P0420 is one of the most common check engine codes, and one of the most dreaded, because people associate it with an expensive catalytic converter. Sometimes the converter really is worn out. But P0420 can also be caused by cheaper problems, so it pays to understand what the code actually means before spending money.
What P0420 means
P0420: Catalyst System Efficiency Below Threshold (Bank 1).
Your car has oxygen sensors before and after the catalytic converter:
- The upstream sensor (before the converter) sees exhaust straight from the engine. Its signal switches up and down rapidly as the engine adjusts the fuel mixture.
- The downstream sensor (after the converter) should see a much steadier signal, because a healthy converter stores and releases oxygen as it cleans the exhaust.
The engine computer compares the two. If the downstream signal starts to look too much like the upstream one, it concludes the converter isn't working efficiently enough and stores P0420.
"Bank 1" is the side of the engine that contains cylinder 1. Four-cylinder engines in a line have only bank 1. V6 and V8 engines have two banks, and the matching code for bank 2 is P0430.
Symptoms
Often there are no symptoms at all apart from the check engine light. You might also notice:
- a failed emissions inspection,
- a rotten-egg (sulphur) smell from the exhaust,
- reduced performance or a rattle from under the car, if the converter is damaged internally.
If the engine is also misfiring or the light is flashing, deal with that first. See steady vs. flashing check engine light.
Common causes of P0420
1. A worn or damaged catalytic converter
Converters lose efficiency with age and mileage. They can also be damaged by:
- misfires, which send unburned fuel into the exhaust where it overheats the converter,
- oil or coolant burning, which coats and contaminates the catalyst,
- physical damage from impacts.
2. A faulty downstream oxygen sensor
If the sensor after the converter is slow or inaccurate, the computer may misjudge the converter's performance. Sensors are much cheaper than converters, so they're worth checking first.
3. Exhaust leaks
A leak near the oxygen sensors can let outside air in and distort the readings. Listen for ticking or puffing sounds from the exhaust, especially when the engine is cold.
4. Engine running problems
Anything that makes the mixture run rich or lean can affect converter readings, including misfires, vacuum leaks and leaking injectors. Fixing these is essential, because a new converter fitted to an engine with an underlying problem can fail again.
5. Software
On some vehicles, manufacturers have released software updates or technical service bulletins related to catalyst monitoring. A dealer can check whether any apply to your car.
How to diagnose P0420
- Read all codes, not just P0420. Misfire, lean or rich codes point to the root cause. See how to read OBD-II codes.
- Fix other faults first, especially misfires. Our P0300 random misfire guide covers the usual causes.
- Check for exhaust leaks around the manifold, joints and sensors.
- Look at oxygen sensor data with a scanner that shows live data. A mechanic will compare the upstream and downstream signals with the engine fully warm.
- Inspect the converter for damage, rattles or overheating discolouration.
- Check for technical service bulletins for your make and model.
How to fix P0420
The fix depends on the cause:
- Exhaust leak: repair the leaking joint, gasket or pipe.
- Faulty downstream O2 sensor: replace the sensor.
- Engine problem: fix the misfire, leak or fuel problem, then re-test.
- Worn converter: replace it.
A note on converter replacement
Catalytic converter prices vary enormously by vehicle. Aftermarket parts are usually cheaper than original-equipment parts, while some vehicles need specific, more expensive units. Emissions rules also differ by country and state. Some places, such as California, only allow approved converters for certain vehicles. Ask your garage which type is legal for your car where you live.
As a rough guide, a sensor or exhaust repair is usually a modest cost. A converter replacement can range from a few hundred to well over a thousand in local currency once labour is included, depending on the car. Get a written quote. GrepKit's free car checker shows typical local price ranges, and its quote checker helps you judge whether a price is fair.
Things that usually don't fix P0420
- Fuel additives marketed as converter cleaners rarely cure a genuinely worn catalyst.
- Clearing the code turns the light off temporarily, but it returns once the monitor runs again.
- Removing the converter or fitting "test pipes" or sensor spacers to fool the system is illegal for road use in many countries, and increases pollution.
Is it safe to drive with P0420?
Usually, for a short time, if the light is steady and the car drives normally. The converter is an emissions device, so a mildly inefficient one doesn't usually stop the car running. However:
- a blocked converter can restrict the exhaust, causing power loss and overheating,
- the underlying cause, such as misfires, can get worse,
- you may fail an emissions inspection.
Get it diagnosed rather than living with the light, because a permanent light also hides any new fault that appears.
Frequently asked questions
Does P0420 always mean I need a new catalytic converter?
No. A worn converter is a common cause, but exhaust leaks, a faulty downstream oxygen sensor and engine running problems can all trigger P0420. Rule those out first.
What's the difference between P0420 and P0430?
They mean the same thing for different sides of the engine. P0420 refers to bank 1 (the side with cylinder 1) and P0430 refers to bank 2, which only exists on V-shaped or flat engines.
Can a bad oxygen sensor cause P0420?
Yes. A slow or faulty downstream sensor can make a healthy converter look inefficient. Replacing the sensor fixes the code in those cases.
Will P0420 fail an emissions test?
In many places, yes, because the check engine light is on and a fault code is stored. Requirements vary, so check the rules where you live.