OBD-II Code P0278 — What It Means & How to Fix It

What Does This Error Mean?

P0278 means Cylinder 6 is not contributing its expected share of engine power — a cylinder contribution or balance fault. The engine computer measures tiny crankshaft speed changes after each cylinder fires. If Cylinder 6 produces a smaller-than-expected speed increase, the ECM stores P0278. The cylinder is still firing — just weakly. You may notice a subtle vibration at idle, slightly reduced power, modest fuel economy drop, and sometimes a rough feel when accelerating from a stop.

Affected Models

All vehicles 1996+ with 6 or more cylinders
Common in V6, V8, V10 engines from GM, Ford, Chrysler
Common in high-mileage vehicles with aging spark plugs or coils
More common in rear-bank cylinders that are harder to service

Common Causes

Worn or fouled spark plug on Cylinder 6 producing weak combustion
Failing ignition coil on Cylinder 6 delivering insufficient spark energy
Partially clogged fuel injector on Cylinder 6 not supplying enough fuel
Low compression on Cylinder 6 from worn piston rings, valves, or a head gasket issue
Vacuum leak near the Cylinder 6 intake runner creating a lean condition in that cylinder

How to Fix It

Remove and inspect the Cylinder 6 spark plug. Compare its condition to plugs from other cylinders. A Cylinder 6 plug that looks much worse — heavily fouled, eroded, or cracked — is likely your answer. Replace the full plug set if they're due for service.

On V6 and V8 engines, Cylinder 6 is usually on the rear bank. Rear bank plugs are harder to reach and often go much longer without replacement than front bank plugs.
Swap the Cylinder 6 ignition coil with a coil from another cylinder. Clear the codes and drive the vehicle. If the contribution fault moves with the coil to its new cylinder location, the Cylinder 6 coil is the problem. Replace it.

This swap test is fast, free, and definitive. Do it before spending money on a new coil.
Have the Cylinder 6 injector flow-tested at a shop. A partially clogged injector can fire without delivering the proper amount of fuel, causing low power output from that cylinder. Cleaning may restore it; replacement may be needed if cleaning fails.

Request a flow-rate test — not just an electrical test. An injector can pass an electrical test and still flow poorly due to deposits.
Look for vacuum leaks near the Cylinder 6 intake runner. A small air leak leans out just that one cylinder, reducing combustion without causing a complete misfire. A smoke test is the most reliable method; carburetor cleaner spray is a lower-tech alternative.

When using carburetor cleaner, spray small amounts and watch the idle speed change on your scan tool — a bigger RPM jump indicates a leak nearby.
Perform a compression test across all cylinders and compare results. Cylinder 6 should read within 10% of the highest cylinder. Low compression means engine wear that requires professional assessment and likely internal repairs.

A leak-down test after a low compression reading tells you exactly where the pressure is escaping — rings, intake valve, exhaust valve, or head gasket.

When to Call a Professional

If the basic tune-up fixes don't clear P0278, have a mechanic run a compression test and cylinder leak-down test on Cylinder 6. Low compression points to internal wear — rings, valves, or a head gasket — that requires professional engine work. Don't drive indefinitely with a contribution fault. It typically progresses into a full misfire and eventual catalytic converter failure.

Frequently Asked Questions

Why is Cylinder 6 a common source of contribution faults?

On many V6 and V8 engines, Cylinder 6 is the rearmost cylinder on the rear bank. It's physically the hardest to access for spark plug and coil service. As a result, it often has the oldest plugs and is most likely to show wear first. Proper, regular maintenance — including rear-bank plugs — prevents most P0278 events.

Will P0278 cause me to fail an emissions test?

Yes. A contribution fault stores an active fault code, which means your OBD-II system will flag it during an emissions inspection. The vehicle will fail until the fault is repaired and the code is cleared.

How much does it cost to fix P0278?

Spark plugs: $50–$150 installed. Ignition coil: $50–$200 installed. Injector cleaning: $50–$100. Injector replacement: $150–$400. Engine compression/internal work: $1,000–$4,000+.