P0420 Code: Catalyst System Efficiency Below Threshold (Bank 1)

What P0420 means

Your car has an oxygen sensor before the catalytic converter (upstream) and one after it (downstream). A healthy converter stores oxygen, so the downstream sensor reads a steady signal while the upstream one switches rapidly. When the downstream signal starts to copy the upstream one, the computer decides the converter’s efficiency has dropped below its threshold and sets P0420. Bank 1 is the side of the engine with cylinder 1, so inline four- and six-cylinder engines only use bank 1.

Symptoms

• Check engine light, often with no other change in how the car drives.
• Failed emissions test.
• Rotten-egg (sulfur) smell from the exhaust.
• Reduced power or a rattle from under the car, if the converter’s inside has broken up or clogged.

Common causes

• A worn or contaminated catalytic converter.
• A faulty downstream (or upstream) oxygen sensor giving a false reading.
• An exhaust leak near the oxygen sensors.
• An engine problem that damaged the converter: misfires, a rich mixture, or oil or coolant burning in the cylinders.
• Engine computer software, on some vehicles, covered by a service bulletin.

Before you replace parts

1. Read the codes with an OBD-II scanner and write down every code, not just this one. Several codes together often point to one cause.
2. Look at the freeze frame data: engine temperature, speed and load when the code set.
3. Check for technical service bulletins (TSBs) for your vehicle; some codes have known fixes or software updates.
4. Clear the code and drive. If it returns quickly, the fault is current; if it takes days, it may be intermittent.

How to diagnose it

1. Fix any misfire (P0300 series), lean or rich codes first.
2. Check for exhaust leaks ahead of and around the oxygen sensors.
3. With a scan tool, watch both oxygen sensors on a warm engine. A good converter gives a fairly steady downstream reading; a downstream signal that switches like the upstream one points to the converter.
4. Check the oxygen sensor response and heater; a lazy sensor can trigger the code.
5. Check whether your vehicle has an extended emissions warranty on the converter before paying for one.

How to fix it

• Repair exhaust leaks.
• Replace a faulty oxygen sensor (oxygen sensors).
• Replace the catalytic converter if it has failed (catalytic converters). Use a converter that is legal for your state; California and some other states require CARB-approved converters.
• Fix the root cause first, or a new converter can fail too.

Related codes

• P0430: the same fault on bank 2.
• P0300: misfires that can damage the converter.
• P0171 and P0174: lean mixture codes.
• P0135 and P0141: oxygen sensor heater codes.

About this guide

Code definitions here are the generic OBD-II meanings. Manufacturers can add their own details, and the same code can have different common causes on different engines, so use a service manual or a qualified technician for a full diagnosis. Parts named are ones we list; check the vehicle, engine and part number in each listing, or send us your year, make, model and engine and we will check before you order.

Questions

Can I drive with a P0420 code?

Usually yes for a short time, as it rarely changes how the car drives. But the car will fail emissions, and if the cause is a misfire or rich mixture, the converter can overheat. Have it checked soon.

Will a new oxygen sensor fix P0420?

Only if the sensor is the cause. Test both sensors before replacing anything, as the converter itself is the most common cause.

Do catalytic converter cleaners fix P0420?

They rarely fix a worn converter. They may help with light carbon build-up, but the code usually returns if the converter has failed.

Is P0420 covered by warranty?

Many vehicles have a longer federal emissions warranty on the catalytic converter. Check your warranty booklet or ask a dealer.

What is the difference between P0420 and P0430?

They mean the same thing on different sides of the engine: P0420 is bank 1 and P0430 is bank 2.