Code
P1340
CITROEN
P — Powertrain
Cylinder 4 ignition fail malfunction
Views:
UK: 3
EN: 5
RU: 7
AI status
Completed
Completed
100%
Causes
- Faulty ignition coil on cylinder 4
- Failed or fouled spark plug on cylinder 4
- Damaged or corroded coil/plug wiring or connector
- Fuel injector fault (stuck, clogged or electrically open) for cylinder 4
- Low compression or mechanical issue on cylinder 4 (valve, piston, head gasket)
- Poor engine or ECU ground, blown fuse or power supply fault to ignition circuit
Symptoms
- Engine misfire or rough idle, often felt as vibration
- Check Engine Light (MIL) illuminated
- Possible loss of power and reduced fuel economy
- Intermittent or constant misfire related to engine speed/load
- Black soot on tailpipe if long-term misfire
What to check
- Read and record freeze frame data and stored DTCs with a good scan tool
- Confirm whether P1340 is current or historic; look for related codes (misfire, coil/injector circuits, cam/crank)
- Visual inspection of spark plug, ignition coil and their connectors on cylinder 4 for damage, carbon tracking or oil contamination
- Check for loose, corroded or damaged wiring and connectors for ignition and injector circuits (pins, pins pushed out, water ingress)
- Check battery voltage and engine ground integrity (during cranking and idle)
- Perform a basic cylinder balance or cylinder contribution test if available on scan tool
Signal parameters
- Misfire counter/trim for cylinder 4 from ECM (counts or misfire rate)
- Ignition coil supply voltage: should be close to battery voltage with key ON/engine running
- Coil primary waveform (oscilloscope): clean switching pulse and dwell consistent with other cylinders
- Injector pulse width and frequency from ECU (ms) during same operating conditions as other cylinders
- Crank and cam position sensor signals: stable timing and correct phase relationship compared to other cylinders
- Relative compression or leak-down result for cylinder 4 compared with others (compression should be within manufacturer tolerance)
Diagnostic algorithm
- Connect scan tool, record freeze frame and live data (misfire counters, injector pulse, ignition dwell) and note related DTCs. Clear codes, run engine and see if P1340 returns.
- Visually inspect coil, spark plug and wiring on cylinder 4. Replace or reseat plug/coil if visibly damaged or contaminated. Re-scan.
- Swap the ignition coil (or coil-on-plug assembly) from cylinder 4 with another cylinder. If the fault moves to the other cylinder, suspect the coil or its connector/wiring. If code remains on cylinder 4, suspect plug, injector, compression or wiring to ECM.
- Check spark plug condition and gap; replace plug if fouled, oil-wet, or outside spec. Re-test for misfire.
- Using a noid light or oscilloscope, verify injector drive signal and continuity to the injector on cylinder 4. Swap injectors with another cylinder to see if the fault follows the injector.
- Perform a relative compression or wet/dry compression test on cylinder 4 and compare to other cylinders to exclude mechanical causes.
- Inspect and test wiring continuity and resistance between the coil/injector connector and the ECU, and verify good grounds and fused power to ignition coils or injector drivers.
- Capture coil primary and secondary waveforms with an oscilloscope while comparing to a known-good cylinder to identify irregular dwell, weak secondary energy, or arcing.
- If wiring, components and compression are OK, check ECU wiring, fuses, relays and perform ECU software update/reflash if available. Consider ECU replacement only after excluding external faults.
- After repairs, clear codes and perform a road/drive cycle to verify the fault does not return.
Likely causes
- Faulty ignition coil or spark plug on cylinder 4
- Connector/wiring fault to coil or injector on cylinder 4
- Fuel injector electrical or mechanical failure on cylinder 4
- Low compression or mechanical problem in cylinder 4
Fault status
Status
Engine control module has recorded an ignition failure for cylinder 4. Investigate ignition coil, spark plug, injector, wiring, power/ground and mechanical condition of cylinder 4. Clear codes after repairs and confirm by test drive.
Repair difficulty: Medium
Diagnostic time: 1-3 hours
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