P1240 — Injector Circuit Cylinder 7 Intermittent

Causes

• Damaged or broken wiring in the injector harness for cylinder 7 (open, short to ground, short to battery)
• Corroded, loose, or contaminated injector connector or terminals
• Failed or intermittent injector (internal coil or intermittently open/shorted)
• Poor or intermittent ground or 12V supply to the injector circuit
• Faulty ECM/PCM injector driver (intermittent)
• Water intrusion, oil contamination, or mechanical damage at the connector/harness

Symptoms

• Intermittent rough idle or hesitation under load
• Intermittent misfire affecting cylinder 7 (may trigger P0307)
• Check Engine Light (MIL) on intermittently or stored as a pending code
• Reduced fuel economy and occasional loss of power
• Engine runs normally at times and then shows symptoms under certain conditions (vibration, temperature, load)

What to check

• Scan for stored, pending, and freeze-frame data; note accompanying codes (e.g., P0307) and operating conditions when fault set
• Visually inspect wiring and connectors for cylinder 7 injector: look for damaged insulation, pin corrosion, water/oil intrusion, and secure mating
• Perform a wiggle test on harness and connectors while monitoring live data or watching for code set/reaction
• Back-probe injector connector and check for constant 12V (or battery voltage) on the supply terminal and pulsed driver signal on the control terminal with engine cranking/running
• Measure injector coil resistance with an ohmmeter (compare to spec in service manual)
• Use a noid light or oscilloscope to confirm injector driver pulse pattern and amplitude during cranking and running

Signal parameters

• Injector supply: near battery voltage (typically ~12V) present at injector + terminal (varies by vehicle)
• Injector control: typically a low-side switch from ECM — control terminal idles near battery voltage and is pulled to ground when energized
• Injector coil resistance: varies by injector type — low-impedance injectors are typically a few ohms, high-impedance types are typically ~10–16 ohms; check specific service spec
• Pulse characteristic: injector is driven with timed pulses (pulse width varies with load/engine speed); use a noid light or scope to verify consistent pulses while running
• Watch for intermittent loss of supply or intermittent grounding on the control line when issue reproduces

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