P0357 Ignition Coil "G" Primary/Secondary Circuit Malfunction

P0357 is an OBD-II diagnostic trouble code indicating that the engine control module (ECM/PCM) has detected a malfunction in the primary or secondary ignition circuit of ignition coil “G.”

The code does not automatically mean that the ignition coil itself has failed. The ECM monitors the electrical behavior of the ignition coil circuit, and an abnormal condition can originate from the coil, connector, wiring, power supply, control circuit, spark plug, or, less commonly, the ECM itself.

The letter “G” identifies a particular ignition coil according to the manufacturer's ignition-system configuration. It should not automatically be interpreted as a particular cylinder number. The exact cylinder or coil position must be confirmed from the vehicle's service information.

What does P0357 actually mean?

An ignition coil has a primary side and a secondary side.

The primary side operates on the vehicle's low-voltage electrical system. The ECM controls the primary circuit so that the coil can build a magnetic field.

When the ECM switches the primary circuit off, the magnetic field collapses and produces the high voltage required to create a spark at the spark plug. This is the secondary ignition side.

P0357 is set when the ECM detects electrical behavior from Ignition Coil G that does not match the expected operating pattern.

Depending on the vehicle, this can involve an open circuit, short circuit, abnormal current flow, excessive resistance, missing control signal, or another electrical fault.

So the correct interpretation is:

There is a problem associated with the primary/secondary circuit of Ignition Coil G.

It is not simply a command to replace Coil G.

What can cause P0357?

A defective ignition coil is certainly one possibility, but there are several other causes.

Common causes include:

• Failed ignition coil G

• Damaged coil wiring

• Broken or partially broken control wire

• Short to ground

• Short to battery voltage

• Excessive resistance in the wiring

• Loose coil connector

• Corroded connector terminals

• Bent or pushed-back connector pins

• Poor terminal contact

• Missing ignition power supply

• Poor ground

• Damaged or incorrectly gapped spark plug

• Oil or water contamination around the coil

• Incorrect replacement coil

• Damaged ECM ignition driver

• Previous wiring repair performed incorrectly

An intermittent wiring fault deserves particular attention when the engine runs normally most of the time but develops a misfire during acceleration, vibration, or high engine temperature.

A wire can appear perfectly intact while being broken internally.

What symptoms can P0357 cause?

The symptoms depend on the severity of the fault.

Possible symptoms include:

• Check Engine Light

• Engine misfire

• Rough idle

• Engine shaking

• Hesitation

• Poor acceleration

• Reduced engine power

• Difficult starting

• Long cranking

• Engine stalling

• Increased fuel consumption

• Unburned-fuel smell

• Intermittent loss of power

Sometimes there may be almost no noticeable symptoms.

This can happen when the ignition coil only begins to fail under specific conditions, such as high engine load, high cylinder pressure, high temperature, or higher RPM.

Why can the engine run normally at idle?

Ignition faults do not always appear under every operating condition.

At idle and light load, the ignition system may have relatively little difficulty producing a spark. During hard acceleration, however, cylinder pressure rises substantially.

The ignition system must then generate enough voltage to overcome the resistance of the spark-plug gap under much more demanding conditions.

A weak coil, excessive spark-plug gap, damaged insulation, or poor electrical connection may therefore produce:

Normal idle → acceleration → misfire → Check Engine Light/P0357

This is why reproducing the fault under the same load and engine-speed conditions can be an important part of diagnosis.

How should P0357 be diagnosed?

The first step is to scan the vehicle for all stored and pending diagnostic codes.

P0357 should not be considered in isolation. Additional misfire, injector, system-voltage, or synchronization codes can change the diagnostic direction.

Freeze-frame data is also valuable. Engine RPM, calculated load, coolant temperature, battery voltage, and other recorded parameters can show whether the fault appeared during starting, idle, acceleration, or heavy load.

The technician should then identify exactly which physical ignition coil is designated “G” for that particular engine.

Inspect the coil and connector

The coil itself and its connector should be inspected carefully.

Look for:

• Cracked coil housing

• Burn marks

• Signs of electrical arcing

• Oil contamination

• Water intrusion

• Corrosion

• Loose terminals

• Damaged connector locks

• Broken wires

• Harness chafing

• Previous repairs

• Poorly crimped terminals

The connector may look normal externally while having poor terminal tension internally.

An intermittent fault can sometimes be reproduced by carefully performing a wiggle test on the harness while monitoring the ignition circuit or misfire data.

Check the coil power supply

The technician should verify that Coil G is receiving the correct electrical supply.

A missing power supply can make a perfectly good coil appear defective.

The relevant power circuit should be checked under the conditions specified by the manufacturer rather than relying only on a static voltage measurement.

