Comprehensive diagnostic guide for P0357 Ignition Coil G Primary/Secondary Circuit Malfunction

1) Code definition and scope

• What P0357 means: P0357 is an ignition coil circuit fault. Specifically, it indicates a malfunction in the primary and/or secondary circuit of ignition coil G (the coil designated as "G" for cylinder 7 in the engine's firing order). In OBD-II terms, this is a Powertrain (P0) code within the P0xxx set. The issue is detected when the PCM/controller detects an abnormal condition in the coil's primary drive or the coil's secondary spark path. Source context: OBD-II Trouble Codes are described as part of the Powertrain Codes in the Wikipedia OBD-II reference, which explains that DTCs are generated by the vehicle's control systems and that powertrain codes fall under P0xxx. The Open Source code definition notes the exact label for this code as "Mau funcionamento do primário/secundário da bobina de ignição G" (Ignition coil G primary/secondary circuit malfunction). Use this as the canonical mapping for cylinder G/Z. Sources: Wikipedia OBD-II - Diagnostic Trouble Codes; OBD-II - Powertrain Codes; Open Source code definition (G coil).
• Practical note: Since engine designs vary, the exact cylinder mapping and coil naming (G) depends on the vehicle. Refer to the vehicle's wiring diagram and service data for your specific make/model.

2) Typical symptoms you may see

• Check Engine Light (CEL) illuminated with P0357 stored.
• Misfire symptoms often associated with a coil fault: rough idle, stumble or shake, especially when the engine is under load or at idle.
• Possible reduced engine power or hesitation during acceleration.
• Potential increased or abnormal fuel consumption due to misfire conditions.
• In some cases, there may be a cylinder 7 misfire code (P0307) or multiple misfire codes if the misfire is severe or persistent.
  Note: These symptom patterns align with general OBD-II powertrain/misfire behavior described in the OBD-II reference and are consistent with coil-related DTCs mentioned in the diagnostic code family.

3) Most probable causes and approximate likelihood and common failures seen with ignition coil circuits:

• Faulty ignition coil G (internal coil failure or degraded secondary/primary insulation): ~40-50%
• Faulty wiring harness or connector to coil G (damaged insulation, corrosion, loose or corroded pins, water intrusion): ~20-30%
• PCM/ignition driver / controller issue or intermittent drive signal to coil G: ~10-15%
• Spark plug, secondary path damage, carbon tracking, or related ignition path issues affecting the same cylinder (or misfire path): ~5-10%
• Other engine problems that momentarily mimic coil faults (rare in isolation for P0357 but possible in mixed DTC scenarios): remaining percentage
  Caveat: The above percentages are qualitative field estimates since do not include NHTSA complaint frequency data for this exact code. They reflect a practical likelihood distribution many technicians observe when diagnosing P0357 in the real world.

4) Diagnostic approach and flow (step-by-step)

A structured approach helps avoid unnecessary parts replacement and ensures safe, repeatable tests.

• Step 1: Confirm the code and context
  • Use a scan tool to confirm P0357 is present and note any accompanying codes (e.g., P0307, P0356, P0358, P0351, etc.).
  • Check freeze-frame data and any misfire data (cylinder 7 misfire events) if the tool provides it.
  • Review any related live data (coil G primary current if your tool can read it; some vehicles expose coil current or spark timing data).
  • Reference: DTC classification.
• Step 2: Visual inspection
  • Inspect coil G and its wiring harness/connector for obvious damage: chafed insulation, exposed wires, corrosion at pins, bent/loose connectors.
  • Inspect the spark plug corresponding to cylinder 7 (if accessible): fouling, oil contamination, excessive gap, worn electrodes.
  • Look for signs of moisture intrusion around the coil pack or connectors (which can cause intermittent faults).
• Step 3: Verify power and ground and signal integrity
  • With ignition ON (engine OFF or as specified by the manufacturer), test coil G primary circuit:
    • Check that the 12V feed to the coil is present and stable.
    • Check that the coil ground is solid (low resistance to chassis ground and noise-free).
  • With the engine cranking, verify that the PCM is driving the coil properly (if the test tool provides ignition on/off signal for coil G).
• Step 4: Electrical measurements on the ignition coil path
  • Primary resistance check: Measure resistance of the coil primary circuit (between the power feed and ground/coil ground). Compare to manufacturer spec in the service data.
  • Secondary resistance check: Measure resistance from the coil secondary output (spark plug boot/coil secondary terminal) to the secondary side; compare to spec.
  • If primary or secondary resistances are out of spec, coil G is suspect.
• Step 5: Circuit troubleshooting and tests
  • If power/ground are solid and primary/secondary resistances are within spec, swap-based testing can help isolate the fault:
    • If feasible, swap coil G with another cylinder's coil (or swap coil packs/coil modules within the same engine design) to see if the P0357 follows the coil. If the code moves to the other cylinder, the coil is the likely fault; if the code remains on cylinder G, the wiring/PCM path may be at fault.
  • Check the harness between the PCM and coil G for continuity (no opens) and for shorts to ground or to 12V or other circuits.
• Step 6: Check for PCM/driver concerns
  • If mechanical coil tests pass but the PCM is not delivering proper drive to coil G (e.g., intermittent ignition control output), investigate wiring integrity, and consider PCM/driver integrity or software/firmware updates as per vehicle maker guidance.
• Step 7: Related components
  • Check the spark plug and seal/boot for cylinder 7; ensure no carbon tracking in the boot or spark plug area.
  • If ignition coil G and its wiring test ok but misfire continues, consider fuel delivery inconsistencies or induction-related issues that may contribute to misfire, while noting P0357 specifically relates to the coil circuit, not purely to fuel.
• Step 8: Verification after repairs
  • Clear codes and perform a road test or controlled engine test to verify the fault does not return.
  • Re-check for any new codes and confirm absence of P0357 after repair.

