P0420 code: Catalyst system efficiency below threshold (bank 1)

What P0420 means

P0420 is a generic powertrain code defined by SAE J2012: Catalyst System Efficiency Below Threshold (Bank 1). The P means powertrain, the 0 means it is a generic code that reads the same on every OBD2 vehicle, and the 4 places it in the auxiliary emissions controls group. Bank 1 is the side of the engine with cylinder 1. On a 4-cylinder engine there is only one bank, so P0420 simply refers to the main converter.

The engine computer judges the converter by comparing the upstream O2 sensor (before the catalyst) with the downstream sensor (after it). A healthy converter stores oxygen, so the downstream signal stays fairly steady while the upstream signal switches rich and lean. When the downstream sensor starts to copy the upstream switching, the computer concludes the converter has lost its oxygen storage capacity and sets P0420. You can confirm the wording for any code with our code decoder.

Symptoms of P0420

Most cars with P0420 feel normal. The code is an emissions efficiency test, not a drivability fault, so the main sign is the check engine light. When symptoms do show up, they often point to the cause rather than the code itself.

• Check engine light on, usually steady rather than flashing
• Failed emissions or smog inspection, or catalyst monitor not ready
• Rotten egg (sulfur) smell from the exhaust on some vehicles
• Rattle from under the car if the converter substrate has broken apart
• Loss of power or poor acceleration if a damaged converter is restricting exhaust flow
• Other codes stored alongside it, such as misfire, lean or O2 sensor codes

Common causes of P0420

Work from the cheapest and most likely cause to the most expensive. A converter rarely fails on its own; something upstream often damaged it, and replacing it without fixing that cause can kill the new one too.

How to diagnose P0420

A scan tool that shows live data and freeze frame makes this job much easier. A basic code reader only tells you the code; graphing the two O2 sensors side by side is what tells you whether the converter or a sensor is at fault.

1. 1

   Read all stored and pending codes. Fix any misfire, fuel trim, O2 sensor or coolant temperature codes first, because they can set P0420 or damage the converter.

2. 2

   Save the freeze frame data. Note engine temperature, speed and load when the code set.

3. 3

   Inspect the exhaust from the manifold back to the downstream sensor for leaks, cracks and loose clamps.

4. 4

   Warm the engine fully, then graph bank 1 sensor 1 and bank 1 sensor 2. The upstream sensor should switch quickly; the downstream sensor should be steadier.

5. 5

   If the downstream sensor switches almost as fast as the upstream one with no leaks and normal fuel trims, the converter is the likely cause.

6. 6

   If the downstream sensor is flat, slow or stuck, test or replace that sensor before condemning the converter.

7. 7

   After any repair, clear the code and complete a drive cycle so the catalyst monitor can run again.

Scanners that graph O2 sensor data →

How to fix P0420 for good

The lasting fix is whatever the diagnosis points to. If the cause was an exhaust leak or a failed downstream sensor, that repair alone often clears the code. If the converter really is worn out, replace it with a part that is legal in your state. Some states, including California, require specific CARB-approved converters, so check the label and listing before you buy.

Avoid shortcuts such as O2 sensor spacers, defoulers or tuning that hides the code. They can make the car illegal to drive on public roads and will not pass a proper inspection. Fuel additives sold as converter cleaners are not a reliable fix for a converter that has lost its catalyst coating.

Note.A catalytic converter runs extremely hot. Let the exhaust cool fully before touching sensors or clamps, and support the car on proper jack stands if you work underneath.

Can you drive with P0420?

In most cases you can drive with P0420 for a short time while you diagnose it, because the code alone does not mean the engine is being harmed. Watch for warning signs that change the picture: a flashing check engine light means an active misfire that can overheat and destroy the converter, and a loss of power or a red-hot converter can mean it is clogged. In those cases, stop driving and get it checked. Read more in can you drive with the check engine light on.

Related codes

P0430 is the same test for bank 2; see P0420 vs P0430. Misfire codes like P0300 and fuel mixture codes like P0171 and P0172 often come before a converter failure. An O2 sensor heater code such as P0141 can also affect the downstream sensor reading.

Frequently asked questions

Will clearing P0420 make it go away? Clearing the code turns off the light, but if the fault is still there the code returns once the catalyst monitor runs again, often within a few drive cycles. Clearing also resets readiness monitors, so the car will not pass inspection right away.

Is P0420 always a bad catalytic converter? No. Exhaust leaks, a worn downstream O2 sensor and engine problems that upset the air-fuel mixture can all set P0420. Check those first because they are cheaper to fix.

Can a bad O2 sensor cause P0420? Yes. A slow or faulty downstream sensor can give a reading that looks like a failing converter. Graphing both sensors in live data helps tell the difference.

What scan tool do I need for P0420? Any tool that shows live O2 sensor data and freeze frame will do. See our best OBD2 scanners for DIY mechanics, or a bidirectional scan tool if you also want to run tests.