P0356 Code: Ignition Coil F Primary/Secondary Circuit
The coil F circuit for cylinder 6 failed its check. On inline sixes it's the coil at the firewall.
At a glance
Severity: Moderate
- Meaning
- Ignition Coil F Primary/Secondary Circuit
- System
- Ignition & Misfire · Powertrain
- Safe to drive?
- With caution
- Typical repair cost
- $90–$1,000
- DIY difficulty
- Moderate
P0356 is set when the sixth coil output does not behave electrically the way the engine computer expects, either in how the primary current builds or in the feedback pulse from the coil. It applies to V6, inline-six and V8 engines. On an inline six, cylinder 6 is the last one, right at the firewall, where under-hood heat builds up and the cowl or cabin filter housing often has to come off first. On almost every transverse V6, whatever the numbering scheme, cylinder 6 ends up on the radiator-side bank at the transmission end, which makes it one of the easier coils to reach. Turbocharged engines put extra strain on coils, because the higher cylinder pressure demands a higher firing voltage.
Can I keep driving? Drive gently and only as far as needed. On turbocharged engines, avoid boost: a dead cylinder under boost can light a flashing check-engine light within seconds.
How to read P0356
- P Powertrain
- 0 Generic (SAE)
- 3 Ignition / misfire
- 56 Specific fault
Symptoms
- Stumble or jerk under boost or hard acceleration, smoother when cruising
- Check-engine light, flashing during acceleration, with P0306 stored
- Pending P0356 on the scan tool before the light comes on
- Engine shake at idle that is felt through the steering wheel
- Reduced power, sometimes with a reduced-power message on turbo engines
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Coil F has failed internally
- Oil in the cylinder 6 well from a leaking valve cover gasket
- Worn spark plug, especially on turbo engines, forcing the coil to fire at excessive voltage
- Loose or corroded terminals in the coil F connector
- Shared waste-spark coil failed (cylinders 1 and 6 on inline sixes, 3 and 6 on GM V6 packs)
- Coil F wiring damaged by heat near the firewall or the exhaust
- Coil F driver in the engine computer has failed
Diagnosis step by step
Locate cylinder 6 and plan access
On an inline six it is the rearmost cylinder at the firewall, and on many longitudinal sixes the cabin air filter housing or a cowl panel has to come off first. On transverse V6s it is on the radiator-side bank at the transmission end. On a GM V8 it is third from the front on the passenger side, and on a Ford V8 second from the front on the driver side.
Tools: Flashlight, Screwdriver
Check which cylinders are affected
If cylinder 6 is the only one misfiring, the fault is local to coil F. If cylinder 1 on an inline six or cylinder 3 on a GM three-coil V6 pack also misfires, the shared coil tower is the suspect.
Tools: OBD-II scanner
Compare current ramps with a low-amp clamp
If you have a scope and a low-amp clamp, clamp the feed wire of coil F and then coil 5 with the engine idling. A healthy coil shows a clean ramp to a steady peak, typically several amps. A ramp that rises far faster than its neighbor points to shorted windings, and no current at all to an open circuit or a missing trigger.
Tools: Multimeter, OBD-II scanner
Pull coil F and read plug 6
Check the boot for oil and carbon tracks, then remove plug 6. On turbo engines, a gap that has opened well past spec or an eroded center electrode is common and puts coils under stress. Replace plugs as a set if they are near the end of their service interval.
Tools: Spark plug socket, Flashlight
Swap coil F with coil 5
Swap the coils on cylinders 5 and 6, clear the codes and drive, including some load. If the code moves to P0355, the coil is bad. If it stays with cylinder 6, check the connector and wiring back to the computer.
Tools: Screwdriver, OBD-II scanner
Confirm the pending code is gone
Many engines need the fault on two consecutive trips before the light comes on. After the repair, check that no pending P0356 is stored after a couple of drives, not just that the light is off.
Tools: 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 F | Yes | $90–$280 |
| Replace coil F plus a full set of spark plugs | Yes | $250–$650 |
| Replace all coils on a high-mileage turbo engine | Yes | $400–$1,000 |
| Replace valve cover gasket (oil in the plug wells) | No | $300–$800 |
| Repair heat-damaged coil F wiring | No | $100–$350 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Notes by brand
BMW
BMW's inline sixes, particularly the turbocharged N54 and N55 in the 335i, 535i and X-series, are well known for wearing out coils and plugs, and many owners replace all six coils together with the plugs. Their plastic valve covers and gaskets are also a common leak point, and oil in a plug well quickly ruins a coil.
Field data
| Toyota | 2 | RAV4 |
|---|
Source: NHTSA Complaints API (US government open data), analyzed by DTC mention.
Emissions / smog test impact
Will fail
P0356 with the check-engine light on means an automatic fail at any OBD-II emissions inspection. A pending code without the light does not fail by itself, but it usually matures into a confirmed code within a trip or two.
Frequently asked questions
Where is cylinder 6 on an inline six?
Counting starts at the timing chain end, which is the front of a longitudinal engine. Cylinder 6 is therefore the rearmost, right by the firewall. On many cars you have to remove the cabin air filter housing or a cowl cover to reach it.
Why do coils fail more often on turbo engines?
The spark has to jump the plug gap against much higher cylinder pressure under boost, so the coil has to produce more voltage. That extra stress, plus under-hood heat, wears the coil's insulation faster, and worn plugs make it worse.
Can I test coil F without a scope?
Yes. The swap test with coil 5 is reliable and needs only a scan tool. A spark tester during cranking and a look at the boot for oil or carbon tracks round it off. The current ramp test is simply faster when you have the equipment.
My scan tool shows P0356 as pending but there's no light. Should I worry?
A pending code means the fault was seen once but not yet confirmed. It often becomes a real code with the check-engine light on the next trip, so it is worth checking coil F now, while the engine is still running well.