Diagnostic Trouble Code P21A0 Definition, Possible Causes, & Diagnosis

Diagnostic Trouble Code P21A0 Definition

The diagnostic trouble code (DTC) P21A0 indicates an ongoing air/fuel imbalance in cylinder 5. The engine control module (ECM) monitors feedback from the air/fuel ratio sensor while the engine is running and in combination with information from the crankshaft position sensor is able to detect the average air fuel ratio of each cylinder in the exhaust stream. If the ECM detects that cylinder 5 is consistently rich or lean compared to the other (bank 1 in a V engine) cylinders, it will set P21A0 and illuminate the check engine light.

P21A0 Possible Causes

Before determining possible causes of P21A0, we need to review some additional information and confirm that P21A0 is the priority DTC. First, we need to check for any additional engine DTC's related to a rich or lean condition, engine misfires, and air/fuel ratio sensor performance. Any such DTC's will typically be prioritized over P21A0 and should be diagnosed first. The air/fuel ratio sensor is not typically a top possible cause of P21A0 as the DTC sets due to abnormal readings only at certain crankshaft positions.

Next, we want to review freeze frame data to see the short term fuel trim at the time the DTC set and check for a lean or rich condition on one or both banks. A lean condition often points to an intake manifold leak, exhaust manifold leak, or fuel injector issue affecting cylinder 5 in the case of P21A0 while a rich condition in the context of P21A0 could be due to an intake or exhaust restriction, or a fuel injector issue. P21A0 with no overall rich or lean condition may point to a mechanical compression imbalance between cylinder 5 and the other cylinders (on bank 1 in a V engine).

Here are some common possible causes of P21A0 to consider:

• Cylinder 5 fuel injector

• Cylinder 5 spark plug

• Engine mechanical (compression imbalance between cylinder 5 and the other bank 1 cylinders)

• Intake manifold leak affecting cylinder 5

• Exhaust manifold leak affecting cylinder 5

• Intake or exhaust restriction

• Exhaust leak near the air/fuel ratio sensor

P21A0 Diagnosis

Here are some general steps to eliminate possible causes and aid in the diagnosis of P21A0:

Test the intake and exhaust systems for leakage: Particularly at the intake and exhaust manifolds, near the air/fuel ratio sensor, and anywhere else that the leak may affect only cylinder 5.

Inspect the condition of the spark plugs: A worn, damaged, or incorrect cylinder 5 spark plug can cause incomplete combustion and potentially lead to P21A0.

Inspect the bank 2 catalytic converter: If a rich condition is present, this can be caused by a restriction in the catalytic converter. Inspection can be done by removing the air/fuel ratio and oxygen sensors and using a borescope to inspect the catalyst for deterioration or damage.

Inspect the intake system for a restriction: Dirty air filters, damaged intake components, or debris in the system can create a restriction and potentially cause P21A0.

Perform a compression test: Focus on the variance in readings between cylinder 5 and the other bank 1 cylinders. Service information will typically provide a compression variance spec between cylinders that should not be exceeded.

Test the response and range of the air/fuel ratio sensor: Use a suitable scan tool to command the fuel trim up and down. Confirm that the air/fuel ratio sensor reading responds as expected and through its full voltage output range.

Replace the fuel injectors: If all other possible causes have been eliminated, it is most likely that the cylinder 5 fuel injector is causing the imbalance.