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P0420: catalyst efficiency below threshold (bank 1)
P0420 means the engine computer has measured that the catalytic converter on bank 1 no longer stores enough oxygen to clean the exhaust properly. The converter is the usual culprit, but an exhaust leak or a tired rear O2 sensor can set the same code for far less money.
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Check your car for P0420
Runs on your device
Connect your OBD2 adapter
- Plug the adapter into the OBD2 port – usually under the steering wheel, near the pedals.
- Switch the ignition on, or start the engine.
- Bluetooth adapter? Turn Bluetooth on. Nothing else to set up.
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Classic Bluetooth adapter (PIN 1234)? Pair it in your system settings first, then choose USB or paired Bluetooth – on a computer or in Chrome 138+ on Android. Wi-Fi adapters cannot work in a browser.
Your car data stays on your device. The page talks to the adapter directly; codes, VIN and values are never uploaded.
Never use it while you drive. Set things up before you set off, or let a passenger handle the phone. Standard OBD2 reads engine and emissions data; ABS, airbag and coding need workshop equipment.
What P0420 means
A catalytic converter stores oxygen and uses it to burn the leftover fuel and carbon monoxide in the exhaust. The engine computer watches it with two O2 sensors: one before the converter, which switches quickly between rich and lean, and one after it, which should stay almost flat because a working converter smooths those swings out.
When the rear sensor starts copying the front one, the converter is no longer storing enough oxygen. Once the computer sees this on two drives in a row, it stores P0420 and switches on the check engine light. Bank 1 is the side of the engine with cylinder 1; a four-cylinder engine has only bank 1. The same fault on the other side of a V6 or V8 is P0430.
Symptoms of P0420
- Usually none: the car drives normally and only the check engine light is on.
- A failed emissions test, because testers read the stored code and the catalyst self-test.
- A smell of rotten eggs (sulphur) from the exhaust, especially after hard acceleration.
- Rattling under the car at idle or when you tap the exhaust: a broken ceramic core.
- Weak acceleration and high fuel use when the converter is partly blocked.
Causes of P0420, cheapest first
Work down the list in order: the first checks cost little or nothing, the last ones are jobs for a workshop.
An exhaust leak before or near the rear O2 sensor
A cracked flange, a leaking gasket or a rusted pipe lets outside air in, and the rear sensor reads it as a weak converter. Listen for ticking or puffing with the engine cold and look for black soot marks around the joints.
A slow or ageing rear O2 sensor
A lazy rear sensor can swing up and down like the front one even when the converter is fine. Compare both sensors in the live graph; an old rear sensor or damaged wiring is a far cheaper part to try than a converter.
A slow front O2 sensor
The test relies on the front sensor switching cleanly. A classic front sensor should flip between about 0.1 and 0.9 V several times a second at a steady 2,500 rpm; a wideband sensor shows a lambda value close to 1.00 instead.
An engine fault that damaged the converter
Misfires, burning oil or a leaking head gasket overheat or coat the converter. Look for other codes such as P0300, check oil and coolant levels, and fix the cause first, or a new converter fails the same way.
A worn-out catalytic converter
After many years the coating simply wears out. If everything above checks out and the catalyst self-test still fails, the converter needs replacing. Choose one approved for your car; cheap universal converters often bring the code back within months.
Can you drive with P0420?
You can usually keep driving. P0420 rarely changes how the car runs, so a few weeks of normal driving are fine. Act sooner if the light flashes, the car loses power, you smell sulphur or hear rattling: then the converter may be overheating or breaking up. Most emissions inspections fail while the code is stored.
Check P0420 with OBD Studio
Connect your adapter above, or try the demo car, then:
- Open Fault codes: the freeze frame shows the speed, load and temperature at the moment P0420 was stored.
- In Live data, graph the front and rear O2 sensor of bank 1 with the engine warm at about 2,500 rpm. A rear sensor that switches almost as fast as the front one confirms a weak converter or a faulty rear sensor.
- In Inspection, tap Read self-tests (cars with CAN): the catalytic converter result shows the measured value against its limit.
- After the repair, clear the code and drive on mixed roads. The catalytic converter monitor in Inspection turns Complete once the car has tested itself again.
More fault code guides
Each of these codes has its own page with causes ranked by cost and the checks that go with them.
- P0171System too lean
- P0300Random or multiple misfire
- P0455EVAP system large leak
- P0442EVAP system small leak
- P0128Engine warms up too slowly
- P0172System too rich
- P0401EGR flow too low
- P0101Air flow sensor implausible
- P0135Front O2 sensor heater
- P0016Crankshaft and camshaft out of step
- P0011Intake camshaft timing over-advanced
- P0141Rear O2 sensor heater
- P0113Intake air temperature sensor high
- P0457Fuel cap loose or missing
- P0299Turbo underboost
- P0700Transmission control system fault
- P0087Fuel rail pressure too low
- P0562System voltage low
- U0100Lost communication with engine computer
More OBD Studio pages
Frequently asked questions
Can a bad O2 sensor cause P0420?
Yes. A rear O2 sensor that has become slow, or whose wiring is damaged, can produce exactly this code while the converter is still fine. That is why it is worth graphing both sensors before you buy a converter.
Will P0420 go away on its own?
If the cause was temporary, such as a short misfire, the light goes out after the car has passed its catalyst self-test on a few drives. A converter that is really worn does not recover, so the light comes back.
Do catalytic converter cleaners fix P0420?
Rarely for good. An additive can burn off light deposits, but it cannot restore a coating that has worn away. Use the live data to find the cause first: an exhaust leak or a sensor costs far less than a converter.
Can I read P0420 without going to a workshop?
Yes. With any ELM327-compatible Bluetooth LE or USB adapter, OBD Studio on this page reads P0420 and every other stored code for free, shows the freeze frame and clears the code after the repair. Nothing to install, no account.
