P0303 on Mazda: How to Diagnose & Fix
Cylinder 3 Misfire Detected
5 Mazda models · 7 documented issues · 2 critical · $50-$1,200 repair
P0303 on Mazda by model: symptoms, diagnosis and fix
The NB Miata (1999-2005) crankshaft position sensor (CPS) is a known failure point. The sensor deteriorates from engine heat, causing intermittent stalling, no-start conditions, and random misfires. Failure is often heat-related - the car runs fine when cold but stalls or dies after reaching operating temperature.
Common Symptoms
- Engine stalls when hot, restarts after cooling down
- Intermittent no-start condition (cranks but does not fire)
- Random misfires and rough running at operating temperature
- Check engine light with P0335 (CPS circuit) code
How to Fix
Replace crankshaft position sensor with OEM Mazda unit (ZL01-18-221A). Aftermarket sensors (Delphi, Dorman) have higher failure rates on this application. The sensor is located at the front of the engine beside the crankshaft pulley, and its gap to the pulley's sensor plate must be set correctly.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Genuine Mazda BP4W-18-221B crankshaft position sensorOEMMazda BP4W-18-221B
Confirm VIN before ordering; thi
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Owner tips & cautions
- TipCarry a spare CPS in the trunk - they fail without warning. Miata.net CPS mega-thread
- TipRoute sensor wiring away from exhaust manifold heat to extend sensor life
Sources for this issue (2)
The 1994-2000 1.8L BP engines use a wasted-spark coil pack with an integrated igniter transistor. The early version (identified by the 4-pin input connector, later revised to 3-pin) is notorious for internal igniter/solder-joint failure, especially if the ignition is left in the ON position with the engine not running - the igniter transistor overheats and either burns out or melts its own solder connections. The result is a dead cylinder pair, a persistent misfire, and unburned fuel dumped into the catalytic converter.
Common Symptoms
- Hesitation or stumble on startup that worsens over time
- Severe misfire under load
- Running rich, fuel smell, black smoke
- Check engine light with misfire codes
- Dead cylinder pair (1&4 or 2&3)
How to Fix
Replace the coil pack with an OEM Mazda unit (owners repeatedly report short life from budget aftermarket coils). Replace the plug wires and spark plugs at the same time since a failing coil stresses them, and inspect the catalytic converter if the car was driven while misfiring. Avoid leaving the key in ON without the engine running.
Sources for this issue (3)
Direct-injection Skyactiv engines accumulate carbon deposits on intake valves due to oil vapors from the PCV system. Without port injection to clean valves, carbon can fuse to valve surfaces over time, typically appearing around 65,000+ miles.
Common Symptoms
- Rough idle and engine misfires
- Reduced power and poor fuel economy
- Check engine light with misfire codes
How to Fix
Walnut blasting intake valves to remove carbon deposits. In-tank fuel additives (Techron, Sea Foam, Lucas) cannot reach the intake valves on a direct-injection engine, so they do little to prevent this buildup.
Owner tips & cautions
- TipWalnut blasting every 60,000-80,000 miles as preventive maintenance
Sources for this issue (3)
Ignition coil-on-plug coils fail causing misfires. Ford 3.0L Duratec V6 coils are prone to cracking and arcing. Usually fails one at a time.
Common Symptoms
- Engine misfire
- Check engine light flashing
- Rough idle
- Loss of power
- Poor fuel economy
How to Fix
Replace failed ignition coil. Replace all 6 spark plugs when replacing any coil. Consider replacing all coils proactively if one fails.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Genuine Mazda AJ09-18-100 direct ignition coilOEMMazda AJ09-18-100
Individual OEM coil replacement after a 3.0L cylinder-specific ignition failure is confirmed Fitment/availability: The inspected direct OEM dealer page identifies AJ09-18-100 and its Tribute fitment through 2001-2010. Confirm VIN/engine …
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On the 2.0L Skyactiv-G direct-injection engine used in the 2016-2021 CX-3, carbon can build up on the backs of the intake valves. Fuel is injected straight into the cylinder, so it never washes over the intake valves. Oil mist from the crankcase ventilation (PCV) system can then bake onto them over time. As deposits grow, they disturb airflow into the cylinders. Mazda has documented this on an earlier Skyactiv engine. Its bulletin 01-012/19 for the 2012-2013 Mazda3 with SKYACTIV says carbon around the intake valves can cause unstable combustion and misfire codes P0300-P0304 while the engine warms up after a cold start. We found no matching Mazda bulletin for the CX-3, and owner reports on Mazda forums are mixed. One owner reports more than 100,000 miles on a Skyactiv car with the same performance and fuel economy as before. Another notes that cars driven mostly on short trips are more likely to build deposits.
