P0420 means the ECM judged Bank 1 catalyst oxygen-storage performance below threshold. It does not prove the converter was the first failure. On 2018–2022 Traverse 3.6L LFY vehicles, GM documents broken or poorly penetrated exhaust welds with rattle/noise, lean codes and P0420/P0430. Repair active misfire, mixture faults, exhaust leakage and contamination before condemning the catalyst.
Match the symptom
Select the closest observation. The result changes the first test; it does not name a part to replace.
Inspect leaks and execute the RPO-specific catalyst test.
When the P0420 repair is actually complete
The repair is complete when the original operating condition is reproduced safely, the monitored values behave normally and no pending P0420 or related fault returns. Turning off the lamp or clearing memory alone proves nothing.
Recheck for oil, fuel, vapor, coolant or boost leaks and inspect anything moved during testing. If the original condition cannot be reproduced because weather, fuel level or temperature changed, document that limitation and continue monitoring rather than claiming certainty.
If the code returns, compare the new freeze frame with the first. The same pattern suggests an incomplete root-cause repair; a different load, temperature or companion-code pattern may reveal a second fault.
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 |
|---|---|---|
| Aged or damaged catalyst | Monitor fails after all upstream faults pass | Run SI catalyst efficiency procedure |
| Misfire or mixture damage | Counters, trims or companion codes abnormal | Correct engine control first |
| Exhaust leak | Soot/tick/smoke ahead of sensors | Inspect cold and leak-test |
| Oil/coolant contamination | Consumption/deposits/fluid loss | Find internal source |
| Sensor, wiring or wrong converter | Implausible response or repair history | Circuit/response and emissions-part verification |
A cracked pipe can distort the monitor
Air entering near the monitored exhaust path can change oxygen-sensor behavior and create lean/post-catalyst evidence. Rattle, exhaust noise and soot make the physical inspection especially important on a covered LFY.
Historical misfire, rich operation or oil/coolant contamination may have damaged a converter even after the initiating fault is gone. Document and correct that cause so a replacement is not immediately exposed to it.
What an owner can check—and where to stop
With the engine cold, an owner can inspect oil and coolant levels, the air-filter housing, intake and charge pipes, battery terminals, accessible grounds, EVAP hoses and connectors disturbed during recent work. Photograph the original condition and avoid unplugging several components at once.
Professional equipment is appropriate for regulated EVAP smoke, bidirectional purge/vent or throttle commands, charge-air pressure testing, oscilloscope cam/crank correlation, mechanical oil-pressure measurement, fuel-pressure work, compression/leak-down and catalyst testing. Turbo plumbing can contain hot oil residue and pressurized air; fuel vapor and moving throttle plates add separate hazards.
Ask the repairer which test failed, under what conditions and which RPO-specific procedure was used. A line reading only ‘code present—replace component’ does not distinguish the component from its wiring, plumbing, calibration or the upstream fault that damaged it.
Repair verification
Clear P0420 only after saving the original evidence and repairing a proven cause. Reinspect every disturbed seal, clamp, connector and fluid level, then monitor the relevant data at idle before attempting a road test.
Repeat the safe portion of the freeze-frame condition. A turbo moisture fault may require climate-specific reasoning; an EVAP monitor needs the correct fuel level and soak; a catalyst monitor will not be meaningful while misfire or fuel control remains abnormal.
Check pending codes after the drive and allow readiness to progress without repeated resets. Permanent codes disappear after the vehicle completes enough passing monitor operation, not because the battery was disconnected. Keep before-and-after reports.
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.
- Confirmed, pending and permanent codes
- RPM, load, coolant and intake temperature
- Vehicle speed and time since start
- Battery voltage and relevant five-volt reference
- Companion codes and recent repair history
- Relevant commanded-versus-actual PIDs
- Upstream/downstream sensor graphs
- Misfire counters and trims
- Oil/coolant consumption history
A practical diagnostic workflow
- 01Save the full report
Record monitor state and companion faults.
- 02Fix active engine faults
Misfire and mixture diagnosis comes first.
- 03Inspect exhaust integrity
Find leakage before interpreting oxygen storage.
- 04Validate sensor operation
Use controlled response and circuit tests.
- 05Run catalyst procedure
Meet enable conditions and use VIN-correct thresholds.
- 06Protect the repair
Correct contamination and verify monitor completion.
Do not replace both converters or oxygen sensors from a Bank 1 code. Inspect the documented exhaust paths on a matched LFY, verify sensor circuits, and prove catalyst failure after upstream faults are corrected.
Why the model year changes the test
Traverse diagnostics divide into the first-generation 3.6L LLT/LFX, the second-generation 3.6L LFY, a limited 2.0L turbo application, and the newer 2.5L turbo generation. Bank layout, fuel pressure, timing hardware, air plumbing and GM bulletins change with engine RPO and build date. Confirm the VIN and emissions label before choosing a test.
- 2018–2022 3.6L LFY is the defined PIP5681C exhaust-weld population.
- P0420 addresses Bank 1; P0430 is Bank 2 on V6 applications. Confirm cylinder-bank and sensor location from VIN-correct diagrams.
- Match federal/California emissions certification before ordering a legal replacement converter.
P0420 FAQ
Does P0420 always mean a bad converter?+
No. Upstream engine, leak and sensor faults can create or damage it.
Can misfire cause it?+
Yes, and active misfire must be repaired first.
Can oil burning ruin a converter?+
Yes. Correct the consumption source before replacement.
Can I drive?+
Usually briefly if alone and smooth; stop for overheating or flashing misfire.
Will an oxygen sensor fix it?+
Only when testing proves sensor/circuit failure.
Can an exhaust leak set it?+
Yes, particularly ahead of or near the monitored sensors.
Does clearing fix it?+
No; it resets evidence and monitor readiness.
What confirms repair?+
Normal upstream systems and a completed catalyst monitor without a pending return.
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.
01GM PIP5681C — Traverse exhaust leaks and lean/catalyst codes2018–2022 Traverse 3.6L LFY guidance for broken/poor welds with P0171/P0174/P0420/P0430 and exhaust noise↗02Chevrolet manuals and guidesOfficial GM model-year and VIN manual lookup; use VIN-correct Traverse procedures and specifications↗