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P1351 Error Code - Diagnosing Spark Issues

Chevrolet S10 pickup second-generation

Earlier this month in our forum, user Lonelyboy was back, this time with a P1351 error code on a Chevrolet Blazer. This is a relatively common code on small General Motors trucks from the late 1990s and early 2000s. Most mechanics start with the easy stuff and replace spark plugs, distributor cap/rotor, etc. This only rarely solves the issue. As Big Block 409 pointed out, the problem is more likely to be a circuit issue, not a bad part.

The P1351 Error Code

The code being thrown by the computer here is an Ignition Coil Control Circuit High Voltage code. The title itself points to the likely issue. The problem could be anything from the ignition control module through the circuit/harness to the battery. In most vehicles, including Chevrolet, the ICM has its own power supply and circuit as it is integral to the starting unit for the vehicle.

The ICM is one of the things that activates when you turn the key to start the car. It controls the spark timing and (often) fuel insertion when the vehicle is cranked to start it. After the engine starts, usually on the third or fourth crank at latest, the ICM gives control to the power control module (PCM) or engine computer (ECM).

Diagnosing the Circuit

Begin by using a multimeter to check the circuit. Begin with the ICM (ignition control module) at its power output point with the other end on a ground point. Check voltage, which should be over 1 and less than 4, usually in the 2.5-3V range, depending on engine size.

Next, use the meter (or a test light) to check the ground. Simply connect the meter to the power input point and to a ground point. The result should be battery voltage (11-12V DC). This should be true with or without the keyswitch being on. There may be a second ICM terminal to test as a second input. Do so there as well.

Now it's the ICM itself. Most likely it is at fault and should be replaced. Unless you have specialized equipment, however, testing the ICM is not possible, so replacement is the course of action here. It's possible, though unlikely, that the ECM is also faulty.