Code
P1366
MINI
P — Powertrain
Ignition coil A, primary/secondary circuit - low
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AI status
Completed
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Causes
- Faulty ignition coil A (internal short/open or short to ground)
- Open or shorted wiring between coil A and the engine control module (ECM)
- Corroded, loose or damaged coil connector or terminal
- Faulty coil driver in the ECM/ignition control module
- Low battery or charging system voltage during diagnostics
- Poor engine ground(s) affecting coil circuit
Symptoms
- Malfunction Indicator Lamp (MIL) illuminated
- Misfire on the affected cylinder (rough idle, surging)
- Reduced engine power and acceleration
- Poor fuel economy and increased emissions
- Hard start or extended crank time
- Occasional backfiring or hesitation under load
What to check
- Read and record freeze frame and live data (cylinder misfire counters, battery voltage, fuel trims)
- Scan for related codes (misfire codes P030x, other coil circuit codes P035x)
- Visually inspect ignition coil A, harness and connector for corrosion, damage, or oil ingress
- Check battery and charging voltage at idle and cranking (should be ~12–14.5V)
- Measure coil primary and secondary resistance with manufacturer spec limits
- Check continuity and for short to ground between coil connector and ECM pin
Signal parameters
- Typical coil primary resistance (generic): ~0.5–2 Ω (manufacturer values vary)
- Typical coil secondary resistance (generic): ~5 kΩ–20 kΩ (varies by design)
- Expected primary supply voltage: near battery voltage (~12–14.5 V) when key on/run
- Driver pulse: square/controlled pulse from ECM; primary waveform should show consistent dwell and switching edges
- Fault condition: primary circuit voltage or waveform amplitude significantly lower than expected or missing pulses
Diagnostic algorithm
- Safety first: disable ignition and follow vehicle-specific procedures before removing coils. Disconnect battery if required.
- Retrieve freeze frame/live data and note battery voltage, misfire counters, and whether code is continuous or intermittent.
- Visually inspect coil A and connector for damage, corrosion, oil, or loose pins. Repair connector issues as needed.
- Measure battery/charging voltage at rest, at key-on, and during crank — confirm adequate supply voltage.
- Measure coil A primary resistance with DMM and compare to manufacturer spec. If out of range, replace the coil.
- Check secondary resistance (if serviceable) and inspect for open winding or short to core.
- Check continuity between coil A connector pins and the ECM coil driver pin; inspect for shorts to ground or other circuits.
- Swap coil A with another cylinder coil (if identical) and clear codes. If code moves to the other cylinder, coil is likely defective. If code remains on A, suspect wiring or ECM.
- Use an oscilloscope on the coil primary to verify driver pulses, dwell time and voltage. Look for low amplitude, clipping, missing pulses or excessive resistance.
- If wiring and coil check good, test or substitute the ECM/ignition driver (follow manufacturer procedures).
- Repair or replace faulty component(s), clear codes and perform a road/test drive to confirm no recurrence.
Likely causes
- Defective ignition coil A
- Damaged connector/wiring (pin corrosion, broken conductor, chafed insulation)
- Poor ground at cylinder head or coil mount
- ECM coil driver failure
- Low system voltage (weak battery/charging)
Fault status
Status
Ignition coil A, primary/secondary circuit — low voltage/signal detected. MIL on.
Repair difficulty: Medium
Diagnostic time: 1–3 hours
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