P0420

What P0420 means

Your car has an oxygen sensor before the catalytic converter and another one after it. The computer compares the two readings to see how much the converter is cleaning up the exhaust. P0420 means the converter on Bank 1 (the side of the engine with cylinder 1) isn't showing enough difference between those two readings. In plain terms, the computer thinks the catalytic converter isn't doing its job as well as it should.

Is it safe to drive

Yes, generally. This is an emissions code, not an engine performance or safety code. Your car will run and drive normally. You won't feel anything different, and you're not going to get stranded because of this one. The main things to watch for: if you start noticing a rotten egg smell from the exhaust, a drop in fuel economy, or the check engine light starts flashing instead of staying solid, that's a step up and worth getting looked at sooner rather than later. Otherwise this can wait a few weeks if that's more convenient. In Washington, a lit check engine light will also fail an emissions test if you're due for one.

What commonly causes it

The obvious answer is a worn-out catalytic converter, and sometimes that is exactly it. But this code gets triggered by other things often enough that we don't want to just quote you a converter without checking first:

• A failing or lazy downstream oxygen sensor giving a false reading, even though the converter is fine
• An exhaust leak upstream of or near the sensors, which throws off the readings
• Engine misfires or a cylinder running rich, which dumps unburned fuel into the converter and damages it over time (this is common on higher-mileage V6 and V8 engines with more than one bank)
• A bad head gasket or oil consumption issue that's slowly killing the converter from the inside
• Simply age and mileage. Converters do wear out.

Which of these is likely depends a lot on the engine. A 4-cylinder with no other symptoms and high mileage often really is just a tired converter. A V6 or V8 throwing this code alongside a rough idle or a misfire code points somewhere else entirely, and replacing the converter first would be the wrong move.

Diagnosis and rough cost

Diagnosis starts with pulling live data from both oxygen sensors to see what the computer is actually seeing, plus a check for exhaust leaks and any misfire codes stored alongside it. That's usually under an hour of shop time. If the sensors and exhaust check out and the converter really is the problem, replacement cost varies a lot depending on the vehicle. Some cars use a simple bolt-on converter, others have it welded into a combined exhaust manifold section, which costs more in parts and labor. We won't know which category your car falls into, or give you a real number, until it's on the lift.

Nick handles diagnostics and driveability work here, and can walk you through what he finds before anything gets ordered. We're open Saturdays too, if that's easier to work around your schedule.