Safe to drive short-term. Repair within a few weeks. P0420 means the catalytic converter on Bank 1 is no longer cleaning exhaust gases efficiently enough to satisfy the ECM's monitoring threshold. Typical fix cost ranges from about $15-$400 if a catalytic-converter cleaner or a faulty oxygen sensor resolves it, up to roughly $900-$2,500 to replace the catalytic converter (aftermarket units and labor often run $500-$1,500) - but confirm the cause first, because a cheaper upstream fault can trigger P0420 without the converter actually being bad.
What P0420 means
The engine control module continuously compares the activity of the upstream (pre-catalyst) oxygen sensor against the downstream (post-catalyst) sensor. A healthy catalytic converter stores and releases oxygen as part of the conversion reaction, which keeps the downstream sensor's voltage relatively stable. When the converter's substrate degrades, the downstream sensor starts mimicking the rapid switching of the upstream sensor — a pattern the ECM interprets as insufficient catalyst efficiency. P0420 is set on Bank 1 (the side of the engine containing cylinder 1) once the calculated efficiency ratio falls below the threshold for a calibrated monitor drive cycle.
Symptoms
- Illuminated malfunction indicator lamp (check engine light)
- Failed emissions or smog inspection
- Faint sulphur or rotten-egg odour from the exhaust, especially at idle
- Slightly reduced fuel economy (5–10%) if a rich mixture is also present
- No noticeable drivability issues in most cases
Common causes
- Degraded catalytic converter substrate — the most common cause on vehicles above 80,000–100,000 miles
- Pre-catalyst exhaust leak drawing in ambient air and distorting O2 sensor readings
- Failed or sluggish downstream (post-cat) oxygen sensor reporting false data
- Sustained rich-running condition (from a separate fuel trim fault) that has overloaded and coated the catalyst
- Engine oil or coolant entering combustion due to worn piston rings or a head gasket leak, poisoning the catalyst substrate
Severity & driving advice
Severity: Moderate — Emissions are uncontrolled and the vehicle will fail an emissions test. A root-cause rich mixture left unresolved can accelerate engine wear.
Can I drive? Safe to drive short-term. Repair within a few weeks.
Diagnostic approach
- Clear and retest after resolving companion codes — Fuel trim faults like P0171 or P0172 and misfires like P0300 can mimic or directly cause P0420. Address those first; the catalyst code may not return.
- Inspect the exhaust system ahead of the front O2 sensor — An exhaust manifold crack or leaking flange gasket draws in outside air before the upstream sensor, corrupting the efficiency calculation. Listen for ticking sounds at cold start and check flanges visually.
- Compare upstream and downstream O2 sensor waveforms on a live data scan — The upstream sensor should switch rapidly between rich and lean (high and low voltage). The downstream sensor on a good converter should hold fairly steady mid-voltage. If it mirrors the upstream pattern, the converter is failing.
- Perform a catalyst efficiency temperature test — Using a non-contact thermometer or thermal imager, the outlet temperature of a working catalytic converter should be measurably higher than the inlet (the exothermic reaction generates heat). Equal inlet/outlet temperatures suggest a dead substrate.
Estimated repair costs
Typical US repair costs (parts & labour) for the faults behind P0420. These are national-average estimates, not a quote— your actual price varies by region, shop labour rate, and your vehicle’s make, model and year. Figures reviewed 2026.
| Repair | Estimated cost (USD) |
|---|---|
| Diagnosis / inspection | $80–$150 |
| Catalytic converter cleaner / fuel additive (DIY, mild cases) | $15–$40 |
| Upstream or downstream oxygen sensor | $150–$400 |
| Exhaust leak / flange gasket repair | $40–$150 |
| Aftermarket catalytic converter (parts + labor) | $500–$1,500 |
| OE-grade catalytic converter (parts + labor) | $900–$2,500 |
Make & model notes
Toyota / Lexus: On Toyota and Lexus, P0420 usually points to an aging original catalytic converter rather than a sensor, and on V6 engines it commonly appears together with P0430 (the opposite bank). A frequent trap: fitting a cheap aftermarket converter whose lower-grade substrate re-triggers P0420 within weeks - an OEM-grade cat is usually what clears it for good.
Subaru: Subaru's 2.5L engines (Impreza, Forester, Outback, Legacy) are well known for premature catalytic-converter failure setting P0420, often well before 100,000 miles. Oil consumption or a head-gasket weep can contaminate the converter, and the close-coupled cat runs very hot - so on these cars a genuinely failed cat is a common finding, not just a lazy sensor.
