P0351 to P0358: ignition coil primary and secondary circuit by cylinder

Coil-on-plug ignition gives every cylinder its own coil, which is good for spark energy and good for diagnostics: a fault can be attributed to a specific cylinder.

P0351 through P0358 map to cylinders 1 through 8 in order.

Code	Cylinder
P0351	1
P0352	2
P0353	3
P0354	4
P0355 to P0358	5 to 8

Related: ignition coil failure symptoms and ignition coil and spark plug diagnostics.

Primary versus secondary

The coil has two windings.

Primary — the low voltage side the ECU switches. A primary circuit fault is electrical and detectable directly by the module: open circuit, short, or a driver failure.

Secondary — the high voltage side that fires the plug. Secondary faults are harder to detect and usually show as a misfire rather than a circuit code.

A P035x code generally means the ECU detected a problem on the primary side, which narrows things considerably. It is a wiring, connector, coil or driver problem — not a spark plug problem.

That is the useful implication: a circuit code points away from the plug.

The swap test, still the best tool

Costs nothing and answers the question definitively.

1. Note the code and its cylinder.
2. Swap that coil with a neighbouring cylinder’s coil.
3. Clear codes and run the engine until the monitor completes.
4. Read again.

If the code moved to the other cylinder, the coil is faulty. If it stayed, the coil is fine and the fault is in the wiring, connector or ECU driver for that cylinder.

Do the same with the spark plug if you also have misfire codes.

What else lives in that circuit

The connector

Coil connectors sit on top of a hot engine and get removed at every plug change. Latches break, pins spread, and seals harden.

A connector that is not fully latched produces an intermittent circuit fault that no amount of coil replacement fixes.

The wiring

Each coil needs power, ground and a switched signal from the ECU. The power feed is often shared across coils — a fault there sets codes on several cylinders at once.

If you have four coil codes, look at the shared feed and its fuse before buying four coils: fuses and relays basics for owners.

The ECU driver

Each coil has its own output transistor inside the module. These do fail, usually after a shorted coil took one out with it.

If a new coil sets the same code immediately, suspect the driver — and consider that the previous coil may have damaged it.

Oil and water in the plug wells

A recurring physical cause worth checking directly.

Valve cover gasket leaks fill the spark plug wells with oil. The coil sits in that oil, its boot degrades, and the high voltage tracks to ground instead of jumping the plug gap.

Similarly, water from a blocked windscreen scuttle drain can find its way into the wells.

Pull one coil and look. If the well is full of oil, the gasket is the repair and the coils are collateral damage.

Oil leak diagnosis causes.

Symptoms

• Misfire on the named cylinder: P0301 to P0304.
• Rough idle, smoother at higher RPM.
• Hesitation under load, which is when the coil works hardest — a marginal coil fails under load before it fails at idle.
• Flashing check engine lamp in severe cases, which means catalyst damage is occurring: check engine blinking vs solid.

Do not replace all of them by default

Replacing every coil “while you are in there” is common advice and often unnecessary. Coils do not wear as a set; they fail individually, usually from heat or oil.

The exception is a high-mileage engine where several are already failing, or where oil contamination affected all of them.

Sequence

1. Note the cylinder and check for misfire codes alongside.
2. Pull the coil and inspect the well for oil or water.
3. Check the connector for latching and pin condition.
4. Swap with a neighbour and re-test.
5. Check for a shared feed fault if multiple cylinders are affected.
6. Replace the coil if the fault followed it.
7. Confirm misfire counters return to zero afterwards.

Watch misfire counters per cylinder while you do the swap test — an ELM327 adapter and the free tier of the app are all it takes. The counter tells you the answer before the code has time to set again.