P0420 is one of the most common check engine codes on a Honda, and also one of the most misunderstood. It points at the catalytic converter — but the converter isn’t always the real problem. This guide explains what P0420 means on a Honda, what actually causes it, how to diagnose it without throwing expensive parts at the car, and whether it’s safe to keep driving.
As an Amazon Associate we earn from qualifying purchases. Some links below are affiliate links, which means we may earn a small commission at no extra cost to you.
What the P0420 code means
P0420 stands for “Catalyst System Efficiency Below Threshold (Bank 1).” In plain English, the downstream oxygen sensor — the one after the catalytic converter — is telling the computer that the converter no longer appears to be cleaning the exhaust as well as it should. The related code P0430 is the same fault on bank 2 of a V6 Honda such as the Pilot or Odyssey.
What causes P0420 on a Honda
- A worn catalytic converter — the obvious and most expensive cause, common on high-mileage Hondas.
- A failing downstream oxygen sensor — a lazy sensor reports bad data and sets P0420 even when the converter is fine. This is the cause people most often miss.
- An exhaust leak near the sensors, which lets in extra air and skews the readings.
- An untreated misfire or rich/lean condition that damaged the converter over time.
- Aftermarket or non-compliant exhaust parts that don’t meet the original converter spec.
How to diagnose P0420 (before buying a converter)
Because a catalytic converter is expensive and an oxygen sensor is cheap, diagnosis order matters. Start by reading the live data with an OBD2 scanner that shows sensor voltages, then:
- Check for any exhaust leaks before the rear sensor.
- Compare the upstream and downstream O2 sensor activity — see our Honda O2 sensor guide for what good vs bad looks like.
- Rule out a worn downstream sensor before condemning the converter.
- Only then, if the converter is confirmed bad, look at our Honda catalytic converter guide for replacement options and cost.
Is it safe to drive with a P0420 code?
A steady check engine light with P0420 is generally not an emergency — you can usually keep driving in the short term. But it won’t pass an emissions test, fuel economy may suffer, and if the underlying cause is a misfire, the damage continues. Get it diagnosed soon rather than ignoring it for months.
Frequently asked questions
What does the P0420 code mean on a Honda?
P0420 means catalyst system efficiency below threshold on bank 1. The downstream oxygen sensor is reporting that the catalytic converter does not appear to be cleaning the exhaust effectively.
Does P0420 always mean I need a new catalytic converter?
No. A worn downstream oxygen sensor or an exhaust leak can set P0420 even when the converter is fine. Diagnose before replacing the expensive converter.
Can I drive my Honda with a P0420 code?
A steady light with P0420 is usually safe to drive short-term, but the car will fail emissions and economy may drop. Get it diagnosed soon, especially if a misfire is present.
What is the difference between P0420 and P0430?
They are the same fault on different banks. P0420 is bank 1 and P0430 is bank 2, which applies to V6 Hondas like the Pilot and Odyssey.
How much does it cost to fix a P0420 on a Honda?
It depends on the cause. A downstream oxygen sensor is relatively cheap, while a catalytic converter is one of the more expensive Honda repairs, so confirming the real cause first can save a lot.