Code
P1231
BUICK
P — Powertrain
Injector Circuit Cylinder 4 Intermittent
Views:
UK: 26
EN: 43
RU: 29
AI status
Completed
Completed
100%
Causes
- Damaged or chafed wiring in the injector harness for cylinder 4
- Corroded, loose, bent or contaminated connector terminals at the injector or wiring harness
- Poor or missing ground(s) or low battery/connector voltage to the injector circuit
- Intermittent internal failure of the injector (coil or connector)
- Faulty PCM/ECM driver for the injector circuit
- Aftermarket alarm/immobilizer or recent repairs that disturbed the harness
Symptoms
- Check Engine Light (MIL) illuminated intermittently
- Occasional or intermittent rough idle or misfire, especially on cylinder 4
- Reduced engine power or hesitation under load
- Possible degraded fuel economy
- May not always be present; fault can be intermittent and return after driving
What to check
- Read and record freeze frame and live data (misfire counters, fuel trim, injector command)
- Retrieve stored codes and pending codes; note if P1231 is intermittent or current
- Perform visual inspection of injector 4 connector and wiring for corrosion, damage, oil or loose pins
- Wiggle test the harness and connectors while monitoring live data or DTC status to try to reproduce intermittency
- Back-probe the injector connector and check supply voltage (battery) and driver signal when commanded
- Measure injector coil resistance and compare to factory spec
Signal parameters
- Injector coil resistance (compare to factory spec; low-impedance injectors typically ~2–4 Ω, high-impedance ~12–16 Ω — verify OEM spec)
- Supply voltage at injector connector (battery voltage, ignition ON)
- Voltage at injector command pin while cranking and while commanded ON (should show a switching waveform)
- Injector pulse width and frequency (ms and duty cycle) while engine running or when commanded by scan tool
- Continuity/ohms from injector connector pin to corresponding PCM pin
- Changes in waveform or voltage during wiggle test (intermittent open/short signatures)
Diagnostic algorithm
- Verify the complaint and capture freeze-frame data and live parameters (misfire counts, scanner fuel trim, injector status).
- Clear codes, perform a road or drive cycle to try to reproduce the fault and note conditions when P1231 sets (hot, cold, vibration).
- Visually inspect the injector 4 connector and wiring harness for damage, corrosion, oil contamination or loose pins. Repair or clean as needed.
- With ignition OFF, disconnect connector and measure injector coil resistance; compare to OEM specification. If out of range, consider replacing injector.
- With a fully charged battery and ignition ON (engine OFF), back-probe the connector: verify constant supply voltage to the injector and verify the PCM driver pin shows proper voltage (open circuit when not commanded).
- Use a scan tool to command injector 4 ON/OFF and observe injector response (noid light or oscilloscope). Look for inconsistent pulses, missing pulses, or abnormal waveform shape.
- Perform wiggle/stress tests on the harness and connectors while monitoring live data or DTC status to identify intermittent wiring faults. Repair any chafed or broken wires and secure harness routing away from heat or moving parts.
- Check related engine/chassis grounds and repair any high resistance grounds. Ensure good battery/charging system condition.
- If wiring, connector and injector check OK, inspect PCM connector and pin for corrosion or damage and test continuity to the PCM. If fault appears to originate in PCM driver, consult manufacturer procedures — PCM replacement or repair may be required.
- After repairs, clear codes and perform a verification drive cycle to confirm the fault does not return.
Likely causes
- Corroded/loose injector connector at cylinder 4
- Broken or intermittent wire in the harness (pinched, chafed, flex-point)
- High resistance connection or poor ground near the injector
- Failed injector coil or internal intermittent short
Fault status
Status
Injector Circuit Cylinder 4 Intermittent — ECM detected an intermittent/open/short condition in the electrical circuit for injector on cylinder 4.
Repair difficulty: Medium
Diagnostic time: 0.5-2 hours
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