P0303 Code: Cylinder 3 Misfire Detected

What the P0303 code means

Every time a cylinder fails to burn, the crankshaft briefly loses speed. The computer measures that variation precisely enough to know exactly which cylinder it happened in, and it stores the matching code.

The P0303 code corresponds to cylinder three. The advantage of a code like this is that it narrows the job: the cause is in the spark, fuel, air or compression of that cylinder, and you rule out those four fronts in that order, from cheapest to most expensive.

If a P0300 also appears, it means other cylinders are misfiring at the same time, and you should look first for a cause common to the whole engine, such as fuel pressure or vacuum leaks.

When the misfire is concentrated in a middle cylinder, like number three on an inline four, there is one cause worth ruling out early: the head gasket. The middle cylinders are the first to show a deteriorated gasket, because the clamping force of the cylinder head is lower between them than at the ends.

The clue is not in the engine but in the coolant reservoir: bubbles at idle, a level that drops with no visible leak, or coolant that looks oily. If the misfire also gets worse with a hot engine, the leak-down test stops being optional.

P0303 code symptoms

• Check engine light flashing during the misfire.
• Clear vibration at idle.
• Jerking and loss of power when you accelerate.
• Hard starting, especially when cold.
• High fuel consumption and a smell of gas at the tailpipe.
• Possible limp mode with limited rpm.
• Bubbles in the coolant reservoir at idle if the head gasket is involved.
• A misfire that gets worse with a hot engine.
• Coolant loss with no visible leak under the vehicle.

Likely causes of the P0303 code

1. Worn spark plug or one with a cracked insulator.
2. Cylinder 3 ignition coil with deteriorated insulation.
3. Dirty injector or one with an uneven flow.
4. Spark plug wire shorting to ground.
5. Vacuum leak in the runner for that cylinder.
6. Low compression from a burned valve or a damaged seat.
7. Oil collected in the spark plug tube from a leaking valve cover gasket.
8. Deteriorated head gasket.
9. Deteriorated head gasket between the middle cylinders, where the clamping force is lower.
10. Injector with a clogged internal filter from dirt in the tank.
11. Injector wiring harness with an intermittent contact from vibration.
12. Crack in the cylinder head over the affected cylinder, letting coolant into the chamber.

How to diagnose the P0303 code step by step

1. Check the freeze frame data to learn whether the misfire happens cold, hot, at idle or under load.
2. Swap the coil with another cylinder and check whether the code moves.
3. Do the same with the spark plug and look at its condition when you take it out.
4. Inspect the spark plug tube. If it is full of oil, the spark leaks away and the cause is the valve cover gasket.
5. Swap the injector if the above has been ruled out.
6. Measure compression and compare it with the other cylinders.
7. Run a leak-down test if compression turns out low.
8. Watch the coolant reservoir with the engine running. Continuous bubbles mean combustion gases are getting into the cooling system.
9. Check whether the coolant looks oily or whether the level drops with no visible leak.
10. Perform a cylinder leak-down test and listen at the oil filler opening, at the intake and at the coolant reservoir to learn where the air escapes.
11. Test the gas in the coolant reservoir with a combustion leak detector. It confirms combustion gases without removing the cylinder head.

How to fix the P0303 code

• Replace the set of spark plugs with the correct part number.
• Replace the faulty ignition coil.
• Replace the valve cover gasket if there is oil in the spark plug tubes, and clean the tubes before you reinstall anything.
• Clean or replace the affected injector.
• Repair the localized vacuum leaks.
• Take on the mechanical repair if compression confirms it.
• Replace the head gasket and resurface the head if the leak-down test confirms a path into the cooling system.
• Have the injectors ultrasonically cleaned and replace their internal filters if the fuel carries dirt.
• Secure the injector harness with zip ties to eliminate the intermittent contact from vibration.
• Replace or machine the cylinder head if the combustion leak detector confirms gases are entering the cooling system.

Technical reference data

• Detection threshold: more than 2 percent misfires over 1,000 revolutions turns the check engine light on. More than 10 percent over 200 revolutions makes it flash.
• Expected compression: 10 to 14 bar (145 to 200 psi).
• Ignition coil primary resistance: 0.5 to 2 ohms.
• Spark plug gap: 0.7 to 1.1 millimeters (0.028 to 0.043 inch).
• Expected compression pressure: 10 to 14 bar (145 to 200 psi).
• In the leak-down test, a loss above 20 percent points to a mechanical problem.
• Where the air escapes in the test: intake means an intake valve, exhaust means an exhaust valve, crankcase means piston rings, and coolant reservoir means the head gasket.
• A combustion leak detector on the coolant reservoir confirms the passage of gases without removing the cylinder head.

Brand-specific notes

Oil collected in the spark plug tubes is a very common cause of single-cylinder misfires, and it goes unnoticed unless you remove the coil to look inside the tube. It is worth checking before you replace components.

The middle cylinders of an inline engine are the first to show a tired head gasket. If the misfire is concentrated there and there is any symptom in the cooling system, put that check ahead of the electrical ones.

Frequently asked questions about P0303