P02A4

Cylinder 3 - Injector Restricted

P02A4 is a generic OBD-II powertrain diagnostic trouble code: Cylinder 3 - 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
P02A4
Group
Powertrain
System
Fuel/Inj
Severity
Warning (MIL on, possible limp mode)
Need P02A4 disabled?
RaceTune permanently disables any OBD-II trouble code on supported ECUs — for motorsport, off-road, and export use.
→

What P02A4 means

P02A4 is a generic OBD-II powertrain diagnostic trouble code indicating that the powertrain control module has detected a possible restriction in the fuel injector serving cylinder 3. Unlike P02A3, which flags a rich over-fuelling condition, P02A4 represents a lean condition — cylinder 3 is not receiving enough fuel. The PCM determines this by monitoring crankshaft acceleration variations between cylinders and correlating them with post-combustion oxygen sensor data. When cylinder 3 consistently produces less power contribution than the other cylinders and the PCM must apply the maximum positive fuel trim correction to compensate, or when the injector's measured response during a balance test falls short of expected flow, P02A4 is stored. The most common cause is physical restriction of the injector nozzle due to fuel deposit build-up — particularly in vehicles using lower-quality fuel or those with long oil change intervals that allow combustion deposits to accumulate around the injector tip. Electrical faults in the injector circuit, including high resistance connectors or a partially failing PCM injector driver, can also reduce pulse-width response. Because a lean cylinder runs hotter than stoichiometry, sustained P02A4 risks elevated combustion temperatures that can damage piston crowns, exhaust valves, and the catalytic converter.

Common causes

Most-frequently reported root causes when P02A4 is logged.

  • 1
    Fuel deposit build-up partially blocking the injector nozzle orifice in cylinder 3, reducing flow rate
  • 2
    Contaminated or degraded fuel causing varnish accumulation on the injector needle and seat
  • 3
    High-resistance or corroded electrical connector at the cylinder 3 injector reducing drive current
  • 4
    Open or partially broken injector wiring reducing the effective injector on-time below commanded duration
  • 5
    Faulty oxygen sensor(s) reporting a falsely rich condition, causing the PCM to retard fuel delivery unnecessarily
  • 6
    PCM injector driver channel degradation resulting in a weaker electrical pulse to the cylinder 3 injector
  • 7
    Low fuel pressure from a weak fuel pump or clogged fuel filter limiting maximum injector throughput
  • 8
    Mass air flow or MAP sensor fault causing the PCM to miscalculate the required injector pulse-width

Symptoms drivers notice

MIL illuminated with P02A4 stored, often accompanied by cylinder 3 misfire code P0303
Rough idle and engine vibration as cylinder 3 contributes less power than the remaining cylinders
Reduced overall engine power and hesitation under acceleration due to a lean-burning cylinder
Decreased fuel efficiency as the PCM compensates by increasing fuel trim across other cylinders
Lean exhaust codes or oxygen sensor activity codes stored alongside P02A4
Possible engine pinging or knock under load if the lean condition promotes abnormal combustion

How to diagnose P02A4

A typical diagnostic flow when this code is present.

  1. 1
    Retrieve all stored codes and freeze frame data with a scan tool; note if a cylinder 3 misfire code accompanies P02A4
  2. 2
    Perform a fuel injector balance test via the scan tool to compare cylinder 3 injector drop against the other cylinders — a restricted injector will show a smaller pressure drop
  3. 3
    Use an automotive stethoscope to listen for the regular clicking sound of the cylinder 3 injector while the engine runs; absent or weak clicking indicates electrical or mechanical injector failure
  4. 4
    Inspect the cylinder 3 injector electrical connector and wiring harness for corrosion, pushed-back pins, or damaged insulation causing high resistance
  5. 5
    Measure injector resistance with a DVOM and compare to manufacturer specifications; out-of-spec resistance indicates an internally faulty injector
  6. 6
    Check fuel pressure at the rail and compare to specification; low pressure compounds restriction issues and must be corrected first
  7. 7
    Remove and have the cylinder 3 injector ultrasonically cleaned and flow-tested if all electrical checks pass and restriction is confirmed

Related powertrain codes

Frequently asked questions

Can injector cleaning fix P02A4?

If the cause is fuel varnish or carbon deposit build-up on the injector nozzle, professional ultrasonic cleaning or fuel system cleaning treatments can restore flow. However, if the injector needle or seat is mechanically worn or the electrical coil is degraded, cleaning will not resolve the fault and replacement is necessary.

Why does a restricted injector trigger a lean code rather than a low-flow code?

A restricted injector delivers less fuel than commanded, creating a lean air-fuel mixture in that cylinder. The PCM detects this through oxygen sensor feedback and crankshaft acceleration analysis, and stores P02A4 to specifically indicate that the restriction is in the injector hardware rather than a general fuel delivery issue.

Is P02A4 dangerous to ignore?

Yes. A persistently lean cylinder runs at higher combustion temperatures, which can damage exhaust valves, piston crowns, and the catalytic converter over time. The associated misfire also accelerates wear and may cause secondary codes.

Could a bad oxygen sensor cause P02A4?

A faulty upstream oxygen sensor that reports a falsely rich signal can cause the PCM to reduce fuel trim below actual requirements, mimicking a restricted-injector symptom profile. Always verify oxygen sensor operation before condemning the injector.

Disabling P02A4 in software

RaceTune can permanently disable P02A4 — 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.

Got P02A4 in your scan?

Upload your ECU file — we'll identify the exact software version and confirm whether a disable is available for your car.

Upload your file