P0303 Code: Cylinder 3 Misfire Detected
Cylinder 3 is misfiring. Check whether it misses at idle or under load before you blame the coil.
At a glance
Severity: Moderate
- Meaning
- Cylinder 3 Misfire Detected
- System
- Ignition & Misfire · Powertrain
- Safe to drive?
- With caution
- Typical repair cost
- $90–$900
- DIY difficulty
- Moderate
P0303 means the engine computer keeps seeing a missing power pulse from cylinder 3. The fix is usually a coil or a plug, but cylinder 3 misfires are also where fuel-side and valve-side problems tend to hide, because technicians often swap the coil, see no change and stop there. When the misfire happens tells you a lot. Misses at idle and light cruise lean toward a clogged injector, an intake leak or a tight valve. Misses only under hard acceleration lean toward ignition, because that is when the coil has to produce its highest voltage. On an inline four, cylinder 3 is a middle cylinder paired with cylinder 2 on wasted-spark systems. On GM V8s it is the middle of the driver's-side bank.
Can I keep driving? Short, gentle trips to a shop are usually fine with a steady light. Avoid towing and hard acceleration, and stop driving if the light begins to flash, because that means the catalytic converter is overheating.
How to read P0303
- P Powertrain
- 0 Generic (SAE)
- 3 Ignition / misfire
- 03 Specific fault
Symptoms
- Rough, lumpy idle that may smooth out at higher rpm (fuel or valve cause)
- Stumble only under hard acceleration or uphill (ignition cause)
- Check-engine light steady, flashing under heavy load
- Lower gas mileage and slight loss of power
- Occasional fuel smell from the exhaust
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Failing ignition coil or boot on cylinder 3 (or the shared 2/3 coil on wasted-spark engines)
- Worn or damaged cylinder 3 spark plug
- Clogged, sticking or leaking cylinder 3 fuel injector
- Valve problem on cylinder 3: tight exhaust valve clearance, burned valve or worn cam lobe
- Intake leak at the cylinder 3 runner or gasket
- Open or shorted wire to injector 3 or coil 3
- Failed injector or coil driver inside the engine computer
Diagnosis step by step
Find out when cylinder 3 misfires
Watch the live misfire counter for cylinder 3 at idle, at a steady 40 mph cruise and during a firm acceleration. Counts that climb mostly at idle point to fuel or valves. Counts that jump only under load point to the coil or plug.
Tools: OBD-II scanner
Test injector 3 electrically and by ear
Listen for a sharp, steady click with a stethoscope or long screwdriver. Measure the injector's coil resistance and compare it with the others; a difference of more than about 10% is suspicious. With the connector unplugged and the engine cranking, a noid light or multimeter should show pulses, which proves the wiring and the computer driver are working.
Tools: Multimeter, Screwdriver
Run an injector balance or cut-out test
Many scan tools can shut off one injector at a time and show the rpm drop. Cylinder 3 should drop rpm about as much as the others. A much smaller drop confirms the cylinder isn't contributing and helps you tell a fuel problem from a mechanical one.
Tools: OBD-II scanner
Swap coil 3 and read plug 3
Swap coil 3 with a neighbor and see whether the misfire follows. Then pull the plug. A dry, chalky-white tip suggests the cylinder is running lean (injector or intake leak), while a wet, sooty tip suggests no spark.
Tools: Screwdriver, Spark plug socket
Check compression and valve clearance
If ignition and fuel check out, test compression. Low compression that doesn't improve with a squirt of oil points to the valves. On engines with adjustable valves, measure the clearance on cylinder 3: an exhaust valve that has lost its clearance won't seal when hot and will eventually burn.
Tools: Compression tester
Consider injector cleaning or replacement
A borderline injector on a port-injected engine can often be restored by off-car ultrasonic cleaning and flow testing. On direct-injection engines, a leaking or clogged injector is normally replaced, and the seals and hold-down parts should be replaced with it.
Tools: Torque wrench
Where the parts are
- 1Ignition coil
- 2Spark plug
- 3Fuel injector
- 4Camshaft sensor
- 5Crankshaft sensor
- 6Engine computer (ECM)
- 7Intake manifold
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Replace cylinder 3 ignition coil | Yes | $90–$300 |
| Replace spark plugs (full set) | Yes | $120–$450 |
| Clean and flow-test injectors off the car | No | $150–$350 |
| Replace cylinder 3 fuel injector (direct injection costs more) | No | $200–$900 |
| Valve clearance adjustment | No | $200–$500 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Notes by brand
Honda
Many Honda four-cylinder engines use screw-type valve adjusters and need periodic valve clearance checks. When an exhaust valve's clearance tightens up, that cylinder misfires, mostly when hot, and new coils or plugs don't help. A valve adjustment done in time usually cures it before the valve burns.
Field data
| Ford | 135 | Escape, Edge, Fusion Energi | |
|---|---|---|---|
| Honda | 66 | ||
| Chrysler | 31 | Pacifica, 300, Town And Country | |
| Kia | 27 | Sorento, Optima, Soul | |
| Hyundai | 15 | ||
| Jeep | 12 | Grand Cherokee, Cherokee, Grand Cherokee Trackhawk | |
| Ram | 7 | 1500 Quad, 1500 | |
| Dodge | 5 | Durango, Challenger, Charger Pursuit |
Source: NHTSA Complaints API (US government open data), analyzed by DTC mention.
Emissions / smog test impact
Will fail
With P0303 stored and the check-engine light on, an OBD smog check will fail. Fix the misfire, then drive normally for several days until the readiness monitors show complete before retesting.
Frequently asked questions
I replaced coil 3 and the plug, but P0303 came back. Why?
Then the problem is most likely on the fuel or mechanical side: a weak injector, an intake leak at runner 3 or a valve that isn't sealing. An injector cut-out test and a compression test will separate those quickly.
Does it matter whether the misfire happens at idle or under load?
Very much. Ignition faults usually show up under load, when the coil needs the most voltage. Lean or mechanical faults show up at idle, when a small shortfall in fuel or compression makes the biggest difference.
Can injector cleaner in the tank fix a cylinder 3 misfire?
Occasionally, when an injector is only slightly varnished, and it's cheap enough to try. A stuck or electrically failed injector won't respond to additives, and on direct-injection engines tank cleaners do nothing for carbon on the intake valves.
Could the engine computer be the cause?
It's possible but uncommon. If the injector and its wiring test fine but the noid light shows no pulse at the computer's connector, the driver circuit inside the computer may have failed, often after a shorted injector. Replace the injector first, or the new computer can fail the same way.