P1361 means low voltage.
Check the wiring before you replace the ignition coil. Look for short circuits, broken or chafed wires, loose connections or corroded terminals.
Inspect the insulator surrounding the plugs, checking for cracks. Cracks usually occur where the porcelain insulator meets the shell (where the plug begins spreading out toward its widest point).
Clean the plugs of any dirt or corrosion. You can use a stiff brush, but taking it to a shop with an available spark plug cleaner works better.
Look over the gap between the side and center electrodes. The side angle should be parallel to the center one. Use a spark plug gapping tool to measure the gap and bend it if needed.
The affected coil control circuit matches the last digit on the code. P0351 means a problem with Ignition Coil 1, while P0354 indicates Coil 4.
Don't bend used spark plugs if the Sierra uses platinum ones. Spark plugs are easy and inexpensive to replace.