Engine codes
P0303: cylinder 3 misfire detected
P0303 points to cylinder 3. Ignition parts remain the most frequent cause, but when swapping them changes nothing, a compression test is the step that tells you whether the engine itself is involved.
P0303: can I drive, and what will the labor cost?
Estimated labor, replace one ignition coil
$22 to $88
| Labor time (estimate) | 0.2 to 0.8 h |
| Rate used | $110/h |
| Technician wage, United States (average) | $27.13/h |
| High: limit driving, fix within days Limit driving; stop if the light flashes. | |
Labor only: parts and the diagnostic fee are not included. Hours are our estimate for a typical car; book times vary by model. Wage: BLS, May 2025.
P0303 is the generic OBD-II code for "cylinder 3 misfire detected". The computer has seen cylinder 3 fail to fire often enough to store a fault. The order of suspects is the same as for any single cylinder: the ignition coil, the spark plug, the fuel injector, then mechanical faults. This page focuses on the last step, because it decides how big the repair is. A compression test measures the pressure each cylinder builds when the engine is cranked; a leak-down test pumps air into a cylinder at top dead center and listens for where it escapes. Air hissing from the exhaust points to an exhaust valve, from the intake to an intake valve, from the oil filler to the rings, and bubbles in the coolant to the head gasket. Each takes about one and two hours of labor. It is worth paying for before replacing an injector when ignition swaps have not moved the misfire.
Coil swap test: does the misfire follow the coil?
Result
Coil
| What it means: The misfire moved with the coil: replace that ignition coil. |
Swap only with the engine off and the coil connectors clipped back. Read the per-cylinder misfire counters, or wait for the new P030x code, before concluding.
Codes, rules and labor data checked by Radif Partners · Editorial policy · Method and sources
P0303 at a glance
| Code | P0303 |
| Standard description | Cylinder 3 Misfire Detected |
| System | Ignition and misfire |
| Severity | High: limit driving, fix within days |
| Common symptoms | rough idle, loss of power |
Reading a compression test
What matters most is the spread between cylinders, not the number itself, which varies with engine design, altitude and the gauge. A cylinder well below the others, with the rest close together, is the classic sign of a local mechanical fault. If adding a little oil to the low cylinder raises its reading, the rings are leaking; if it does not, a valve or the head gasket is. A shop follows up with the leak-down test to say which.
What a burned valve feels like
A valve that no longer seals gives a steady misfire at idle that does not change much with load, and often a rhythmic puff at the tailpipe. It can follow long periods of lean running, or carbon on the valve seat. It is not common on a well-maintained engine, which is why it comes after the ignition tests.
Before going that far
Run the coil swap test on this page; move the plug too. If the misfire follows either part, the problem is solved for the cost of that part. If P0300 or a neighbor's code such as P0302 or P0304 appears as well, a shared cause is more likely than a mechanical one, and the misfire counters on a scanner show how the problem is spread.
Likely causes of P0303, in order
| Cause | How often |
|---|---|
| Ignition coil on cylinder 3 | most frequent |
| Spark plug | most frequent |
| Injector on cylinder 3 | frequent |
| Valve or head gasket problem | less frequent |
P0303 repairs and labor time
| Repair | Labor (estimate) | At $110/h |
|---|---|---|
| Replace one ignition coil | 0.2 to 0.8 h | $22 to $88 |
| Replace spark plugs (inline 4 cylinder) | 0.5 to 1.5 h | $55 to $165 |
| Compression and leak-down test | 1 to 2 h | $110 to $220 |