P219C: Cylinder 1 Air-Fuel Ratio Imbalance

What it means

Cylinder 1 fuel trim deviates significantly from the bank average — a single-cylinder injector or intake fault.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

The control module evaluates cylinder 1’s combustion balance against the other cylinders on its bank using engine and exhaust data. P219C sets when cylinder 1’s inferred air-fuel contribution differs significantly from the bank average. It points to cylinder 1 for diagnosis but does not prove whether the cause is its injector, intake airflow, ignition, compression, or sensor input.

Common causes

Most likely first.

1. Clogged or leaking cylinder 1 injector
2. Cylinder 1 intake leak or airflow issue
3. Cylinder 1 ignition misfire
4. Low compression in cylinder 1

Symptoms

• Check-engine light
• Rough idle or engine vibration
• Engine hesitation or reduced power
• Increased fuel consumption

How to diagnose it

1. Read freeze-frame data, cylinder-specific misfire counts, and available fuel-trim or balance data to confirm when cylinder 1 stands out.
2. Inspect the cylinder 1 injector connector and nearby wiring, then check for an intake leak at the runner serving cylinder 1.
3. Compare cylinder 1 injector operation with the other cylinders using available scan data or an appropriate injector test.
4. Inspect cylinder 1 ignition components and check for a misfire that could create an apparent air-fuel imbalance.
5. If fuel, air, and ignition checks are inconclusive, perform a compression comparison including cylinder 1 and assess relevant sensor inputs against the manufacturer’s specification.

Common fixes

• Repair an intake leak or damaged connection serving cylinder 1.
• Clean or replace the cylinder 1 injector if testing confirms restricted flow or leakage.
• Repair the cylinder 1 ignition fault causing uneven combustion.
• Address low compression in cylinder 1 if confirmed by testing.

Can I keep driving?

If the warning light is steady and the engine runs smoothly, limit hard acceleration and arrange diagnosis soon because an ongoing cylinder 1 imbalance can raise emissions and fuel use. Stop driving if the light flashes or the engine shakes severely.