OBD-II Car P0420 Error
What it means
P0420 means the powertrain control module has decided the catalytic converter on Bank 1 (the engine bank that contains cylinder #1) is no longer cleaning the exhaust efficiently enough. The computer compares the upstream oxygen sensor (before the converter) with the downstream sensor (after it); when the two signals look too similar, it concludes the converter has lost efficiency. Typical symptoms are a steady check engine light, slightly reduced fuel economy or power, sometimes a faint rotten-egg (sulfur) smell from the tailpipe, and a guaranteed failed emissions test.
The bottom line on the P0420 error
P0420 almost always means the catalytic converter has lost efficiency after high mileage, and while it is safe to drive short-term it is the most misdiagnosed OBD-II code there is. The common trap is buying a downstream oxygen sensor: it is only the messenger reporting a worn converter, so replacing it rarely clears the fault.
Start cheap and smart โ read live O2 sensor data and rule out exhaust leaks and misfires before spending anything. If the converter itself is truly worn, replacement runs $900โ$2,500, so confirm the diagnosis before authorizing that bill.
P0420 snapshot
Our own at-a-glance triage of the P0420 error, from the verified data on this page. See error-code rankings โ ยท How we score โ
Common causes of the P0420 error
- Worn-out or failing catalytic converter (most common after high mileage)
- Faulty or aged downstream (post-cat) oxygen sensor giving misleading readings
- Exhaust leak near the manifold or before the downstream sensor, letting in extra air
- Engine misfire or rich/lean condition that overheated and damaged the converter
- Leaking fuel injector or contaminated converter (oil or coolant burning in the engine)
- Aftermarket or low-quality converter that does not meet OEM efficiency
Which OBD-II cars get the P0420 error
P0420 appears on virtually every OBD-II vehicle once mileage climbs past 100,000, but it is famously frequent on high-mileage Toyota/Lexus (the Camry, Corolla, and Prius are among the most-reported), Honda/Acura, Subaru, Nissan (Altima), and Ford (F-150) and Chevrolet (Silverado 1500) models where the original catalytic converter simply ages out. Vehicles that have suffered a long-running misfire or an oil/coolant leak wear their converters out much sooner.
How to fix the OBD-II Car P0420 error
- 1Read live oxygen sensor data firstBefore replacing anything expensive, use a scan tool to watch both O2 sensors. A healthy downstream sensor should read a fairly steady voltage while the upstream switches rapidly. If the downstream sensor mirrors the upstream, the converter is likely weak; if the downstream sensor is dead or erratic, replacing that sensor may clear the code.
- 2Inspect for exhaust leaks and misfiresCheck the exhaust manifold, gaskets, and pipe joints ahead of the downstream sensor for leaks, and clear any pending misfire (P030x) or fuel-trim codes first. A leak or misfire will falsely trip P0420 and will quickly ruin a new converter if left unfixed.
- 3Replace the oxygen sensor if it tests badIf the downstream O2 sensor is sluggish or out of range, replacing it is far cheaper than a converter and sometimes resolves P0420 completely. Use an OEM-grade sensor and clear the code, then drive a full warm-up cycle to confirm.
- 4Replace the catalytic converter as a last resortIf sensors, leaks, and misfires all check out and the code returns, the converter itself is worn. Fit an OEM or properly emissions-compliant direct-fit converter; cheap universal units often re-trigger P0420 within weeks.
๐งฐ When to call a professional
Call a professional if the code returns after you have replaced the downstream O2 sensor and fixed any leaks/misfires, if you cannot read live sensor data yourself, or if your area requires a certified emissions repair. Catalytic converter replacement also often needs welding or specialized tools.
OBD-II P0420 vs OBD-II P0430 vs OBD-II P0442: how they compare
How this code stacks up against related error codes for difficulty, urgency, and cost.
| OBD-II P0420 | OBD-II P0430 | OBD-II P0442 | |
|---|---|---|---|
| DIY difficulty (1โ5) | 3 | 3 | 2 |
| Urgency (1โ5) | 2 | 2 | 2 |
| Typical cost | $$$ | $$$ | $ |
How much does it cost to fix the P0420 error?
| DIY parts only (O2 sensor) | $50โ$250 |
| Professional diagnosis & O2 sensor repair | $150โ$500 |
| If catalytic converter replacement | $900โ$2,500 |
Repair or replace?
This is the OBD-II code where the repair-or-replace decision matters most. P0420 is the single most commonly MISdiagnosed code โ shops often sell a $200โ$400 downstream O2 sensor that almost never fixes it, because the sensor is the messenger reporting a worn converter, not the fault itself. Insist on live O2-sensor data and rule out exhaust leaks and misfires before authorizing anything. If the converter truly is worn, replacement is the expensive path โ RepairPal averages over $2,000, and even a direct-fit aftermarket unit runs $900โ$1,800; cheap universal cats often re-trigger P0420 within weeks. It is safe to drive short-term, but you will fail every emissions test until it is fixed, so it cannot be ignored indefinitely.
Common replacement parts for the P0420 error
| Downstream (post-cat) O2 sensor | $100โ$300 | Cheapest possible fix, but rarely the real cause of P0420 | |
| Catalytic converter (direct-fit) | $900โ$2,500 | Use OEM or emissions-compliant; avoid cheap universals | |
| Exhaust gasket / clamp | $20โ$100 | Fix leaks near the sensor that falsely trip P0420 |
Prices are typical retail ranges and vary by model and supplier โ always confirm the exact part number for your appliance before buying.
OBD-II P0420 error โ FAQ
๏ผ โ Is it safe to drive with this code?
Yes, P0420 alone is not an immediate safety risk โ the car will usually run normally for short-term driving. However, you should not ignore it long-term: a failing converter can hurt fuel economy, increase emissions, and may eventually clog and cause power loss. You will also fail any emissions/smog test until it is fixed.
๏ผ โ How much does it cost to fix?
It varies widely by cause. A downstream oxygen sensor replacement typically runs about $150โ$300 including labor. If the catalytic converter itself must be replaced, expect roughly $800โ$2,500 depending on the vehicle and whether an OEM part is required. Always rule out cheaper causes (sensor, leak, misfire) first.
๏ผ โ Will the code clear itself if I just replace the gas cap or reset it?
No. P0420 is about catalytic-converter efficiency, not the fuel cap (that would be an EVAP code like P0442). Clearing the code with a scan tool only turns the light off temporarily; it will return once the computer re-runs the catalyst monitor unless the underlying issue is repaired.
Sources & manufacturer references
This guide is independently written and not affiliated with OBD-II. Always unplug appliances before servicing and follow your model's manual. Error codes and steps can vary by model โ when in doubt, consult a qualified technician.