Voltage-drop testing can be particularly useful because a circuit can show acceptable voltage with little or no load but develop a significant voltage drop when the coil is operating.

Check the ECM control circuit

The control wire between the ECM and ignition coil is another critical part of the diagnosis.

The ECM must provide the appropriate control signal to operate the coil.

Simply checking whether the wire has continuity is not always enough.

A technician may need to check:

• Control-wire continuity

• Resistance

• Short to ground

• Short to voltage

• Voltage drop

• Control waveform

• Signal behavior while the engine is running

An oscilloscope can be extremely useful because it allows the technician to see whether the ECM is actually producing the expected ignition-control waveform.

Check the spark plug

The spark plug associated with Coil G should also be inspected.

A worn plug, excessive electrode gap, cracked insulator, carbon fouling, oil contamination, or incorrect plug can increase the voltage demand placed on the ignition system.

A weak ignition coil may work with a new plug but break down with a worn plug under heavy load.

However, the reverse is also possible: a spark-plug problem may cause a misfire without the coil itself being electrically defective.

Therefore, a spark plug should not automatically be blamed simply because P0357 is present.

The coil swap test can provide a strong clue

If the coils are interchangeable, a technician can swap Coil G with another suitable coil and monitor the result.

For example, suppose the engine originally reports a misfire associated with the cylinder controlled by Coil G. If the misfire moves to the cylinder receiving Coil G after the swap, the coil becomes a strong suspect.

If the fault remains at the original location, attention should shift toward:

• Wiring

• Connector

• Power supply

• Control circuit

• Spark plug

• ECM driver

• Other cylinder-specific factors

This makes a coil swap a diagnostic test, not simply a parts-replacement strategy.

Can the ECM cause P0357?

It can, but an ECM failure is generally much less common than a coil, connector, or wiring problem.

The ECM contains the ignition driver circuitry on many ignition-system designs. A failed driver can prevent the coil from receiving the correct control signal.

Before condemning the ECM, the complete external circuit should be proven good.

The technician should verify the coil's power supply, ground, control wiring, connector condition, and control waveform first.

Replacing or cloning an ECM without proving a driver fault can result in an expensive repair that does not solve P0357.

Can a bad battery cause P0357?

Low system voltage can interfere with ignition operation, particularly during engine cranking.

However, low battery voltage is not the direct meaning of P0357.

If several unrelated low-voltage, communication, and ignition codes appear simultaneously, the battery and charging system should certainly be tested.

But if P0357 appears by itself, the Coil G circuit should remain the primary diagnostic focus.

What should be replaced?

The correct repair depends on what testing finds.

If Coil G has an internal electrical failure, the coil should be replaced.

If the coil is good but the control wire is damaged, the wiring should be repaired.

If the connector has loose or corroded terminals, the terminal or connector may need repair.

If the spark plug is worn or damaged, it should be replaced according to the manufacturer's specification.

If the ECM control driver has been conclusively proven defective, ECM repair or replacement may be necessary.

The important point is that P0357 alone does not identify which of these components has failed.

P0357 does not automatically mean “bad ignition coil”

This is the most important distinction when diagnosing this code.

P0357 tells you that the ECM has detected an abnormal condition involving Ignition Coil G's primary/secondary circuit.

It does not tell you:

“The coil is definitely defective.”

A professional diagnosis should establish whether the fault is inside the coil or somewhere in the circuit controlling it.

The logical diagnostic path is:

Identify Coil G → scan all codes → examine freeze-frame data → inspect coil and connector → verify power and ground → test control circuit → inspect spark plug → compare ignition signals → perform a coil swap where applicable → evaluate the ECM driver if everything else is proven good.

If P0357 is accompanied by a flashing Check Engine Light, severe shaking, or a major loss of power, continued driving should be avoided where possible. A severe active misfire can send unburned fuel into the catalytic converter and cause expensive secondary damage.

P0357 compared with other ignition coil codes

P0357 belongs to the ignition-coil circuit family and specifically identifies Coil G.

The letter designation is manufacturer-dependent, so the physical cylinder associated with Coil G must be verified for the particular engine.

This is important because two different manufacturers can use different coil-lettering strategies.

The same diagnostic principle applies to the neighboring codes: the code identifies the affected ignition-coil circuit, while the actual failed component still has to be established through testing.

Final diagnostic conclusion

P0357 – Ignition Coil “G” Primary/Secondary Circuit Malfunction means that the ECM has detected abnormal electrical behavior in the ignition circuit associated with Coil G.

The fault may be caused by the coil itself, but it can just as easily involve the connector, power supply, control wiring, spark plug, or ECM driver.

The best repair is therefore not the fastest part replacement. It is the repair that follows the fault through the circuit and identifies where the electrical behavior becomes abnormal.