Common Symptoms
- Check engine light with misfire codes, especially on cold starts
- Gradual loss of low-end power and throttle response
- Reduced fuel efficiency over time
- Rough idle after extended parking
- Fouled spark plugs
- Hesitation during acceleration from stop
How to Fix
1. Rule out the cold-weather muffler problem first. Does the rough idle or check engine light (P0300-P0304) appear only on the first start after a night parked below 32°F (0°C)? Mazda bulletin 01-007/19 covers the 2016-2019 CX-3 for exactly this. Water collects and freezes in the main muffler and blocks the exhaust. The fix is an updated main muffler with two drain holes (part PEZ2-40-100 for 2WD, PEZ3-40-100 for AWD). It is covered under Mazda's New Vehicle Limited Warranty term. The dealer also changes the oil if the level reads above the F mark. 2. Get the stored codes and freeze-frame data before any parts are replaced. Mazda's carbon bulletin for the Mazda3 applies only when the misfire was logged within about the first 45 seconds after a cold start (coolant below 100°F, under high load). In one Skyactiv Mazda3 case, the coils, MAF sensor and O2 sensors were all replaced and the misfire stayed. 3. Confirm the carbon before paying for cleaning. In Mazda's Mazda3 procedure, the technician removes the intake manifold and checks for deposits around the intake valves. 4. If deposits are confirmed, the valves need cleaning. Mazda's bulletin 01-012/19 (2012-2013 Mazda3 only) has the technician remove the intake manifold and spark plugs. The valves are then soaked with a commercial top-end decarbon solvent and scraped with a plastic or wooden tool, never a metal one. A columnist at The Truth About Cars warns that chemical cleaning or walnut-shell blasting may not be enough on its own, and that heavy deposits may need the intake removed and cleaned on the bench. 5. Coverage: we found no recall, warranty extension or bulletin that covers intake-valve cleaning on the CX-3. Bulletin 01-012/19 applies only to the 2012-2013 Mazda3. Expect carbon cleaning on a CX-3 to be a paid repair unless the dealer finds a covered cause, such as the muffler bulletin above.
Sources for this issue (5)
- Reddit r/MechanicAdvice Thread Discussing 2016 Mazda CX-3 Skyactiv 2.0L Intake Valve Carbon Buildupforum
- Mazda TSB 01-012/19: Check engine light on with DTC P0300-P0304 (2012-2013 Mazda3 SKYACTIV)tsb
- Mazda TSB 01-007/19: Rough idle and/or MIL on with P0300-P0304 in cold temperaturetsb
- Piston Slap: Misfiring on Mazda3 Skyactiv's Diagnosis?article
- Intake valve carbon buildup on 2017 and later Skyactiv engines (Mazdas247)forum
The 2.5L Skyactiv-G with cylinder deactivation uses hydraulic valve clearance adjusters to drop and re-engage two cylinders. A PCM software fault in the control of those adjusters during the transition back from deactivated to full four-cylinder operation can let an intake valve rocker arm come out of position and strike internal engine parts. The result is a rough-running engine, misfire, loss of power and in some cases a stall in traffic. Mazda recalled about 17,134 Mazda3s (plus CX-5 and Mazda6) under NHTSA campaign 19V497000. Because a rocker that has already jumped can damage the valvetrain, cars that ran with the bad calibration for a long time may need hardware, not just a flash.
Common Symptoms
- Engine runs rough or shakes at light throttle
- Misfire and flashing check engine light
- Sudden loss of engine power
- Engine stall while driving
- Check engine light with misfire codes
How to Fix
Have the dealer perform recall 19V497 - the remedy is a PCM software reflash, free of charge. If the car has already exhibited misfires, a rough idle or a stall, insist on a valvetrain inspection before or alongside the flash; a dislodged rocker arm can require rocker/HLA replacement and, in the worst cases, cylinder head work. Do not dismiss an intermittent MIL with misfire codes on these cars as a coil or plug fault until the recall status is checked.
Call to confirm they can assess your vehicle and perform the stated work. Search results do not confirm qualification, eligibility or coverage.
Sources for this issue (4)
P0303 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0303 mean on a Mazda?
P0303 stands for "Cylinder 3 Misfire Detected." This code means your engine's computer detected a misfire specifically in cylinder number 3 — that cylinder isn't burning its fuel-air mixture properly or completely. The computer senses misfires by watching for tiny fluctuations in crankshaft speed each time a cylinder is supposed to fire. A misfire wastes fuel, can cause a noticeable shake, rough idle or loss of power, and can flood raw fuel into the exhaust where it may overheat and damage the catalytic converter. Because the fault is isolated to one cylinder, the cause is usually something specific to that cylinder's ignition, fuel delivery, or compression. On Mazda it is documented across 5 models: MX-5 Miata, CX-5, Tribute, CX-3, Mazda3.
What are the most common causes of P0303?
The most common causes of P0303 are: Worn or fouled spark plug in cylinder 3, Failed or weak ignition coil on cylinder 3, Bad spark plug wire or boot (where applicable), Clogged or leaking fuel injector for cylinder 3, Vacuum or intake air leak affecting that cylinder, Low compression (worn rings, burnt/leaking valve, head gasket), Wiring or connector fault at the coil or injector. The specific cause varies by vehicle.
How much does it cost to fix P0303 on a Mazda?
Published repair estimates on Mazda range from $50 to $1,200 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.