You plug in your OBD scanner and it spits out P0420. The full name is "Catalyst System Efficiency Below Threshold (Bank 1)", which tells you roughly nothing useful. Here's what it actually means and what you should do about it.
What the code is telling you
Your car has a catalytic converter in the exhaust system that cleans up combustion gases before they leave the tailpipe. To know if it's working properly, the engine management system uses two oxygen sensors, one before the cat, one after.
When everything is fine, the post-cat sensor should read a fairly steady voltage because the cat is processing the exhaust cleanly. When P0420 triggers, the downstream sensor is seeing too much variation, meaning the converter isn't doing its job anymore.
P0420 is specifically Bank 1, on four-cylinder cars there's only one bank, but on V6 and V8 engines Bank 1 is the side that contains cylinder 1.
Why it's so common
P0420 is one of the most frequently scanned codes, and there are a few reasons for that. Catalytic converters wear out over time, most last 150,000 to 200,000km, but that drops fast if the engine runs rich, burns oil, or has a coolant leak. Anything that dumps unburnt fuel or contamination through the exhaust will poison the cat.
But here's the thing: P0420 doesn't always mean the cat is dead. Other problems can trigger the same code.
Causes, start here before buying a new cat
Failed catalytic converter, the most obvious cause, but check everything else first because a new cat costs $400 to $2,000+ depending on your car.
Bad oxygen sensor (downstream), if the post-cat O2 sensor is reading incorrectly, the ECU sees bad data and throws P0420 even though the cat is fine. An O2 sensor is $60 to $180 and is worth checking before condemning the converter.
Exhaust leak before the downstream sensor, a crack or loose joint anywhere between the engine and the rear sensor can let in fresh air and mess up the reading. Run the engine with it cold and listen for ticking at the manifold or along the pipe.
Engine misfires, if the engine is misfiring and sending unburnt fuel into the exhaust, it will damage the converter quickly and trigger P0420. Check for misfire codes (P0300 series) alongside this one.
Wrong fuel, using E85 in a car not tuned for it, or low-quality fuel with sulfur content, degrades catalytic converters faster than normal.
Oil burning, if your engine is burning oil and sending it through the exhaust, the cat gets contaminated. Blue smoke from the exhaust and a dropping oil level are the signs.
How to diagnose it yourself
With a basic OBD scanner, you can watch the live data from both oxygen sensors simultaneously. The upstream sensor should fluctuate rapidly between roughly 0.1v and 0.9v as the engine management system hunts for the right fuel mixture. The downstream sensor should stay relatively flat around 0.6-0.7v if the cat is doing its job.
If the downstream sensor is oscillating just as fast as the upstream one, the cat isn't converting anything. If the downstream sensor is flat but at the wrong voltage (consistently near 0 or consistently near 1v), the sensor itself is probably faulty.
That cat is definitely dead.
Can you drive with P0420?
Technically yes, the car will run. P0420 doesn't affect drivability directly, you won't feel any different performance (assuming no other codes are causing it). However:
- You'll fail an emissions test
- If a misfire or rich running condition is causing the code and you ignore it, the underlying problem will get worse and damage an expensive component further
- In some states and countries, driving with a known emissions fault can result in fines
If P0420 is the only code and the car runs normally, you have time to diagnose it properly rather than rushing a repair. If it came in alongside misfire codes or you can smell fuel or see odd smoke, sort those out first.
Repair costs
| Repair | Cost (AUD) |
|---|---|
| Downstream O2 sensor | $180-$350 |
| Exhaust leak repair | $150-$400 |
| Aftermarket catalytic converter | $400-$900 |
| OEM catalytic converter | $900-$2,500+ |
| Full replacement inc. Labour | $600-$3,000 |
Costs vary significantly by car make and model. European brands with tight under-bonnet packaging and proprietary parts sit at the top end. Japanese economy cars are usually on the cheaper side.
What makes P0420 worse on certain cars
Some platforms are notorious for P0420. Toyota and Lexus vehicles from the early 2000s triggered it so often that Toyota issued technical service bulletins with software updates to raise the threshold before condemning the converter. If you have one of those, an ECU reflash can clear persistent false codes.
BMWs are prone to triggering P0420 when the high-pressure fuel injectors start leaking slightly and running the engine rich. On those cars, sort the fuel system first.
Subarus with head gasket issues routinely burn coolant through the exhaust and kill converters, if you have a Subaru with P0420, check for white exhaust smoke and whether your coolant level is dropping.
Ask TorqueBot
Got P0420 on your specific car? Tell TorqueBot the year, make, and model, we can tell you which cause is most common for that engine, what sensors fit your car, and get you a parts estimate before you spend anything.