P02AC

Cylinder 5 - Injector Restricted

P02AC is a generic OBD-II powertrain diagnostic trouble code: Cylinder 5 - Injector Restricted. It is logged by the engine control unit when the fuel/inj monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
P02AC
Group
Powertrain
System
Fuel/Inj
Severity
Warning (MIL on, possible limp mode)
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What P02AC means

Code P02AC is stored when the powertrain control module (PCM) detects a possible restriction in the fuel injector for cylinder 5. Using oxygen sensor data and crankshaft/camshaft position timing, the PCM identifies a persistent lean combustion event localised to cylinder 5 and attributes it to insufficient fuel delivery from that injector. This code is only applicable to engines with five or more cylinders, including inline-five (I5), inline-six (I6), V6, V8, V10, and V12 configurations. A restricted injector delivers less than the commanded fuel quantity, leaving excess oxygen in the combustion charge and elevating combustion temperatures beyond safe limits. Causes range from carbon or varnish deposits clogging the injector nozzle — especially common in engines running low-quality fuel or extended oil change intervals — to injector circuit faults that reduce the effective energising voltage or duty cycle. If related MAF or MAP sensor codes are present, these should be diagnosed first as sensor faults can mimic injector restriction by skewing fuel calculations. Early intervention prevents the lean combustion damage to pistons, valves, and cylinder head surfaces.

Common causes

Most-frequently reported root causes when P02AC is logged.

  • 1
    Clogged or varnish-fouled fuel injector nozzle on cylinder 5 reducing delivered fuel volume
  • 2
    Open or high-resistance fault in the cylinder 5 injector wiring reducing effective drive voltage
  • 3
    Faulty oxygen sensor on the affected bank providing inaccurate lean/rich feedback to the PCM
  • 4
    Failed or contaminated MAF sensor causing the PCM to under-calculate fuel requirements
  • 5
    Failed MAP sensor resulting in incorrect load-based fuel delivery calculations
  • 6
    Low fuel system pressure due to a weak pump or blocked fuel filter limiting injector flow
  • 7
    PCM driver fault causing reduced injector pulse width on cylinder 5

Symptoms drivers notice

Illuminated MIL with lean fuel trim deviation isolated to cylinder 5
Engine misfire or rough running under light load or at idle on cylinder 5
Reduced power output and throttle hesitation, particularly noticeable in higher-cylinder-count engines at highway speeds
Decreased fuel economy as fuel trim attempts system-wide compensation
Stored misfire code for cylinder 5 (e.g. P0305) accompanying P02AC
Possible engine knocking or pinging from the lean-side combustion overheat

How to diagnose P02AC

A typical diagnostic flow when this code is present.

  1. 1
    Connect an OBD-II scan tool and verify P02AC is present; check for companion codes such as P0305 (cylinder 5 misfire) or lean bank codes and record all freeze frame data
  2. 2
    Diagnose any stored MAF or MAP sensor codes first — these can mimic a cylinder-specific injector fault by globally skewing fuel calculations
  3. 3
    Perform an injector balance test to compare fuel contribution from each cylinder; a restricted cylinder 5 injector will show a measurably lower contribution
  4. 4
    Use a mechanic's stethoscope against the cylinder 5 injector with the engine running and confirm a regular, rhythmic click — absent or erratic clicking suggests injector failure
  5. 5
    Test the injector connector with a digital multimeter for proper resistance (typically 12–16 Ω) and confirm voltage and ground pulse are present during cranking
  6. 6
    Check fuel rail pressure against manufacturer specification; low pressure affects all injectors but can push a marginally restricted injector below the threshold first
  7. 7
    Remove and flow-test or ultrasonically clean the cylinder 5 injector; replace if flow rate is more than 5–10% below specification after cleaning

Related powertrain codes

Frequently asked questions

Which engines can trigger P02AC?

P02AC can only occur in engines that have a cylinder 5, meaning five or more cylinders: inline-5 (I5), inline-6 (I6), V6, V8, V10, and V12 engines. Four-cylinder engines do not have a cylinder 5 and will never generate this code.

Should I fix MAF or MAP codes before addressing P02AC?

Yes. A faulty MAF or MAP sensor affects the PCM's fuel mass calculations for all cylinders, which can drive one cylinder's trim to its limit before the others — creating a false impression of a cylinder-specific fault. Always resolve sensor codes first and retest.

Can fuel injector cleaner resolve P02AC?

For mild carbon or varnish deposits, a high-quality fuel system cleaner may reduce the restriction enough to clear the code. However, heavily clogged injectors require professional ultrasonic cleaning or outright replacement to restore adequate flow.

Is P02AC more serious on a V8 than an I5?

The underlying fault severity is comparable, but the impact on drivability differs. On a V8 or larger engine, losing effective fuelling on one cylinder represents a smaller percentage of total power, so symptoms may be subtler — potentially allowing the condition to go unnoticed longer, which increases the risk of cylinder damage.

Disabling P02AC in software

RaceTune can permanently disable P02AC — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.

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