P0351
- Powertrain
- Generic (SAE)
- Emissions
Get it looked at now
A cylinder that does not fire pumps raw fuel and oxygen into the catalytic converter, where it ignites and overheats the substrate. This is the classic catalyst-destroying scenario, and a converter costs several times what a coil costs. If the MIL is flashing, that damage is happening now, not eventually.
- Safe to drive
- Flashing light: stop and tow. Steady light: drive only far enough to reach repair, gently, no heavy load.
Ignition Coil "A" Primary/Secondary Circuit Malfunction (Cylinder 1)
The computer detected an electrical fault while driving ignition coil A, the coil that fires cylinder 1 on most vehicles. Either the coil is not responding correctly or its control circuit is broken, so that cylinder is misfiring or not firing at all.
The PCM drives the coil primary through a low-side driver and monitors that circuit electrically. It watches the current ramp as the coil charges and the inductive voltage spike when the driver switches off. An open primary winding, a shorted winding, an open or shorted control wire, a missing ignition feed, or a failed driver all change that electrical behaviour in ways the module can detect. Despite the SAE title naming both primary and secondary, most production systems only directly monitor the primary side and infer secondary condition from it. Coil A also does not universally mean cylinder 1 — on wasted-spark coil packs a single coil serves two cylinders, and coil lettering follows the manufacturer's convention.
How the car detected itDuring each coil charge event the module compares the primary circuit's electrical behaviour against expected limits. No current flow indicates an open circuit. Excessive current indicates a short. A missing or malformed flyback spike at turn-off indicates the coil is not storing and releasing energy correctly. The code typically sets quickly, often within a few seconds of running, and is frequently accompanied by a misfire code for the same cylinder.
A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.
- Safe to drive
- Flashing light: stop and tow. Steady light: drive only far enough to reach repair, gently, no heavy load.
- Urgency
- Get it looked at now
- Will it pass emissions
- No
- If ignored
- Catalytic converter failure is the expected expensive outcome, and it will then set its own codes. The engine will run rough and lose noticeable power. Raw fuel washing down the cylinder wall thins the oil film and accelerates bore and ring wear. If the fault is a short in the control circuit, continued operation can destroy the PCM's ignition driver, converting an inexpensive repair into a module replacement.
- Check engine light
- Solid
- Clearing the light
- A completed drive cycle is normally required before the monitor re-runs and the light can clear.
These are the causes this code can have. For the order to work through them in, see what to check first.
| # | Cause | How often | Component |
|---|---|---|---|
| 1 | Coil primary winding open or shorted | Very common | — |
| 2 | Wiring or connector fault | Common | — |
Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.
In order. Each of these is cheaper or faster than what follows it, and each one can make the rest unnecessary.
Whether the MIL is flashing
Free, instant triage. Flashing means catalyst damage is in progress and the answer changes from drive it in to tow it in.
Which cylinder coil A actually serves on this engine
It is usually cylinder 1 but not always, and on a wasted-spark pack it may serve two cylinders. Replacing the wrong coil is a completely avoidable mistake.
The coil connector and the coil boot
Water in the plug well, oil intrusion from a leaking valve cover gasket, and a carbon-tracked boot are common, visible, and cheap to fix.
Whether the coil is a passive design or has an integrated igniter
It decides which tests mean anything. A passive coil's pins land on the primary winding and can be measured with an ohmmeter; a coil with an internal driver, typically three or four wires, cannot, and must be judged by swap and scope instead.
Whether other coil circuit codes are stored alongside it
Multiple coil codes usually mean a shared ignition feed, a shared ground, or a failed relay rather than several coils dying simultaneously.
Ignition supply voltage at the coil connector, key on
A coil with no power cannot fire and will report as an electrical fault. Checking the feed before buying a coil takes one minute.
Confirm which cylinder coil A serves and record freeze frame
Tools Scan tool, service data
Check the service data for the coil lettering convention on this engine. Record the conditions at set time and note whether a companion misfire code is stored for the same cylinder.
ExpectedPositive identification of the correct coil and a companion misfire code that matches
Inspect the coil, boot, connector, and plug well
Tools Hand tools, inspection light
Pull the coil and look for water or oil in the well, a cracked or carbon-tracked boot, a burnt or melted connector, and green or spread terminals. Oil in the plug well from a leaking valve cover gasket is a very common cause and needs its own repair.
ExpectedA dry well, an intact boot with no carbon tracking, and clean terminals
Verify ignition feed voltage and ground at the coil connector
Tools DMM, wiring diagram
With the key on and the coil unplugged, confirm battery voltage on the supply pin, and confirm a good ground where the design uses one. A blown fuse or a failed relay explains a coil that appears dead, and costs a fraction of a coil.
ExpectedBattery voltage on the supply pin and a low-resistance ground path
Check the PCM control signal to the coil
Tools Oscilloscope or noid light, back-probe leads
With the coil unplugged, crank or run the engine and check for a switching signal on the control wire using a scope, or a logic probe. A scope also lets you compare the drive waveform against a known-good coil circuit on the same engine.
