OBD-II fault code

P21A5: Cylinder 10 Air-Fuel Ratio Imbalance

What it means

Cylinder 10 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 engine control module compares cylinder 10's contribution to the air-fuel correction for its bank with the other cylinders on that bank. This code sets when cylinder 10's fuel-trim contribution differs substantially from the bank average, which can result from fuel delivery, air leakage, ignition, or compression problems.

Common causes

Most likely first.

  1. Clogged or leaking cylinder 10 injector
  2. Cylinder 10 intake leak or airflow issue
  3. Cylinder 10 ignition misfire
  4. Low compression in cylinder 10

Symptoms

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

How to diagnose it

  1. Check freeze-frame data and live fuel-trim and misfire counts to see when cylinder 10's imbalance occurs and whether it is isolated.
  2. Inspect cylinder 10's injector connector, wiring, intake runner, and nearby hoses for looseness, damage, or signs of leakage.
  3. Check for an intake leak at cylinder 10 and compare its injector operation with the other cylinders using the vehicle's approved test method.
  4. Inspect cylinder 10's ignition components and check for a cylinder-specific misfire before attributing the imbalance to fuel delivery.
  5. If those checks do not identify the fault, perform a compression or leak-down test on cylinder 10 and compare results with the manufacturer's specification.

Common fixes

  • Repair a loose or damaged cylinder 10 injector connector or wiring.
  • Repair a leak at the cylinder 10 intake runner or its connected hose.
  • Clean or replace a restricted or leaking cylinder 10 injector as testing indicates.
  • Repair the cylinder 10 ignition fault or address low compression if confirmed.

Can I keep driving?

If the engine runs smoothly and the warning is steady, limit driving and arrange diagnosis soon; avoid hard acceleration. If it runs roughly, loses power, or the check-engine light flashes, stop driving when safe because continued misfire can damage the catalytic converter.

A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together

This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.