What does it mean?
The control module switches ignition circuit "J" to ground for a calculated dwell period and then releases it, and the voltage pattern on that wire tells the module whether its own driver and the external circuit are behaving. This code is stored when the module measures a voltage below the calibrated low limit on the "J" command line at a time when the output should be released and sitting high.
A line that will not rise means current has an unintended path to ground, either through damaged insulation, a contaminated connector or a coil that has broken down internally. Since the dwell can no longer be controlled, spark energy and timing for that cylinder become unreliable and the coil is at risk of overheating. The module typically shuts the output down, records the fault and turns on the MIL (check engine lamp).
Symptoms
- MIL (check engine lamp) on with a misfire code for the cylinder served by that coil
- Uneven idle with a rocking motion felt through the body of the car
- Loss of pulling power and poor response when overtaking or climbing
- Popping or spitting from the exhaust as unburned mixture leaves the cylinder
- Slow, hesitant starts and a stumble during the first seconds of running
- Increased fuel consumption and, on some applications, a burnt smell from an overheating coil
Likely causes
- Command wire rubbed through to a grounded surface such as the cylinder head, a heat shield or a mounting bracket
- Oil or coolant leaking into the coil connector and bridging the command pin to ground
- Ignition coil with an internal short from the primary winding to its core or housing
- Harness section crushed under a clamp or a component that was refitted incorrectly after previous work
- Moisture and salt in a connector with a damaged seal, producing a low resistance leakage path
- Control module driver transistor shorted internally, considered only after the external circuit is isolated and measured clean
How to fix it
- Note the freeze frame values and check for companion coil or misfire codes recorded in the same event
- Unplug the coil on the "J" circuit and measure the harness side command pin to chassis ground; a low resistance with the coil removed confirms the short is in the wiring
- If the harness is clean, measure the coil primary winding and test it for insulation breakdown to the case
- Follow the harness branch physically along the head, the loom clips and any heat shield, looking for flattened or melted insulation
- Check the connector cavities for oil and coolant and confirm the seal, the terminal locks and the housing are intact
- Renew the damaged wire section properly, refit the loom clips in their original positions and keep the branch clear of hot surfaces
- Clear the memory and confirm the output switches normally on live data before completing the repair with a road test