P0351: P0351 Diagnostic Guide

What this code means

P0351 may indicate a possible issue in an ignition coil primary or control circuit where the commanded circuit state and the actual circuit behavior may not agree.

What the vehicle may do

The vehicle may run rough or misfire.
The check engine light may come on.
The engine may crank or run with an ignition-related fault present.

Possible fault areas

Possible ignition coil control circuit problem.
Possible wiring, terminal, or connector issue.
Possible ground or ignition feed concern.
Possible engine control module side circuit concern.
Possible ignition coil output concern.

Diagnostic path

Opening context

On this 2014 to 2018 GMC Sierra 1500 with the 5.3 gas engine, P0351 may point to a problem in an ignition coil primary or control circuit. The truck may run rough, misfire, crank longer than expected, or turn the check engine light on. Broadly, the fault area can be the ignition coil circuit, the coil connector, related wiring and terminals, the ground or ignition feed side, or the engine control module side of the circuit. If other ignition or misfire codes are present, use them as context, but keep this diagnostic path focused on P0351. The logic here is simple: the control circuit is being commanded one way, and the actual circuit state is not matching what the module expects.

Start with verification, not parts

Start with the basic system checks, then follow a structured diagnostic approach. With the engine running, look at the scan tool status for the Ignition Coil 1–8 Control Circuit High Voltage Test Status, Low Voltage Test Status, and Open Test Status. Each one should read OK or Not Run. If any of those statuses are not in that specified state, move into circuit testing. If they are in the specified state, do not call it fixed yet. Keep watching those same parameters while you wiggle the harness and connector at the T8 Ignition Coil and at the K20 Engine Control Module. If the status changes during the wiggle test, repair the wiring, terminals, or electrical connectors as needed.

Reproduce the operating conditions

If the wiggle test stays clean, reproduce the operating conditions for the monitor. This code is evaluated with the engine cranking or running, and ignition voltage Greater than 11 V, after the running conditions are met. You can also reproduce the captured conditions from the scan data. The goal is to confirm whether P0351 resets. If the code sets again, go to circuit testing. If it does not set, the circuit is considered OK at that time.

Ground side testing first

Before the continuity checks, remember that it may take up to 2 min for all vehicle systems to power down before an accurate ground or low reference circuit continuity test can be made. Turn the ignition and vehicle systems off, then disconnect the appropriate T8 Ignition Coil connector. Test for less than 10 Ω between ground circuit terminal 1/A and ground. If it is 10 Ω or greater, disconnect the ground connection and test the circuit end to end: ground circuit terminal 1/A at the component harness to the other end of the circuit at the ground terminal. That test should be less than 2 Ω. If it is 2 Ω or greater, repair the open or high resistance in the circuit. If it is less than 2 Ω, repair the open or high resistance in the ground connection.

Check for a short to ground on the control circuit

If the ground check is less than 10 Ω, turn the ignition on or put the vehicle in service mode. Now verify that a test lamp does not turn on between control circuit terminal 3/C and B+. If the test lamp turns on, turn the vehicle off, remove the test lamp, disconnect the K20 Engine Control Module connector, and test for infinite resistance between control circuit terminal 3/C at the component harness and ground. If resistance is less than infinite, repair the short to ground on that circuit. If resistance is infinite, the path leads to replacing the K20 Engine Control Module.

Check for a short to voltage

If the test lamp does not turn on, remove the test lamp, start the engine, and check the Ignition Coil 1–8 Control Circuit High Voltage Test Status again. It should be OK or Not Run. If it is not, turn the ignition and vehicle off, disconnect the K20 Engine Control Module, and test for less than 1 V between control circuit terminal 3/C at the component harness and ground. If the reading is 1 V or greater, repair the short to voltage on the circuit. If it is less than 1 V, the path leads to replacing the K20 Engine Control Module.

Check control circuit continuity under the jumper test

If the High Voltage Test Status is OK or Not Run, turn the ignition and vehicle off. Connect a 3 A fused jumper wire between control circuit terminal 3/C and ignition circuit terminal 4/D. Run the engine and check the Ignition Coil 1–8 Control Circuit High Voltage Test Status again. If it is not OK or Not Run, turn the vehicle off, remove the jumper, disconnect the K20 Engine Control Module, and test for less than 2 Ω between control circuit terminal 3/C at the component harness and the other end of the circuit at the control module harness. If it is 2 Ω or greater, repair the open or high resistance in the circuit. If it is less than 2 Ω, the path leads to replacing the K20 Engine Control Module. If the status is OK or Not Run with the jumper installed, move on to testing or replacing the T8 Ignition Coil as directed by the diagnostic path.

Coil component test and final verification

Only do the component test after the circuit testing has been performed. Turn the ignition and vehicle off, remove the appropriate T8 Ignition Coil, and leave its electrical connector connected. Install the J-26792 Ignition Spark Tester, then run the engine and verify spark at the tool. A weak or erratic spark counts the same as no spark. If there is no spark, replace the T8 Ignition Coil. If there is spark, that part of the test is OK. After any repair, verify the repair and confirm P0351 stays gone. The takeaway is to prove the ground, control circuit, shorts, opens, module-side logic, and coil output in order—do not skip straight to the coil just because the code names the ignition circuit. For more diagnostic training, visit stepdiagnostics.com.