Ford Check Engine Code
P0303 on Ford
Cylinder 3 Misfire Detected
Also covers: Lincoln, Mercury
P0303 on Ford: what makes it different
P0303 (Cylinder 3 Misfire) is commonly reported on Ford V8 Triton engines and EcoBoost V6s, where ignition components and spark plugs on that specific cylinder are known trouble spots.
Most-affected engines
- 4.6L/5.4L Triton V8 (F-150, Expedition, Crown Victoria)
- 3.5L EcoBoost V6 (F-150, Edge, Explorer)
Common model years: 2004–2014
Most likely cause on Ford
Failing ignition coil or worn spark plug on cylinder 3, sometimes compounded by loose spark plug threads on Triton V8s
Known Ford engine-family issues
Some older Triton V8 engines are known for spark plugs that can work loose in the cylinder head over years of heat cycling, occasionally leading to the plug backing out partially; this can cause a persistent misfire on the affected cylinder well before any visible damage occurs. EcoBoost V6s can also develop carbon buildup on intake valves since fuel isn't washing over them like in a port-injected engine.
Ford-specific causes (most common first)
- Worn or fouled spark plug on cylinder 3
- Failing coil-on-plug ignition coil for that cylinder
- Spark plug threads in the cylinder head loosening over time on older Triton V8s, causing poor sealing and misfire
- Carbon buildup on intake valves reducing airflow to that cylinder (common on direct-injection EcoBoost engines)
- Vacuum leak or injector issue isolated to cylinder 3
- Damaged or oil-contaminated ignition coil boot allowing spark to arc externally
Ford-specific diagnostic tip
Before replacing coils or plugs on an older Triton V8, physically check that the spark plug in the misfiring cylinder is torqued correctly and hasn't started to back out of the head — this simple check can save an unnecessary parts swap.
Symptoms drivers report
- CEL
- Rough idle
- Loss of power
Typical repair cost on Ford
Most Ford owners fix P0303 for between $150 and $550, depending on which underlying cause turns out to be at fault. Start with the most-likely cause for your vehicle — Failing ignition coil or worn spark plug on cylinder 3, sometimes compounded by loose spark plug threads on Triton V8s — before throwing parts at it.
Ford P0303 FAQ
Is P0303 on Ford Triton engines related to spark plugs coming loose?
On some older Triton V8s, spark plugs can gradually loosen in the cylinder head over years of use, which can cause an intermittent or worsening misfire on that specific cylinder.
Can carbon buildup cause a single-cylinder misfire on EcoBoost engines?
Yes, because EcoBoost engines are direct-injected, carbon can accumulate unevenly on intake valves, sometimes affecting one cylinder more than others and triggering a misfire code.
Should I replace all the coils or just the one on cylinder 3?
Start by testing or swapping just the cylinder 3 coil with a known-good one to confirm the diagnosis before replacing coils on cylinders that aren't showing a fault.
Related codes
Drive a different make? See the general P0303 guide for cross-vehicle causes and symptoms.
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Most common fix for P0303:
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