Honda / Acura: On many Hondas and Acuras a worn downstream (rear) oxygen sensor is a common cause of P0420 before the converter itself - confirm the rear O2 sensor's switching behaviour on a scan tool before condemning the cat.
Hyundai / Kia: Hyundai and Kia have documented catalytic-converter and oil-consumption issues (notably some 1.6T and 2017-onward engines) that lead to P0420. Importantly, in the US the catalytic converter is covered by the federal emissions warranty for 8 years / 80,000 miles (longer in some states, and it transfers to later owners) - so a failed cat on a newer Hyundai or Kia may be a dealer warranty repair rather than an out-of-pocket one.
Ford / Chevrolet (GM): On Ford and GM vehicles, check for an exhaust leak ahead of the rear oxygen sensor before blaming the converter - a small leak draws in air that skews the rear O2 reading and mimics a failed cat. Otherwise an aged original converter is the usual cause on higher-mileage engines.
FAQ
How much does it cost to fix a P0420 code?
It depends entirely on the cause. If a catalytic-converter cleaner or a failed oxygen sensor resolves it, expect roughly $15-$400. A replacement catalytic converter runs about $900-$2,500 OE-grade, or $500-$1,500 for an aftermarket unit plus labor. Always diagnose before buying a converter, because upstream faults often set P0420 without the cat being bad.
Will a catalytic converter cleaner fix P0420?
Sometimes, on an early or borderline P0420 caused by mild contamination - a fuel-additive cleaner followed by a sustained highway drive can lower the readings enough to clear it. It will not fix a physically failed or melted converter, and it won't help if the real cause is a misfire, a rich mixture, or an oxygen-sensor fault. It's a cheap first try, not a guaranteed fix.
Can I drive with a P0420 code?
Yes, in most cases the vehicle will drive normally. However, the failing catalyst is no longer reducing harmful emissions, and you will fail an emissions inspection. If the underlying cause is a rich mixture, continued driving may cause additional engine damage.
Will clearing the code fix P0420 permanently?
No. The ECM will run its catalyst monitor during the next complete drive cycle and set the code again if the root cause has not been repaired. Clearing the code only turns off the light temporarily.
Why does P0420 come back after fitting a new catalytic converter?
The most common reasons are an unresolved exhaust leak before the upstream O2 sensor, a faulty downstream oxygen sensor that was not replaced, or a continuing rich fuel trim condition that will re-coat the new converter.
Does P0420 affect performance or fuel economy noticeably?
Usually not directly. However, if the root cause is a rich-running condition or an oil-burning engine, you may notice slightly worse fuel economy or a faint exhaust smell. The converter itself does not restrict exhaust flow until it physically collapses internally.
P0420: cleaning vs replacement - which do I need?
Try cleaning (additive plus a highway drive cycle) only if the converter is otherwise healthy and the code is borderline. Replace the converter when it's physically failed, contaminated by oil or coolant, or the readings are well past threshold. Either way, fix the upstream cause first - a misfire, rich fuel trim, oxygen sensor, or exhaust leak - or a new converter will fail again.
Is it worth fixing P0420?
If you need to pass an emissions test or the converter is genuinely failing, yes. If P0420 is caused by a cheaper upstream fault such as an oxygen sensor, an exhaust leak, or a rich mixture, fixing that is well worth it and far cheaper than a converter. The car usually still drives normally in the meantime, but the emissions problem remains until it's repaired.
What does it mean if I have P0420 and P0430 at the same time?
P0420 is bank 1 and P0430 is bank 2 - the two sides of a V6 or V8. Both setting together rarely means two converters failed at once; more often there is a shared cause such as an exhaust leak, a rich or lean fuel-trim condition, or simply both original converters aging together. Find the shared cause before buying two cats.
Will a new catalytic converter fix P0420?
Usually, if the converter is genuinely the problem - but two cautions. First rule out a worn rear oxygen sensor or an exhaust leak, which both mimic a bad cat. Second, a low-grade aftermarket converter can re-trigger P0420 within weeks, so an OEM-grade unit is often needed to clear it for good.
Is P0420 the catalytic converter or the oxygen sensor?
It can be either. P0420 means the rear oxygen sensor no longer sees the converter cleaning the exhaust - which can be a tired converter or a lazy rear O2 sensor giving a false reading. Check the rear sensor's switching and rule out exhaust leaks before replacing the expensive converter.