Fix P0357: Mechanic’s solutions that WORKED

The OBD2 code P0357 indicates “Ignition Coil G Primary/Secondary Circuit Malfunction,” focusing specifically on cylinder number 7 in the firing order. This code suggests that there’s an electrical issue within the ignition coil circuit, preventing the coil from firing properly.

As a result, it can lead to engine misfires, poor performance, and even an inability to start the engine, as the spark needed to ignite the air-fuel mixture in cylinder 7 is compromised or absent.

P0357 quick overview

Meaning	P0357: Ignition Coil G Primary/Secondary Circuit Malfunction (Cylinder 7)
Is it serious?	Yes, it is serious as it can lead to engine misfires, stalling, and potential damage if not addressed promptly.
Possible causes	– Faulty ignition coil on cylinder #7 – Wiring issues or shorts in the coil circuit – Poor electrical connections to the coil – Control module issues, including the PCM
How to diagnose?	Inspect and test ignition coil: For cylinder #7 to verify its condition. Check wiring and connectors: For damage or shorts in the coil circuit. Swap coils: Between cylinders to see if the issue follows the coil. Use diagnostic tools: Like a labscope to analyze the coil’s electrical signal. Search for external damage: Like coolant leaks or mechanical wear that could impact the circuit. Examine the PCM and fuse block: For faults affecting the ignition circuit.

How to fix P0357 – learn from mechanics

I looked into P0357 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Ford

Vehicle information
2002 Ford Expedition Eddie Bauer 5.4L

Issue + repair history

• Vehicle starts to misfire then stalls and won’t restart
• Diagnosis revealed blown #30 fuse in central fuse block
• Cylinder #7 Coil Over Plug is blown and does not spark, despite PCM triggering
• Wiring checked for short to ground from coil connector to PCM with none found

Other OBD2 Codes + Test Results

• P0357- cylinder 7 circuit fault

Mechanic suggestions

• Check for coolant leak from heater hoses onto COP, a known issue that could cause short
• Swap coil #7 with another to see if the code follows
• Use a breakout box and labscope to check signal waveform and current ramp to #7 coil
• Check connector/wiring near number 4 coil for possible short to ground strap
• Consider a bad coil drawing too much current, potentially overheating and causing the fuse to pop
• Look around the passenger side rear of engine near AC evaporator for a shorted signal wire to coil #7
• Replace the plug alongside the coil as a preventative measure

Final fix

• Found the wiring harness was rubbing on the head of a bolt behind the intake manifold at #8 cylinder, causing a short. Repaired by retaping the harness and securing it away from the manifold.

Case #2: Lexus

Vehicle information 2003 Lexus GX470 4.7L

Issue + repair history

• Misfire on cylinder #7
• Ignition primary circuit issue
• Previous repairs:
  • Replaced coil
  • Replaced injector

Other OBD2 Codes + Test Results

• OBD2 Codes:
  • P0307 – Cylinder 7 Misfire
  • P0357 – Coil Circuit Cylinder 7
• Compression test: Cylinder 7 and 8 have 185 psi
• Checked injector pulse with noid light, seems okay
• Fuel pressure is good

Mechanic suggestions

• Perform a leak down test on cylinder 7
• Conduct a relative compression test and current ramp all coils
• Check for issues with valves, potentially bad valve seats
• Test coil output with a spark tester or test light, check for poor combustion
• Check the wiring to all ignition coils, inspect for carbon buildup or restricted fuel injectors
• Consider getting a factory scanner for more accurate diagnostics, or use a scope shot of the IGF/IGT to identify the correct cylinder

Final fix Replaced another coil as indicated by pending code #5.

Case #3: Chevrolet

Vehicle information 2003 Chevrolet Express 3500 6.0L

Issue + repair history

• MIL Lamp on
• DTC P0357 (Ignition coil G primary circuit problem)
• Previous repairs:
  • Customer replaced #7 ignition coil
  • Customer replaced ECM
  • Replaced #7 ignition coil
  • Replaced ECM after checking IC circuits

Other OBD2 Codes + Test Results

• OBD2 Codes:
  • P0357 – Ignition coil G primary circuit problem

Mechanic suggestions

• Check the alternator for bad diodes that may be feeding back through the system
• Consider using OEM coils instead of aftermarket ones
• Inspect wiring harness near the left front control arm for damage, even if it’s hidden by the wire loom

Final fix Discovered a rubbed-through wire harness near the front suspension, repaired it, and the issue was resolved.

Diagnostic Step	Brief Explanation
Inspect and test the ignition coil on cylinder 7	Confirm if the coil is operational or needs replacement.
Check wiring and connectors to the coil	Look for shorts, damage, or poor connections in the coil’s circuit.
Swap ignition coils	Determine if the problem is with the coil by swapping it with another cylinder’s coil.
Use diagnostic tools to analyze signals	A labscope can help visualize the electrical performance of the coil.
Search for external factors affecting the coil	Coolant leaks or mechanical wear could also cause circuit issues.
Examine PCM and associated fuse block	Faults here can impact the ignition circuit’s functionality.

Drawing on the insights from real service cases, these steps outline a systematic approach to diagnosing and resolving issues indicated by code P0357. Addressing the root causes, such as faulty coils, damaged wiring, or PCM issues, is crucial for eliminating the code and restoring engine performance.