Ignition Coil D Primary/Secondary Circuit
What it means
The engine computer has detected an electrical fault in the circuit of ignition coil D. On most engines with one coil per cylinder, coil D serves cylinder 4, the last cylinder on a four-cylinder engine, but the order should be confirmed in the service manual.
Symptoms4
- Check engine light on, possibly flashing during misfires.
- Engine shudder at idle and at traffic lights.
- A misfire code for the cylinder served by coil D, often P0304.
- Sluggish response and higher fuel consumption.
Possible causes6
- Failed ignition coil D.
- Loose or corroded coil D connector.
- Chafed or broken control wire, especially where the harness runs near a hot end of the engine.
- Fault in the shared supply, such as a fuse or relay.
- Worn spark plug or wide plug gap straining the coil.
- Failed driver circuit inside the engine computer (rare).
How to diagnose5
- Confirm which cylinder coil D serves in the service manual.
- Inspect the coil connector and its harness for chafing or heat damage.
- Swap coil D with another cylinder's coil, clear the codes and note whether the fault follows the coil.
- Measure supply and ground at the connector and check control pulses with a test light or oscilloscope.
- Inspect that cylinder's spark plug and compare it with the others.
How to fix5
- Clean the connector and repair its terminals.
- Repair damaged wiring and secure the harness away from heat.
- Replace ignition coil D if the fault follows it.
- Replace the worn plug, ideally all plugs if they are past their service life.
- Clear the codes and road test to confirm the code does not return.
Repair cost
In Saudi riyals; the highlighted part is the expected range for this fault.
The cost is small for a connector or wire repair, rises when the coil is replaced with the plugs, and rarely exceeds this range if the engine computer is at fault.
Prices are rough estimates for common cars at independent workshops and vary with the car and the source of the part.
Prices last updated: September 2026
Frequently asked questions
Can a spark plug cause this code?
It can contribute. A worn plug needs higher voltage, which strains coil D until it fails, so check the plug when replacing the coil.
The code appeared after an engine wash. Why?
Water in the coil connector or plug well can cause spark leakage or a poor connection. Dry the area and check the connector; if the code stays, test the coil.
This guide is for information only and does not replace an inspection by a qualified technician. If the fault involves brakes, steering, engine temperature, electrics or refrigerant, do not attempt the repair yourself.
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