OBD-II trouble code
P0357: Ignition Coil 'G' Primary/Secondary Circuit
The seventh coil in the manufacturer's mapping. Seven outputs means a V8, and V8s spend their lives towing, hauling and idling — so this is the letter where a coil that passes every bench test still fails on the road.
Quick facts
- System
- Powertrain
- Category
- Ignition
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $55 – $600
- DIY difficulty
- Intermediate DIY
What does P0357 mean?
A module needs seven separately driven ignition outputs to name a seventh coil, which in practice means a V8. That is not a trivial observation, because the population of V8-powered vehicles is skewed hard toward pickups, full-size SUVs, vans and heavy-duty applications — vehicles that spend a large fraction of their lives doing something a compact car never does. They tow. They carry load. They idle for long periods with the air conditioning working hard. And that operating pattern is what makes this code behave differently from the same fault on a small engine.
Here is the mechanism, and it is the reason so many of these faults look like nothing is wrong. A coil is a transformer, and the voltage it must produce to jump the plug gap is not a fixed number. It rises with cylinder pressure, because a denser mixture is harder to ionise. At idle, cylinder pressure is low and a tired coil can still make enough voltage to fire cleanly. Put the engine under sustained high load — a loaded trailer on a long grade, in top gear, at low engine speed — and cylinder pressure climbs sharply while the time available to charge the coil between sparks stays limited. A coil with degraded insulation will hold up in the first condition and break down internally in the second, arcing to ground somewhere inside itself instead of at the plug.
That is why cold resistance testing is such a poor screening tool here. Primary and secondary resistance measured on a cold coil at the bench tests the copper, not the insulation, and insulation failure is the dominant end-of-life mode. A coil that reads perfectly in specification can still be the one that set this code. The measurement that matters is made under the conditions the fault appears in: hot engine, real load, ideally with the fault reproduced.
That makes freeze-frame data more valuable on this code than on almost any other in the family. When the module stores the fault, it captures a snapshot — engine speed, calculated load, coolant temperature, vehicle speed, intake air temperature. Read it. A fault that set at 2,000 rpm, 85 percent load, fully warmed up, at highway speed is telling you it happened while the truck was pulling something, and that tells you to test hot and loaded rather than at idle in a workshop bay. A fault that set at idle in a cold engine is a different story entirely and points more toward a connector or a broken conductor than toward insulation breakdown.
There is a second consequence of the towing pattern. Under sustained load an engine bay runs hotter for longer than it ever does in normal driving, because the cooling system is working at its limit and there is no cold-air recovery period. Heat is what ages coil potting compound, and heat that never lets up ages it faster. A V8 that has spent five years towing has effectively older coils than an identical engine that has done the same mileage commuting, and treating the mileage figure as the whole story understates where those coils are in their life.
Common causes
- Insulation breakdown inside a coil that still passes a cold resistance test but arcs internally under high cylinder pressure
- Accelerated heat aging of the coil on an engine used regularly for towing or sustained heavy load
- Worn spark plug with an opened gap, which raises required voltage most severely exactly when the engine is loaded
- Corroded or heat-damaged coil connector on a V8 with a densely packed engine bay and little airflow
- Chafed ignition harness where it runs across the engine valley or near an exhaust manifold heat shield
- Coil boot degraded by prolonged high underhood temperature, allowing the spark to track to the head instead of the plug
- Failed coil driver inside the engine control module
- Marginal charging system voltage, which reduces the current available to charge the coil between firings
Symptoms
- Check engine light on, often after a towing or heavy-load trip rather than during normal driving
- Misfire that appears only under load and disappears at light throttle
- Loss of power on grades, sometimes accompanied by the transmission hunting for a gear
- Engine that idles and drives normally in the workshop but faults on the road
- Flashing check engine light under sustained pull, clearing once the load is removed
- Reduced fuel economy and a fuel smell from the exhaust after hard work
Diagnostic steps
- 1.Read the freeze-frame data before anything else. Engine speed, calculated load, coolant temperature and vehicle speed at the moment the code set tell you whether this is a load-related fault or an idle-and-cold one, and those lead to different tests.
- 2.Ask how the vehicle is used. Regular towing or hauling changes the expected coil condition at a given mileage and makes insulation breakdown the leading hypothesis.
- 3.Do not clear the code on the strength of a cold bench resistance test. Resistance measures the windings, not the insulation, and insulation failure is what this pattern usually is.
- 4.Reproduce the fault under load — a road test with the trailer attached, or a loaded dynamometer pull — with live data recording, rather than trying to diagnose it at idle.
- 5.Check battery and charging voltage under load. Low system voltage limits how much current the coil can store between firings and produces load-related misfires with healthy coils.
- 6.Inspect the coil boot and the top of the plug for evidence of tracking — a grey or carbonised trail down the ceramic means the spark has been finding an easier path than the gap.
- 7.Check the spark plug gap against specification. An opened gap costs the most voltage exactly when cylinder pressure is highest, which is why worn plugs surface as load-only misfires.
- 8.Inspect harness routing across the valley and near exhaust heat shields, where a V8's packaging puts wiring closest to sustained heat.
Repair cost
$55 – $600
A V8 coil is typically $55 to $220 in parts. Labour on most pushrod V8s is modest because the coils sit on top of the valve covers with clear access — often half an hour. Overhead-cam V8s and engines with the coils under a cover or beneath the intake run higher. If the diagnosis requires a loaded road test or dynamometer time to reproduce the fault, expect an extra hour of diagnostic labour, and that is money well spent compared with replacing parts on guesswork. A full set of eight coils is commonly $300 to $900.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with ignition coil replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.