P0351 Code: Ignition Coil A Primary/Secondary Circuit
The engine computer found an electrical fault in coil A, the coil that fires cylinder 1 on most engines.
At a glance
Severity: Moderate
- Meaning
- Ignition Coil A Primary/Secondary Circuit
- System
- Ignition & Misfire · Powertrain
- Safe to drive?
- With caution
- Typical repair cost
- $60–$1,500
- DIY difficulty
- Easy
Every time the engine computer switches coil A on, it checks that the primary circuit behaves as expected: current has to build up while the driver is on, and coils with a built-in igniter send a confirmation pulse back. P0351 means that check failed, so this is an electrical fault in the coil or its wiring, not a fuel or compression problem. On coil-on-plug engines coil A serves cylinder 1, which sits at the timing-belt or chain end of an inline engine. On older waste-spark engines the A output often drives one half of a coil pack that fires cylinders 1 and 4 together, so both can go dead at once. Many engine computers also cut the injector of the affected cylinder to keep raw fuel out of the catalytic converter.
Can I keep driving? Keep driving to a minimum: the engine is running a cylinder short, and if the injector keeps spraying, the catalytic converter can overheat. If the check-engine light flashes or the engine bucks under load, park it and have it towed.
How to read P0351
- P Powertrain
- 0 Generic (SAE)
- 3 Ignition / misfire
- 51 Specific fault
Symptoms
- Engine shakes at idle and feels weak because cylinder 1 is not firing
- Check-engine light on, usually with P0301 stored alongside
- On waste-spark engines, cylinders 1 and 4 misfire together
- Fault showed up right after spark plug or valve cover work
- Raw fuel smell at the tailpipe on engines that don't cut the injector
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Coil A has failed internally: open or shorted primary winding, or a cracked housing
- Coil A connector not fully latched, locking tab snapped off, or terminals corroded
- Oil in the cylinder 1 plug well from a leaking valve cover gasket or tube seal, which destroys boot and coil
- Waste-spark coil pack serving cylinders 1 and 4 has failed
- Coil A trigger or feed wire chafed where the harness passes the timing cover or engine lifting eye
- Coil driver inside the engine computer burned out by a shorted coil
Diagnosis step by step
Work out what coil A is on your engine
Coil A is the first coil output of the engine computer. On coil-on-plug engines that is the coil on cylinder 1. On waste-spark engines with one pack on the valve cover, the A output usually fires cylinders 1 and 4 together, so check a wiring diagram for your engine before ordering parts.
Tools: Flashlight
Look before you pull anything
With the engine off, make sure the coil A connector is pushed fully home and its locking tab is intact. Brittle tabs on older coils snap and let the plug vibrate loose. Then pull the coil and look down the well: oil, water or white corrosion on the spring and plug terminal needs its own repair.
Tools: Flashlight, Screwdriver
Swap coil A with the cylinder 2 coil
On coil-on-plug engines swap the cylinder 1 and 2 coils, clear the codes and drive until the code would normally reset. If the fault moves to P0352, the coil is bad. If P0351 returns, the coil is fine and the wiring or computer is next.
Tools: OBD-II scanner, Screwdriver
Measure feed and ground at the connector
With the key on and the engine off, the feed pin should be within half a volt of battery voltage, about 12.4 volts. Measure the ground pin against the battery negative post while cranking. More than about 0.2 volt means a poor ground connection.
Tools: Multimeter
Ohm-test only two-wire coils
Coils with just two wires, common on Ford, have no electronics inside, so the primary winding can be measured: roughly 0.3 to 1.0 ohm is normal and an open reading means a dead winding. Coils with three or four wires contain an igniter and can't be judged with an ohmmeter, so rely on the swap test for those.
Tools: Multimeter
Suspect the computer last
Only when the coil, connector and the wire from the computer pin to the coil all test good should you suspect the driver. A shorted coil is what usually kills that driver, so never install a replacement computer without a known-good coil on cylinder 1.
Tools: Multimeter, OBD-II scanner
Where the parts are
- 1Ignition coil
- 2Spark plug
- 3Fuel injector
- 4Camshaft sensor
- 5Crankshaft sensor
- 6Engine computer (ECM)
- 7Intake manifold
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Replace coil A (coil-on-plug, easy access) | Yes | $80–$250 |
| Replace waste-spark coil pack for cylinders 1 and 4 | Yes | $120–$350 |
| Repair coil A connector or pigtail | No | $60–$200 |
| Valve cover gasket and plug tube seals (oil in the well) | No | $180–$500 |
| Replace and program the engine computer | No | $600–$1,500 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Notes by brand
Volkswagen
The 1.8T four-cylinder used in Jetta, Golf, Passat and the Audi A4 and TT has pencil coils that were a well-known weak point. Several coils failing one after another was common, so many owners replace the full set together with the plugs once the first coil code appears.
Field data
| Ford | 2 | Edge, Escape Hybrid |
|---|
Source: NHTSA Complaints API (US government open data), analyzed by DTC mention.
Emissions / smog test impact
Will fail
P0351 lights the check-engine light and is an emission-related fault, so it fails any OBD-based smog check. The misfire monitor re-runs soon after the repair, but clearing codes also resets the other monitors, so allow some mixed driving before the test.
Frequently asked questions
Does coil A always mean cylinder 1?
On coil-on-plug engines it practically always does. On waste-spark systems the letter names a driver that can fire two cylinders, and a few manufacturers label their outputs differently, so a wiring diagram for your engine settles it.
What is the difference between P0351 and P0301?
P0301 means the crankshaft sensor saw cylinder 1 not firing, for any reason at all. P0351 means the computer measured a fault in the coil circuit itself. When both are stored, repair the coil circuit first and see if the misfire code returns.
Should I replace all the coils at once?
Not automatically. On high-mileage engines where coils tend to fail in a row, doing the set saves labor, especially if the plugs are due. On a waste-spark pack the question does not arise, because the whole pack is one part.
My new coil failed within weeks. Why?
Usually oil or water in the plug well, or a worn plug with a wide gap that forces the coil to produce more voltage than it is built for. Fix the leak, fit correctly gapped plugs and use a quality coil.