P0420 means the WRX ECM calculated insufficient Bank 1 catalyst oxygen storage; it does not prove the converter alone failed. Subaru bulletin 09-110-23R covers 2018–2023 WRX, screens P0300–P0304, P0171/P0172 and P0137-series faults, checks oil smoke and exhaust leakage, and uses different SSM thresholds for 2018–2021 2.5L turbo and 2022-and-newer 2.4L DIT manual/CVT applications. Repair catalyst-damaging causes before replacing exhaust parts.
Match the symptom
Select the closest observation. The result changes the first test; it does not name a part to replace.
Preserve monitor data.
Before authorizing a catalytic converter
Ask the shop to document three things: the companion-code status, exhaust integrity, and the evidence that Bank 1 sensor behavior fails the VIN-correct catalyst test. A quotation that begins only with 'P0420 means converter' skips the root-cause work that protects the replacement. Review misfire history, fuel trims, oil use and coolant loss before approving an expensive part.
Confirm the proposed converter matches the truck's model year, engine, wheelbase/exhaust configuration and underhood emissions certification. Federal and California-emissions applications may not use the same part. Ask whether new seals, fasteners or sensors are required, what warranty applies, and whether the estimate includes correcting the condition that damaged the original unit.
After repair, verify there are no leaks, rising misfire counts or abnormal trims. Complete the applicable Ford drive cycle so the catalyst monitor can run. A permanent P0420 cannot simply be erased; it clears after the OBD system observes enough passing operation. Keep the invoice and part certification for future emissions inspection or warranty questions.
What can set P0420—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 |
|---|---|---|
| Catalyst deterioration | Prerequisites pass | Engine-specific catalyst test |
| Misfire or rich operation | P030x/rich/fouled plugs | Repair combustion |
| Oil/coolant contamination | Consumption/deposits | Mechanical diagnosis |
| Exhaust leak | Soot/tick | Cold leak test |
| O2/A/F circuit fault | Response/electrical failure | Graph/circuit test |
The WRX threshold is engine and transmission specific
Subaru publishes different SSM catalyst-deterioration values for 2018–2021 2.5L turbo and 2022-plus 2.4L DIT. The later manual and CVT verification values also differ, so a generic rear-sensor graph is incomplete.
Identify WRX versus STI, engine, transmission and ECM calibration before running the road pattern. Modified exhaust or software must be documented because factory monitor expectations may no longer match the installed system.
Turbo problems can make the catalyst a victim
Review boost-related misfire, injector balance, fuel correction, oil consumption and turbo oil leakage. Raw fuel and oil ash can damage a replacement converter if the upstream cause remains.
Inspect cylinder-head joints, turbo/exhaust connections, front pipe, converter and sensor bungs cold for soot or air leakage. Subaru's bulletin requires the general and exhaust inspections before its active A/F test.
Use Subaru's active test and monitor proof
The published procedure commands injection quantity and checks whether the A/F sensor reaches target before choosing the sensor or exhaust branch. It then uses Inspection Mode V and SSM monitor data rather than visual waveform resemblance alone.
Complete the prescribed verification with no pending P0420. Clearing the lamp, installing an extender or seeing incomplete readiness does not prove catalyst efficiency.
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
Layouts differ.
- 02Preserve readiness and codes
Do not clear first.
- 03Repair catalyst killers
Misfire, mixture and fluids first.
- 04Inspect exhaust cold
Check for leaks.
- 05Test sensors and catalyst
Use correct enable conditions.
- 06Complete monitor
Confirm no pending code.
Do not replace the most commonly named component from P0420 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.
- 09-110-23R names 2018–2023 WRX; other years require their current STIS route.
- The bulletin separates 2.5L turbo from 2.4L DIT and even MT from CVT thresholds.
- Aftermarket downpipes or tuning can invalidate stock monitor assumptions and emissions legality.
- A flashing MIL requires reducing load immediately.
P0420 FAQ
Does P0420 prove the catalyst failed?+
No.
Can I drive?+
Briefly with steady lamp; stop for flashing, rattle or power loss.
Will a rear O2 sensor fix it?+
Only if proven faulty.
Which side is Bank 1?+
Use the exact engine diagram.
Can oil use cause it?+
Yes.
Can an exhaust leak cause it?+
Yes.
Why is P0430 useful?+
It provides the other-bank context.
What confirms repair?+
Completed monitor without pending P0420.
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↗