P0420 means the engine computer thinks the catalytic converter on bank 1 isn't cleaning the exhaust as well as it should. Often the converter really is worn out, but exhaust leaks, a faulty downstream O2 sensor or an engine problem like misfires can set the same code. Test before replacing an expensive converter.
What P0420 means
Your catalytic converter cleans the exhaust. There's an oxygen sensor in front of it (upstream) and one behind it (downstream). A healthy converter stores and uses oxygen, so the downstream sensor reading stays fairly steady while the upstream one switches back and forth.
When the downstream sensor starts mirroring the upstream one, the computer decides the converter isn't doing its job and sets P0420: catalyst system efficiency below threshold (bank 1). On V6 and V8 engines, P0430 is the same code for bank 2.
Causes, from most to least likely
| Cause | Notes |
|---|---|
| Worn-out catalytic converter | Common on high-mileage vehicles |
| Exhaust leak near the O2 sensors | Lets in outside air and throws off readings |
| Faulty downstream O2 sensor | Slow or biased sensors can fake a bad converter |
| Engine misfires or rich running | Damage the converter over time and must be fixed first |
| Oil or coolant burning | Contaminates the converter |
| Software issue | Some manufacturers have updates for false P0420s |
Why you should confirm before replacing
Catalytic converters are one of the more expensive exhaust parts, and replacement prices have climbed. If the real problem is an exhaust leak or a lazy O2 sensor, a new converter won't fix the code. And if misfires or rich running killed the old converter, a new one will fail again unless that's fixed first.
How we test it
- Check for other codes. Misfire, fuel trim and O2 sensor codes change the picture.
- Watch both O2 sensors in live data at operating temperature.
- Inspect for exhaust leaks ahead of and around the sensors.
- Check the downstream sensor's response.
- Look for causes upstream: misfires, oil burning, coolant loss.
- Check for manufacturer service bulletins related to P0420 on your vehicle.
Only once those are ruled out do we call the converter bad.
Is it safe to drive with P0420?
Usually, in the short term. The car typically runs normally. But a failing converter can eventually restrict exhaust flow and cost power, and the root cause (like misfires) may be doing more damage. Get it checked.
About converter theft
If your car suddenly sounds like a race car on startup, and especially if it's a truck, SUV or hybrid, the converter may have been cut out. That's a different situation; check under the car and file a police report.
Frequently asked questions
Can I fix P0420 with a fuel additive?
Additives rarely fix a truly worn converter. If the converter is fine and something else caused the code, an additive won't fix that either.
Will P0420 cause my car to fail inspection?
In places that require emissions testing, an active check engine light typically means a failed test. South Carolina doesn't require routine emissions inspections for passenger vehicles, but the code still deserves attention.
Does a P0420 mean I need a new converter right away?
Not necessarily. Confirm the cause first. Many P0420s turn out to be sensors or leaks.
Can you diagnose a P0420 at my house?
Yes. Live-data testing and exhaust inspection can be done on-site.
Want a mechanic to confirm it?
We come to you in Lexington, Irmo, Chapin, Cayce, West Columbia and Columbia, and we test before we replace anything.
