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P1231

Injector Circuit Cylinder 4 Intermittent.

  • Oldsmobile
  • Powertrain
  • Manufacturer code
  • Medium severity

How this code is built

P
SystemPowertrain
1
Defined byThe manufacturer
2
AreaFuel and air metering (injector circuit)
31
FaultThe specific fault in that area

What it means

P1231 on an Oldsmobile means the engine control module has detected an intermittent fault in the fuel injector circuit for cylinder 4. The module monitors the injector driver and circuit voltage, and it saw an open circuit, short or unstable signal that came and went rather than staying present.

Symptoms

  • Intermittent misfire or rough running, often on cylinder 4
  • Engine hesitation or stumbling under load or acceleration
  • Occasional hard starting or momentary cutting out
  • Check engine light on, sometimes going out and returning
  • Poor fuel economy or a fuel smell from unburnt fuel

Possible causes

  • Loose, corroded or damaged connector at the cylinder 4 fuel injectorlikely
  • Chafed, broken or intermittently open injector wiring between the injector and the control modulelikely
  • Faulty cylinder 4 injector with an intermittent internal coil faultpossible
  • Poor connection at the control module connector or a poor earth or power feed to the injector circuitpossible
  • Wiring harness damage from heat, vibration or rubbing against engine componentspossible
  • Fault within the control module injector driverrare

How to diagnose

  1. Read all stored and pending codes and note the freeze frame data, including engine speed, load and temperature when the fault set
  2. Visually inspect the cylinder 4 injector connector and surrounding harness for damage, corrosion, melted insulation or poor retention
  3. With the engine running, gently wiggle the injector connector and harness while watching for a change in idle quality or misfire
  4. Measure the injector coil resistance and compare it with the specification for the specific engine
  5. Check the injector supply voltage and earth or driver signal at the connector, using a suitable test light or lab scope if available
  6. Perform a continuity and short-to-earth or short-to-voltage test on the wiring between the injector and the control module, following the wiring diagram for the model
  7. Inspect the control module connector pins for spread terminals, corrosion or moisture
  8. Swap the cylinder 4 injector with another cylinder to see if the fault or misfire follows the injector

How to fix

  • Clean, repair or replace the damaged injector connector and ensure it locks securely
  • Repair or replace the faulty section of wiring harness, using proper soldered and sealed joints
  • Replace the cylinder 4 injector if its resistance or operation is out of specification
  • Repair terminals or earth and power connections affecting the injector circuit
  • Replace or repair the control module only after all wiring and injector checks have been proven good

Can I keep driving?

Medium severity

The car can usually be driven short distances, but a misfiring cylinder can lead to unburnt fuel reaching the catalytic converter and causing damage. Get it checked soon and avoid heavy loads or long journeys until it is fixed.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.

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