Code
P1240
BUICK
P — Powertrain
Injector Circuit Cyllinder 7 Intermittent
AI status
Completed
Completed
100%
Causes
- Corroded, loose or damaged wiring or connector at injector 7
- Faulty fuel injector (internal short or intermittent open)
- Poor ground or supply voltage to the injector circuit
- Faulty PCM/BCM injector driver (intermittent)
- Intermittent connector contact due to heat/vibration or moisture
- Aftermarket or incorrect replacement harness/connectors
Symptoms
- Check Engine Light (MIL) may be on or flash intermittently
- Intermittent rough idle or hesitation under load
- Occasional single-cylinder misfire (possible P0307)
- Reduced fuel economy or stumble during acceleration
- Fault may clear between starts or after movement/wiggling of harness
What to check
- Read and record freeze frame and DTC history; note if P1240 is current or intermittent
- Visually inspect injector 7 connector, wiring harness and nearby grounds for damage, corrosion, pinched areas
- Back-probe injector connector and monitor injector command while cranking and running
- Measure injector resistance and compare to specification for that engine/injector
- Perform wiggle test on harness and connector while monitoring DTCs and injector signal
- Swap injector 7 with another cylinder (if practical) to see if fault follows injector
Signal parameters
- Injector command: PWM/ground-side switching pulse from PCM; pulse width varies with load — typically 1–10+ ms during running
- Expected supply voltage at injector positive terminal: ~12 V (battery voltage) with ignition ON
- Injector resistance: check service manual; common ranges: low-impedance ~2–3 Ω or high-impedance ~12–16 Ω — verify correct spec for engine
- During injector pulse the low-side driver should pull the circuit to near 0 V (ground); open/short conditions will show abnormal voltages
- Noisy, intermittent or missing pulses on oscilloscope indicate wiring, connector or driver issues
Diagnostic algorithm
- Retrieve DTC(s) and freeze frame. Confirm P1240 is stored and note operating conditions.
- Visually inspect harness and connector at injector 7 for corrosion, bent pins, water intrusion or mechanical damage.
- With ignition OFF, disconnect connector and measure injector coil resistance. Compare to manufacturer's spec. Replace if out of range.
- Reconnect, back-probe positive and negative injector terminals. With ignition ON/engine cranking or running, verify 12V supply and PCM command pulses using a multimeter or noid light. Use an oscilloscope for waveform analysis if available.
- Perform wiggle test: move wiring and connector while monitoring the injector signal and DTC status to reproduce intermittent fault.
- Check ground integrity for engine harness/PCM grounds and repair any high-resistance grounds.
- If injector and wiring check OK, swap injector 7 with another cylinder. Clear codes and run; if code follows injector, replace injector. If not, problem is wiring/driver.
- If wiring suspected, inspect along harness run, repair damaged wires, clean/replace corroded connectors, and re-secure routing to prevent chafe.
- If wiring and injector are good but intermittent persists, test PCM/driver output. Replace PCM only after confirming driver failure and following OEM procedures.
- After repairs, clear codes and perform test drive to verify no recurrence and monitor for related misfire or fuel trim issues.
Likely causes
- Corrosion or bent/loose terminal at the cylinder 7 injector connector
- Broken/shorted wiring between PCM and injector 7 (chafing, pinched)
- Injector 7 developing intermittent internal fault
- Poor ground at engine harness/grounds affecting injector return path
Fault status
Status
MIL (Check Engine) may be illuminated. P1240 stored as intermittent — fault may not be present continuously. Freeze frame data and live data can help reproduce conditions.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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