P0300 means several WRX cylinders misfired or no one cylinder dominated. Save P0301–P0304 companions, roughness counts, fuel correction, boost, rail pressure, temperature and load before moving parts. Cold-only, boost-only, one-cylinder, one-bank and all-cylinder patterns require different tests. Spark-plug gap or weak ignition may appear under cylinder pressure, but injector, fuel delivery, intake leakage, compression and AVCS timing still need evidence. Stop loading the engine when the MIL flashes.
Match the symptom
Select the closest observation. The result changes the first test; it does not name a part to replace.
Stop loading to protect catalysts.
What can set P0300—and how to separate it
The order below is evidence-driven, not a universal probability ranking. Vehicle year, repair history, companion codes and the operating condition in freeze frame change where diagnosis should begin.
| Possible cause | Evidence that supports it | First useful test |
|---|---|---|
| Worn/fouled plug or ignition fault | Cylinder leader and controlled swap follows | Labelled plug/ignition test |
| Injector or fuel-pressure fault | Contribution/pressure mismatch | Injector/fuel tests |
| Intake, purge or boost leak | Trim/smoke pattern | Smoke and purge test |
| Compression, valvetrain or timing fault | Count stays with mechanical weakness | Compression/leak-down/timing |
| Shared voltage or contaminated fuel | All cylinders or post-refueling | Voltage/fuel-quality test |
Preserve roughness counts under the actual condition
Capture cold-start, idle and controlled-load counters before cycling the key or moving coils. Label every plug and make one comparison at a time so the cylinder pattern remains usable.
One-bank accumulation raises local intake, exhaust or cam-control causes. Counts across all cylinders raise shared fuel pressure, MAF, voltage, contaminated fuel or calibration questions.
Boost raises ignition and delivery demand together
Inspect gap, insulator cracks, fuel wetness and oil ash on labeled plugs. A breakdown that appears only under boost can be ignition reserve, but commanded versus actual rail pressure must be recorded at the same time.
If the fault stays on a cylinder after a valid ignition test, check injector contribution and compression/leak-down. Direct-injection equipment and depressurization procedures are mandatory on FA engines.
Mechanical and calibration context matters
AVCS errors, valve sealing, timing indexing and engine modifications can create structured misfire. Do not replace four coils because a generic description says random.
Verify stable counts under the original safe condition and no pending P0300. Repair combustion first, then assess any P0420 catalyst consequence.
Save these scan-data clues
Freeze frame is a snapshot of conditions when the PCM matured the fault. It can distinguish an idle-only problem from a load, cold-start or monitor-completion problem. Clearing first throws that context away.
- Exact VIN, platform, model year and engine sales code
- All confirmed, pending and permanent companion codes
- RPM, load, coolant temperature and vehicle speed
- Relevant commanded and actual scan data
- Recent service, repairs and symptom conditions
- Manufacturer bulletin and current software/VIN status
A practical diagnostic workflow
- 01Identify engine and bank layout
Cylinder patterns differ.
- 02Preserve counters
Save cold/load evidence.
- 03Inspect plugs in order
Deposits direct testing.
- 04Target ignition
Use one controlled test.
- 05Test fuel and air
Graph pressure/trims and smoke paths.
- 06Prove mechanical health
Use compression/leak-down/timing.
- 07Verify original condition
Avoid a flashing-MIL road test.
Do not replace the most commonly named component from P0300 alone. Confirm the exact engine, preserve evidence and complete the manufacturer test sequence before ordering parts.
Why the model year changes the test
WRX diagnostics span EJ20/EJ255 turbo models, FA20DIT direct injection and the newer FA24DIT, while STI variants retain different EJ257 hardware in many years. Intake/exhaust AVCS coverage, turbo and EGR plumbing, cylinder banks, fuel systems, catalysts and ECM monitor thresholds are not interchangeable. Confirm VIN, model year, WRX versus STI, engine code, transmission, emissions label and ECM calibration before using any test or bulletin.
- EJ coil-on-plug, FA direct injection and STI hardware use different tests and cylinder layouts.
- Boost-only misfire should not be reproduced repeatedly with a flashing lamp.
- Modified calibration, fuel and spark-plug specification must be recorded.
- P0011/P0021 companions move AVCS and oil control higher.
P0300 FAQ
Does P0300 mean all coils failed?+
No.
Can I drive?+
Not with flashing MIL or strong shaking.
Can bad fuel cause it?+
Yes.
Should I replace plugs first?+
Inspect and record them first.
Can vacuum leaks cause it?+
Yes.
Can timing cause it?+
Yes, especially with cam/crank codes.
Why save counters?+
They reveal cylinder and bank structure.
What confirms repair?+
Stable counters under original conditions.
Ford diagnostic references
These documents explain Ford monitor operation. Exact pinpoint tests, connector views and specifications must be selected for the truck's VIN and model year in current service information.
01Subaru Bulletin 09-110-23R — P0420 diagnostic procedureCovers 2018–2023 WRX and requires misfire, mixture, sensor, oil-smoke and exhaust checks plus model-specific SSM verification↗02Subaru Technical Information System (STIS)Current VIN-, engine- and model-year-specific diagnostic procedures↗03Subaru owner manuals and vehicle resourcesOfficial model-year operating, warning, fuel and maintenance information↗