P0420 Honda: Causes, Diagnosis & Whether You Can Drive
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What it meansCatalyst efficiency below threshold (bank 1) — the converter isn’t cleaning the exhaust well enough.
Most likely fixA lazy downstream O2 sensor — or an aging catalytic converter.
Safe to drive?Yes, short-term — but it won’t pass emissions.
Typical cost$200–$2,200 (sensor vs. converter).
Diagnose before you buy a converter — a worn downstream O2 sensor or an exhaust leak can set P0420 even when the converter is fine.
What P0420 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.
Because a catalytic converter is expensive and an oxygen sensor is cheap, diagnosis order matters: rule out a lazy downstream O2 sensor and any exhaust leak before condemning the catalytic converter.
Common causes most common first
Failing downstream O2 sensor (the missed cheap cause)
Worn catalytic converter
Exhaust leak near the sensors
Untreated misfire / rich mixture that cooked the cat
Aftermarket / non-spec exhaust parts
Do these checks first
Read live O2 data with a scanner: a healthy downstream sensor is steady; if it swings like the upstream one, suspect the converter.
Check for exhaust leaks before the rear O2 sensor — a leak can fake a P0420.
Rule out the cheap downstream O2 sensor before condemning the expensive converter; clear any misfire codes first.
A downstream O2 sensor is $200–$350 installed. A catalytic converter is one of the priciest Honda repairs at $900–$2,200 — so confirm the cause before replacing it.
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FAQ
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.