ExpectedA repeating drive pulse present on the control wire. No pulse with good supply points at the harness or the PCM driver.
Swap coil A with a coil from another cylinder
Tools Hand tools, scan tool
The fastest way to separate a bad coil from a bad circuit, and on coils with an integrated igniter it is the primary test because resistance tells you nothing. Move the coil, clear codes, run the engine, and see whether the code follows the coil to its new position or stays on position A.
ExpectedIf the fault follows the coil, the coil is confirmed bad. If it stays on position A, the fault is in the wiring or the PCM driver.
Measure primary resistance on a passive coil, and compare across the engine
Tools DMM, service data, wiring diagram
With the coil out, measure across the primary terminals and, on coils with accessible secondary terminals, across the secondary. Compare all coils on the engine to each other. Absolute specifications are vehicle-specific, but a coil that is a clear outlier or reads open or shorted is the finding. This test only applies to passive coils whose connector lands directly on the primary winding. On coils with an integrated igniter — typically three or four wires including a supply, ground, and logic-level trigger — the pins connect to internal electronics and an ohm reading is meaningless. Identify the design from the wiring diagram before measuring; on smart coils, rely on the swap test and the scoped drive waveform instead.
ExpectedOn passive coils, readings closely matched across all coils on the engine. On coils with an integrated igniter, resistance yields no valid conclusion — use the swap and waveform results instead.
Test the control circuit end to end for opens and shorts
Tools DMM, wiring diagram
With both the coil and the PCM connector disconnected, check the control conductor for continuity, for a short to ground, and for a short to voltage. Wiggle-test the harness with a meter connected, paying attention to where it passes near the exhaust or moves with the engine.
ExpectedContinuity end to end, no shorts, and no change during the wiggle test
Consider the PCM ignition driver only after the circuit is proven
Tools Oscilloscope, DMM, wiring diagram
A known-good coil, verified supply and ground, and a verified-intact control wire with no drive pulse points at the driver inside the module. Confirm PCM power and grounds first, and repair any short in the control circuit before fitting a module, or the new one will fail the same way.
ExpectedEverything upstream verified with no drive pulse present at the PCM output pin
| Repair | Only after you confirm | Typical cost | Who |
|---|---|---|---|
| Replace ignition coil A | The fault follows the coil when swapped to another position — or, on a passive coil only, primary resistance reads open, shorted, or clearly out of line with the other coils | $70–$320 | DIY |
| Repair or replace the coil connector and terminals | Burnt, melted, corroded, or spread terminals, or the fault changes during a connector wiggle test | $80–$280 | DIY |
| Replace a blown fuse or failed ignition relay feeding the coil | No battery voltage at the coil supply pin with the key on | $40–$250 | DIY |
| Repair the coil control wire — open, chafed, or shorted section | Continuity, short-to-ground, or short-to-voltage testing on the isolated control conductor identifies the fault | $100–$500 | DIY |
| Replace a leaking valve cover gasket and clean oil from the plug wells | Oil found pooled in the plug well or wicked into the coil boot | $200–$800 | DIY |
| Replace the spark plug if it is damaged or at its service interval | Plug shows a cracked insulator, excessive gap, or heavy fouling that overloaded the coil | $90–$450 | DIY |
| Replace the engine control module for a failed ignition driver | Known-good coil, verified supply, ground and control wire, and no drive pulse at the PCM output pin with module power and grounds confirmed | $700–$1,900 | Shop |
US independent-shop ranges, parts plus labour. Coil-on-plug replacement on an accessible cylinder is one of the cheapest ignition repairs there is. Cost climbs when the intake manifold covers the coil, when the coil is a pack serving several cylinders, or when a valve cover gasket has to come off. Module replacement almost always requires programming, and sometimes an immobilizer relearn, which adds labour or a dealer visit.
- Buying a coil before checking the ignition feed fuse and the control wire, which are far cheaper causes.
- Replacing coil A without confirming which cylinder coil A serves on that specific engine.
- Ohm-testing a coil with an integrated igniter and condemning it because it reads open. Those pins go to internal electronics, not to the primary winding, so the reading means nothing.
- Ignoring oil in the spark plug well. Replacing the coil without replacing the leaking valve cover gasket means the new coil fails too.
- Fitting a replacement PCM without first finding and repairing the short that destroyed the original ignition driver.
- Driving with a flashing MIL and adding a catalytic converter to a repair that started as a single coil.
- Assuming coil resistance specifications carry over between vehicles. Coil design and specified values vary widely, so where the measurement is valid at all, compare coils on the same engine to each other.
- Ignition Coil
- Ignition Coil Wiring
- Ecu Ignition Driver
- Definition
- Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.
- Diagnostic guidance
- Written and reviewed in-house. See our editorial policy.
- Review status
- Reviewed
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