Code
P1231
CADILLAC
P — Powertrain
Injector Circuit Cylinder 4 Intermittent
Views:
UK: 26
EN: 44
RU: 33
AI status
Completed
Completed
100%
Causes
- Damaged, frayed, or chafed wiring in injector harness to cylinder 4
- Corroded/loose/contaminated injector connector or terminal
- Faulty fuel injector (internal intermittent short/open) on cylinder 4
- Poor or intermittent ground at injector or harness ground strap
- Intermittent short to battery or to ground at connector or harness
- Faulty or marginal ECM injector driver for cylinder 4
Symptoms
- Check Engine Light (MIL) illuminated, usually intermittent
- Misfire on cylinder 4 — rough idle or stumble
- Intermittent loss of power or hesitation under load
- Reduced fuel economy and increased emissions
- Possible fuel smell from cylinder area (if injector leaking)
- Faults may come and go, sometimes clearing after engine off/on
What to check
- Read and record all stored codes and freeze-frame data with a scan tool
- Check for related codes: injector circuit codes, misfire codes (P0304), low/high fuel pressure codes
- Visually inspect wiring and connectors at cylinder 4 injector for damage, corrosion, and secure fit
- Wiggle harness and connector with engine running or while monitoring live data to try to reproduce the fault (use caution)
- Back-probe injector connector to check for battery voltage at key-on and injector drive signal while cranking/idle
- Measure injector resistance with the connector disconnected — compare to spec and other injectors
Signal parameters
- Injector coil resistance: typically ~10–20 Ω (varies by application) — compare to factory spec and other injectors
- Injector drive: low-side switching (ECM grounds injector) — waveform should switch between ~0 V (ground) and battery voltage when energized
- Injector pulse width: idle typically ~2–4 ms (varies by engine load/conditions) — use scan tool to view commanded pulse widths
- No-load battery voltage should be stable near battery voltage (12–14 V) during tests
- Intermittent faults may show open/short spikes or missing pulses on a lab scope
Diagnostic algorithm
- Retrieve DTCs and freeze-frame with a scan tool; record vehicle history and any recent repairs.
- Verify P1231 is current or history. Clear codes and perform a monitored drive until failure recurs if possible.
- Visually inspect cylinder 4 injector connector, wiring, and nearby harness for damage, heat, or rubbing points. Repair obvious issues.
- With ignition OFF, disconnect the injector connector and measure injector resistance. Compare to spec and to other injectors. Replace if out of range or open intermittently.
- Check for good battery voltage at the connector with ignition ON (power feed). Back-probe the injector connector and start engine; monitor the driver side for switching to ground using a DVOM or oscilloscope.
- Perform a wiggle test of the wiring and connector while monitoring live data or watching for the fault to set. Look for intermittent changes in voltage/pulse or for the MIL to illuminate.
- Check continuity between the injector connector pin and the ECM injector driver pin (ignition OFF). Repair any high resistance or intermittent continuity.
- If wiring and connector good, swap the suspect injector with another cylinder of same type and clear codes. If fault follows injector, replace the injector. If fault remains on cylinder 4, focus on wiring/ECM driver.
- Use an oscilloscope to capture the injector driver waveform to identify short spikes, missing pulses, or driver clipping indicative of ECM driver problems.
- If wiring is good and injector replacement does not resolve the intermittent fault, consider ECM/driver replacement or reprogramming as a last step after verifying all wiring and grounds.
- After repairs, clear codes and confirm via road test that the fault does not return; re-scan for pending codes.
Likely causes
- Open or intermittent connection at the cylinder 4 injector connector
- Broken or intermittent wiring between injector and ECM (pinched, chafed, or broken strand)
- Corroded or pushed-out terminal in the connector causing intermittent contact
- Failed or intermittent injector internal electronics
Fault status
Status
P1231 - Injector Circuit Cylinder 4 Intermittent: ECM detected an intermittent fault on the injector circuit for cylinder 4.
Repair difficulty: Medium
Diagnostic time: 0.5-3.0 hours
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