What does it mean?
The control module switches each ignition coil or igniter through its own low current command line and watches the voltage on that line to confirm the driver stage and the coil are actually connected. Ignition circuit "I" is one of those individually addressed outputs, so the letter identifies a specific coil position in the manufacturer wiring designation rather than a general coil group.
This code is stored when the module energises the "I" output and sees no current flow at all, a feedback pattern that only an interrupted path can produce. Because the module cannot tell a broken wire apart from a disconnected or internally open coil, it reports the whole command path as broken. After the fault repeats on the number of drive cycles the strategy requires, that output is normally shut down and the MIL (check engine lamp) is turned on.
Symptoms
- MIL (check engine lamp) illuminated, very often together with a cylinder specific misfire code
- Clearly rough idle and a shake felt through the steering wheel because one cylinder contributes nothing
- Stumble, hesitation or a flat spot when the throttle is opened under load
- Long cranking before the engine catches, worst on a cold start
- Raw fuel smell at the tailpipe and a jump in fuel consumption
- Noticeable power loss and, on some applications, limp mode once the misfire counter is exceeded
Likely causes
- Disconnected, loose or corroded connector at the coil or igniter served by the "I" command line
- Broken or chafed command wire between the module and that coil, typically where the harness crosses a hot manifold or a moving bracket
- Open primary winding inside the coil, which produces exactly the same no current feedback as a cut wire
- Missing supply feed to the coil because of a blown fuse, a burnt relay contact or a failed power splice shared by several coils
- Backed out or spread terminal in the module connector that loses contact when the engine heats up or vibrates
- Rodent damage or a previous repair splice that has corroded inside the insulation
- Failed driver transistor inside the control module, which is the least likely cause and is only accepted after the wiring is proven good
How to fix it
- Read the freeze frame and note engine speed, load and coolant temperature when the code set, then check for companion misfire or coil codes that point at the same cylinder
- Inspect the coil connector and the harness branch for the "I" circuit for green corrosion, melted insulation, rub through and terminals that have backed out of the housing
- With the ignition on, measure supply voltage at the coil connector; a missing feed sends the diagnosis to the fuse, relay or power splice instead of the command line
- Check the command wire for continuity from the coil connector to the module pin and measure its resistance, since anything well above a fraction of an ohm confirms the break
- Measure the primary winding resistance of the suspect coil and compare it directly with a coil taken from another cylinder of the same engine
- Wiggle the harness and connector while watching live data or a test lamp to catch a break that only opens under vibration
- Repair the damaged harness section or replace the proven faulty coil, clear the memory and road test through the conditions recorded in the freeze frame