A failing coil can cause trouble across thousands of miles with no obvious alarm before it dies completely. Besides poorer fuel economy and weaker power there are further symptoms to look out for, and ways of confirming a coil is responsible.
How coil-on-plug ignition works
The standard layout for a couple of decades now has been coil-on-plug, with a separate coil sitting on every spark plug. Every European engine uses it, as do most others. Spark timing is adjusted very finely by a computer, and that precision allows makers to run higher compression ratios and more aggressive ignition timing, particularly on turbocharged engines.
How long coils last
Even so, a coil is a part that wears out. Average life is 60,000 to 80,000 miles, varying with how the engine is modified, its condition and how it is driven. A coil that lasts past 100,000 miles, or gives out at only 30,000, is possible but rare. The likeliest outcome is one coil failing inside the typical range and the whole set being replaced.
Some models have a reputation for coil trouble. Those mentioned include the Porsche 996/986 and older VWs and BMWs. Several parts makers sell their own coils, and certain aftermarket coils are designed to give a stronger spark for engines with other modifications. Installing one is no different from fitting an OE coil, so a coil replacement is a natural time to make that upgrade. You can browse NGK ignition parts to compare makers. Bosch ignition parts are another range. Delphi ignition parts are a third.
Signs of a failing coil
The main symptom of a failing coil is an engine misfire, and it is hard to catch early. A misfire means a cylinder fails to fire during its compression stroke because the spark is weak or absent. On a weakening coil it can come and go, and it often sets no code. It will not improve; eventually the coil gives up completely and a code is set.
Other signs include long cranking, poorer fuel economy, and weaker power when misfires hit under load. Long cranking often comes from the electrical system rather than the coil, so check battery voltage to see whether the car simply lacks power. A fuel system fault can produce the same poor economy and weak power, though it usually sets a fault code just as quickly or sooner.
Reading misfire codes
P0300 is the general misfire code. The computer should usually pinpoint the cylinder in the final digit, so P0304 means a misfire on cylinder four.
Symptoms indicate the general area, not always the precise cause. The coil rides on its spark plug inside the cylinder head, alongside the direct fuel injector.
Diagnosing a coil, step by step
Four checks, done in this order, can confirm whether a coil is bad.
- Read the engine computer. If the check-engine light is steady, connect an OBD-II scanner and retrieve the code. You should see at least P0300, and with luck a second code naming the cylinder involved. Entry-level OBD-II scanners cost little and are a basic tool for modern cars, but cars from roughly the last decade have advanced functions, and reading those needs a more capable, more expensive tool.
- Inspect the coils. Reaching the coils is easier or harder depending on how the cylinders are arranged and on the plastic covers. On inline and V engines the coils should be reachable from above, while horizontally opposed engines can be tougher, sometimes needing access from underneath. Unplug the suspected coil, take it out and look it over.
- Swap coils. Remove the suspect coil along with a coil you know is good. Put the known-good coil on the cylinder that was misfiring and move the suspect coil to one that was running normally. Run the engine. Should the misfire follow the suspect coil, that coil is the fault.
- Use a multimeter. This step needs one. Take the coil out, then turn the meter to its lowest ohms range and test the primary and secondary sides as shown in the table below.
| Side | Probe placement | Generally you want |
|---|---|---|
| Primary | The connector's positive and ground pins | 0.3 to 1.0 ohm |
| Secondary | Ground probe stays on the ground pin; the other probe goes to the coil's lower end, the part that meets the spark plug | 8.0k to 15.0k ohms |
Each coil has its own exact range. A reading outside specification on either side points to an internal fault, so the coil should be replaced.
Visual damage
A crack of any kind is grounds for immediate replacement, particularly when metal is visible through the crack. Cracks are fairly common low on the boot and stay hidden until the coil is out. Burnt connectors or wires also happen; they are a serious matter but rarer, and they usually need another problem as well, for instance a poor ground that adds resistance.
Replacing coils
Once you have found one bad coil, replace them all: when one has gone, the rest are probably close behind. New spark plugs are commonly fitted at the same time as new coils.
Key takeaways
- A coil is a wear item with an average life of 60,000 to 80,000 miles.
- A misfire is the main symptom, and it may not set a code at first.
- P0300 is the general misfire code; the last digit of a cylinder code names the cylinder.
- Check codes, inspect, swap, then measure with a multimeter.
- Generally expect 0.3 to 1.0 ohm on the primary side and 8.0k to 15.0k ohms on the secondary side.
- If one coil has failed, replace the whole set.
General information only. Always check the specification for your own vehicle.