P0420
Catalyst System Efficiency Below Threshold (Bank 1)
P0420 is one of the most expensive codes to guess at — and one of the easiest to over-repair. This guide covers what the code actually measures, the cheaper causes to rule out first, and how to know when the converter really is the problem.
Exhaust components run extremely hot. Never inspect the catalytic converter or oxygen sensors on a warm engine. Removing or "gutting" a catalytic converter on a road-legal vehicle is illegal in most jurisdictions. This article is educational and does not replace a certified mechanic.
What P0420 means in simple words
Modern vehicles have two oxygen sensors per exhaust bank: one upstream of the catalytic converter and one downstream. The upstream sensor swings rapidly as the ECU adjusts the air-fuel mixture. The downstream sensor should be much steadier, because a healthy converter buffers those swings.
When the downstream sensor starts mirroring the upstream sensor's activity too closely for too long, the ECU concludes the converter isn't doing its job and stores P0420. "Bank 1" simply refers to the side of the engine that contains cylinder 1.
Notice what this measurement is not: it is not a direct read of the converter itself. Anything that makes the downstream sensor look like the upstream sensor — including a bad sensor, an exhaust leak, or a converter that has been contaminated by another problem — can trigger P0420.
Common symptoms
- Check engine light with no drivability change (most common).
- Slightly reduced fuel economy.
- Occasional rotten-egg smell from the exhaust in advanced cases.
- Failed emissions test.
Possible causes
Oxygen sensor(s), especially downstream
The downstream sensor slows down with age. A lazy or aged O2 sensor can generate P0420 with a perfectly healthy converter. On many vehicles this is the most common actual cause.
Exhaust leak before or between the sensors
A leak in the manifold, at the flex pipe, or around the converter's welds lets fresh air into the exhaust. That fools the downstream sensor into looking too "active." Fixing a $30 gasket is a lot cheaper than a converter.
A history of misfires or oil burning
Raw fuel or oil dumped into the exhaust from long-running misfires or worn valve seals contaminates the catalyst substrate. Sometimes fixing the misfire alone won't recover the converter — but you have to fix the misfire first, or a new converter will die too.
Aftermarket or damaged converter
Cheap universal catalytic converters often fail P0420 monitors within months of install. Road impact damage or overheating can also crack the internal substrate.
ECU tune / modified exhaust
Aggressive tunes, straight pipes, or removed pre-cats often produce P0420. If the vehicle has been modified, that is your primary suspect.
What to check first
- Scan for other codes first. Misfire, fuel-trim, or sensor codes point to the real root cause before you spend anything on a converter.
- Check fuel trims and O2 sensor patterns. With a scanner. If the downstream sensor swings up and down like the upstream one, the converter or downstream sensor is suspect. If both look sluggish, look at the upstream sensor first.
- Visually inspect the exhaust. For soot streaks, cracks, or missing fasteners near the converter — signs of a leak.
- Tap-test on a cool converter. A healthy converter is quiet; a rattle points to a broken substrate. Do not do this while the exhaust is hot.
Test before replacing parts. A code points at a system, not at a specific failed component, so confirming the fault with a measurement is what keeps the repair bill low. An affordable scanner is enough for most of these checks — see our OBD2 scanner guide.
Common mistakes when diagnosing P0420
- P0420 is one of the most expensive codes to guess at — and one of the easiest to over-repair.
- Replacing an expensive converter without ruling those out is a common mistake.
When to stop driving
- Exhaust components run extremely hot.
- Never inspect the catalytic converter or oxygen sensors on a warm engine.
If any of the above applies, park somewhere safe and arrange a tow or a professional inspection. Review our safety guidelines before doing any work yourself.
Risk level: Low. Exhaust components run extremely hot. Never inspect the catalytic converter or oxygen sensors on a warm engine. Removing or "gutting" a catalytic converter on a road-legal vehicle is illegal in most jurisdictions.
Approximate repair cost ranges
Prices vary a lot by vehicle, region, and shop rates. These are rough ranges for context only — not a quote, and not a prediction of what your repair will cost.
| Repair | Typical range | Notes |
|---|---|---|
| Oxygen sensor | — | Replacing an oxygen sensor is a common DIY on many cars. |
| Exhaust leak repair | — | Often needs a shop with a lift and welder. |
| Genuine OEM catalytic converter | Well over a thousand dollars | One of the most expensive routine parts on a modern vehicle — that's exactly why getting the diagnosis right matters here. |
Related codes
- P0300 — Random Misfire
- P0171 — System Too Lean
- Full OBD-II code index
- Best OBD2 scanners to read this yourself
Related symptoms
Frequently asked questions
Can I drive with a P0420?
In most cases, yes — a P0420 is usually not an immediate safety issue. But it will fail an emissions test in states that run them, and the underlying cause can get worse. Address it within a few weeks rather than months.
Does P0420 always mean a new catalytic converter?
No. Oxygen sensors, exhaust leaks upstream of the sensor, and a history of misfires often cause P0420 without the converter itself being the root fault. Replacing an expensive converter without ruling those out is a common mistake.
What is "Bank 1"?
Bank 1 is the side of the engine that contains cylinder number 1. On inline engines there is only one bank. On V6 and V8 engines your service manual identifies which side is Bank 1 — do not guess.
Will fuel additives fix P0420?
Fuel or catalyst cleaners occasionally help if the converter is only mildly contaminated, but they do not fix bad sensors, exhaust leaks, or a mechanically failed converter. Treat them as a low-cost first try, not a solution.
Is it legal to "delete" the catalytic converter?
In the United States and many other countries, removing or gutting a catalytic converter on a road-going vehicle is illegal and can carry significant fines. Address the root cause instead.
Exhaust components run extremely hot. Never inspect the catalytic converter or oxygen sensors on a warm engine. Removing or "gutting" a catalytic converter on a road-legal vehicle is illegal in most jurisdictions. This article is educational and does not replace a certified mechanic.
Last reviewed and updated: . Reviewed following the WrenchPulse editorial process for technical accuracy.