What are the common causes of P21A4?

A weak oxygen sensor. A wiring fault between the oxygen sensor and the ECM. A dirty or faulty fuel injector on cylinder 9. A clogged fuel filter or low fuel pressure. Low compression on cylinder 9.

Read the cylinder 9 fuel trim on a scan tool. Swap the cylinder 9 injector and see if the fault follows. Check the oxygen sensor and its wiring. Check the fuel pressure and the fuel filter. Run a compression test on cylinder 9. Clear the code and retest.

P21A4

Cylinder 9 Air-Fuel Ratio Imbalance

System: Fuel and air metering (P code)
Lamp: Check Engine Light
Category: Powertrain - Fuel/Air Metering

What it means

P21A4 means Cylinder 9 Air-Fuel Ratio Imbalance.

Check these first

A weak oxygen sensor
A wiring fault between the oxygen sensor and the ECM
A dirty or faulty fuel injector on cylinder 9
A clogged fuel filter or low fuel pressure
Low compression on cylinder 9

What to do next

Read the cylinder 9 fuel trim on a scan tool
Swap the cylinder 9 injector and see if the fault follows
Check the oxygen sensor and its wiring
Check the fuel pressure and the fuel filter
Run a compression test on cylinder 9
Clear the code and retest

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Updated

Generic SAE J2012 definition. Last updated 2026-10-10.