Number P0420: Catalyst System Efficiency Below Threshold Bank 1
The engine module checks whether the catalyst of bank 1 still converts enough. On petrol engines it estimates the oxygen storage capacity by comparing the sensors before and after the catalyst – a good catalyst smooths the rear signal. GM uses a test on overrun (ratio below 0.35 sets the code on the
Can you drive with P0420?
Severity: Plan the repair. Yes — this is an emissions-system fault. The car will generally drive normally, but it will fail an emissions inspection with the light on.
If you ignore it: Fuel economy and emissions creep up, and some faults in this family (rich/lean running) stress the catalyst over time.
This is the #1 most common code in our top-150 library, in the catalyst & exhaust efficiency family (Powertrain system). Severity is Number Nest's editorial triage — if the light is flashing or the car feels unsafe, stop driving it whatever any page says.
What P0420 means
The engine module checks whether the catalyst of bank 1 still converts enough. On petrol engines it estimates the oxygen storage capacity by comparing the sensors before and after the catalyst – a good catalyst smooths the rear signal. GM uses a test on overrun (ratio below 0.35 sets the code on the first trip), Ford an index ratio above 0.75 or fuel integration after fuel cut-off. On diesels P0420 concerns the oxidation catalyst, checked by its heat release during filter regeneration (GM Duramax LML, Ford below 45 % of the expected heat). Ford names oil-borne poisoning as the most common cause.
Systems involved: catalytic converter, downstream oxygen sensor, upstream oxygen sensor, engine control module (ECM/PCM).
Causes, ranked by likelihood
| # | Cause | Likelihood |
|---|---|---|
| 1 | Catalyst aged or poisoned (oil consumption, silicone, coolant) | most likely |
| 2 | Catalyst overheated by misfire or rich running | possible |
| 3 | Rear oxygen sensor sluggish, dirty or unsuitable | possible |
| 4 | Exhaust leak ahead of or at the rear sensor | possible |
| 5 | Diesel: oxidation catalyst sooted or poisoned, post-injection faulty | possible |
Check them in order — the list is ranked so the likely, cheaper causes come before the expensive part.
Symptoms you may notice
- Check engine light on, otherwise often no change in driving
- Emission test failed (higher HC, CO, NOx)
- Rattling or loss of power if the catalyst has broken up
Misdiagnosis traps
- Replacing the converter for a dead or lazy oxygen sensor — the sensor reports the converter's work, and a bad reporter convicts a good part. Sensors first, converter second.
- An exhaust leak ahead of the downstream sensor pulls in fresh air and mimics a failing converter on the scope.
- Clearing the code and passing a drive cycle once is not a fix — the catalyst monitor is one of the slowest tests the car runs on itself.
How a good shop confirms it
- Live-scan both oxygen sensors: the downstream signal should sit fairly steady around 0.6–0.8V, not mirror the upstream sensor's switching.
- Check for exhaust leaks ahead of the converter, then confirm engine basics — misfires or rich running will poison a new converter too.
- Compare both banks and check for catalyst-related TSBs before authorizing a converter.
Repair options & costs
| Repair | Shop total (parts + labor) | DIY (parts only) | Difficulty |
|---|---|---|---|
| Oxygen sensor — downstream (post-cat) | $110 – $305 | $60 – $180 (parts only) | 2/5 |
| Catalytic converter replacement | $850 – $1,913 | $700 – $1,600 (parts only) | 4/5 |
National ranges from Number Nest's cost model (midsize car, independent shop, US-average labor). Published cross-check — Oxygen sensor — downstream (post-cat): CarMD 2026 national average $287; Catalytic converter replacement: CarMD 2026 national average $1,511.
OBDex repair data for this code rates the job difficulty medium with a typical labor time in the 0.5–3 hour range and DIY possible: yes.
Price this repair in your state
Estimated total: — (midsize car, independent shop)
Check the federal emissions warranty first
Before you pay: federal law (the Clean Air Act emissions warranty) requires the manufacturer to cover the catalytic converter and the engine computer (PCM) for 8 years or 80,000 miles on 1995-and-newer cars, whichever comes first. California-emissions (CARB) states cover a longer list of parts for 7 years/70,000 miles, and PZEV-rated vehicles sold in CARB states carry 15-year/150,000-mile emissions coverage on many components. Check your warranty booklet or a dealer lookup before authorizing this repair out of pocket.
Frequently asked questions
Will a new oxygen sensor fix P0420 by itself?
Only if the sensor was the problem — which is why the sensors are tested before the converter is condemned. Sensor replacement is the cheap first step when the sensor's signal is lazy, biased or dead; a healthy sensor reporting a bad converter means the converter.
Can a fuel additive or "cat cleaner" clear P0420?
Occasionally, briefly, if deposits were the marginal cause — but a converter that has lost efficiency does not regain it from a bottle. Treat additives as a lottery ticket, not a repair, and never reset the light before an inspection hoping the monitor stays incomplete.
Is P0420 covered under warranty?
On 1995-and-newer cars, the catalytic converter carries a federal emissions warranty of 8 years/80,000 miles, and PZEV vehicles in CARB states extend many emissions parts to 15 years/150,000 miles. Check before paying.
What happens if I keep driving with P0420?
Usually nothing dramatic — the car drives fine. Over months, a converter that is degrading can eventually melt or plug, hurting performance, and the car will not pass an emissions inspection with the light on.
How much does it cost to fix P0420?
The fix most associated with this code — oxygen sensor — downstream (post-cat) — runs $110 – $305 at an independent shop for a midsize car in our cost model (parts + labor, US average). The code names a system, not a part, so confirm the cause before spending: the ranked causes above are ordered by likelihood.
Can I drive with P0420?
See the can-you-drive verdict above — it is based on the system involved and the cascading-damage risk, and it is our editorial triage, not a manufacturer instruction. If the light is flashing or the car feels unsafe, stop driving it regardless of any code.
Will P0420 clear itself?
If the fault was a one-off (a loose cap, a cold-start glitch), the light can go out after several clean trips and the code ages into history. A fault that is still present re-sets the code as soon as the car runs that self-test again — monitors run under specific conditions, which is why "it hasn't come back in two days" is not proof of a fix.
Where this data comes from
Code facts, causes and symptoms come from OBDex — an open dataset of independently authored descriptions of the standardized generic OBD-II codes (CC0 1.0, public domain; github.com/foerbsnavi/OBDex). The SAE J2012 document itself is copyrighted and is not reproduced here. Cost ranges are Number Nest's own US planning model: national parts and labor ranges cross-checked against the CarMD Vehicle Health Index 2026 published averages, with labor adjustable by the BLS OEWS May 2025 state wage index for auto technicians (occupation 49-3023) — a wage index, not a surveyed shop rate. Severity and can-you-drive notes are our editorial triage, not a manufacturer instruction.