5) Tests and measurement details (practical guidance)

• Visual and wiring tests
  • Inspect connector pins for corrosion, corrosion resistance, and proper seating.
  • Inspect wires for insulation damage, routing, and abrasion near moving parts.
• Resistance tests (coil G)
  • Primary: measure across the primary coil terminal and ground or across the two primary terminals as specified by the manufacturer.
  • Secondary: measure from the high-voltage output (spark plug boot/coil secondary terminal) to the primary side ground or appropriate reference as per service data.
  • Compare both results against the manufacturer's specifications. If out of spec, replace the coil G assembly.
• Power/ground tests
  • With ignition ON, verify 12V supply on the coil G feed and ensure a solid ground path when the PCM commands ignition (engine cranking data, if available).
• Functional/drive tests
  • If equipped, observe cylinder-specific misfire data (P0307) and cross-check with P0357 to confirm cylinder G is consistently affected.
  • If your tool supports coil current monitoring, compare coil G current waveform during cranking and running to the expected pattern. Abnormal waveform or lack of drive indicates an electrical or PCM driver fault.

6) Repair options and vehicle-relevant actions

• Replace ignition coil G
  • If primary/secondary resistance tests are out of spec, or swap test indicates the coil follows the fault, replace the ignition coil G (coil-on-plug or coil pack as designed for your engine).
• Repair or replace wiring/connector
  • If inspection shows damaged wiring, repair or replace the harness segment and ensure proper continuity and secure, moisture-free connections.
• Spark plug replacement
  • If the cylinder 7 spark plug shows wear, fouling, or carbon tracking, replace it and re-seat the boot to ensure a solid secondary connection after coil replacement.
• PCM/driver concerns
  • If electrical tests (power/ground/drive signal) pass and coil-related tests are still inconclusive, consult service data for PCM diagnostics, possible updates, or tests to verify that the ignition driver output is being commanded correctly. In some cases, software updates or PCM replacement can be indicated by the vehicle maker.
• Post-repair checks
  • Clear DTCs, perform a thorough road test (including load and acceleration), and re-scan for any recurring P0357 or related codes.

7) Important safety notes

• Ignition systems operate at high voltage. Always follow standard shop safety procedures when working near ignition components.
• Disconnect the battery or de-energize the system as recommended before disconnecting ignition wiring or coils.
• When performing resistance tests, ensure the engine is off (or in accordance with manufacturer guidance) to avoid erroneous readings and personal injury.
• Use insulated tools and avoid shorting harnesses to ground during testing.

8) How the sources map to the guidance

• Code definition and classification: Diagnostic Trouble Codes and Powertrain Codes provide the general classification of DTCs and confirm that P0xxx codes are Powertrain-related. This supports the assertion that P0357 is a Powertrain (ignition) code.
• Specific coil/generator mapping: The Open Source code definition explicitly labels the code as a malfunction of the primary/secondary ignition coil G, aligning with the cylinder-specific ignition coil fault concept.
• Symptom and diagnostic workflow alignment: While the sources don't provide a vehicle-specific fault tree for P0357, the general DTC and ignition coil fault concepts yield the expected symptoms (misfire-like behavior, check engine light) and a practical diagnostic workflow (visual inspection, resistance/continuity checks, swap/confirmation testing, PCM-driver considerations).

7) Quick-reference checklist

• Confirm P0357 with scan tool; note any accompanying codes (P0307, P0356, P0358, etc.).
• Visually inspect coil G, its connector, and the spark plug for cylinder 7.
• Check power and ground to coil G and test primary/secondary resistance against manufacturer specs.
• If available, perform a swap test with coil G or compare ignition signals/current with another cylinder's coil.
• Inspect wiring harness for continuity and absence of shorts; repair as needed.
• If coil path tests pass but code persists, evaluate PCM/driver signals and consider software/firmware updates per vehicle maker guidelines.
• After repair, clear codes and perform a road test; re-scan to confirm P0357 does not return.

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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