P0420 fault code: what it means, what causes it and whether you can keep driving

What does the P0420 fault code mean?

The P0420 code is the generic OBD2 trouble code for "Catalyst System Efficiency Below Threshold (Bank 1)", as diagnostic data provider Identifix words it (September 2025). The catalytic converter is the part of the exhaust that turns harmful gases into less harmful ones, and the engine computer has judged that the converter on bank 1 is doing that job below the level it was programmed to accept.

Bank 1 is the side of a V6 or V8 engine that holds cylinder number one, and bank 2 is the other side, according to Identifix. A straight four or six cylinder engine has only one bank, so on those engines a catalyst code is always bank 1 (AA1Car). P0430 is the same fault on the bank 2 converter of a two-bank engine.

The letter P stands for powertrain and the 0 that follows marks a standardised SAE code rather than a manufacturer-specific one, according to Haynes. The rest of the decoding, and the routine for reading any code, is in how to use an OBD2 scanner.

How does the car decide the catalytic converter is failing?

A car checks its catalytic converter with two sensors in the exhaust. Sensor 1, the upstream one, sits closest to the engine; sensor 2, the downstream one, sits in or behind the converter (AA1Car). The engine computer reads both and uses them to estimate how much oxygen the converter can still store, according to Samarins (2022).

A working converter stores oxygen, so the signal after it stays much steadier than the signal before it. As the converter loses that capacity, the downstream signal starts to follow the upstream one. When the estimate falls below the standard value, or the two readings fall outside expected parameters, the computer sets P0420 and turns on the check engine light (Samarins; Identifix, 2025).

What causes a P0420 fault code?

A failed catalytic converter causes most P0420 codes, according to Samarins, but it is not the only part that can set the code. Identifix lists a defective oxygen sensor, an exhaust leak, a cracked exhaust manifold, a leaking fuel injector, the wrong fuel type, a faulty coolant temperature sensor or thermostat, a bad spark plug or misfire, high fuel pressure and outdated engine software.

Sources: Samarins (2022), Identifix (September 2025).

• A converter fails for a reason: oil burning, misfires, injector or coolant leaks and a lean mixture wear it out early, and Samarins notes that the cause must be repaired or the new converter fails again.

Can you keep driving with a P0420 fault code?

With only P0420 stored, a steady check engine light and no drivability symptoms, a car can often be driven a short distance to be diagnosed, according to Identifix (September 2025). That is a short-term answer, not a reason to ignore the code: whatever is wearing the converter keeps wearing it.

A flashing check engine light, a misfire, a sharp loss of power or an exhaust that overheats is different. Identifix advises reducing load and getting the car serviced immediately. The reason is the converter itself: the US EPA warns that misfiring can lead to catalytic converter damage.

How much does it cost to fix a P0420 fault code?

The cost of fixing a P0420 code depends on what caused it. An oxygen sensor costs a few hundred dollars; a catalytic converter costs one to several thousand. The table below lists the published US figures, all in US dollars, which come from two sources measuring different things: AAA gives the range of what owners pay, Identifix an average.

Sources: AAA, Identifix (September 2025). Labor at $160 to $200 per hour (AAA).

• What moves the price: the make, model and location, the engine size, OEM versus aftermarket parts (an OEM converter costs about 40 to 70% more, per AAA) and state emissions rules such as California's.
• The cheapest fix: the one that matches the cause, which is why a shop rules out leaks, sensors and misfires before it orders a converter.

Is the catalytic converter covered by warranty in the US?

On 1995 and newer US cars and light trucks, the catalytic converter is covered for 8 years or 80,000 miles (about 129,000 km), whichever comes first. The US EPA lists it with the engine computer and the onboard diagnostic computer as the three specified major emission control components, which the manufacturer must repair or replace free if they fail from a defect in materials or workmanship.

The EPA also says original converters are designed to last the life of a properly tuned and maintained vehicle (EPA fact sheet). The EPA's warranty fact sheet adds a condition: skipping spark plug changes at the scheduled interval can lead to misfiring and converter damage, and the manufacturer may deny coverage when the lack of maintenance caused the failure. Proof of maintenance is not required to make a claim, but the manufacturer may ask for it, which is why keeping your maintenance records pays off.

This warranty is US federal law (Identifix cites 40 CFR 85.2103) and applies to US vehicles. In the UK and elsewhere outside the United States, the coverage is whatever the vehicle's warranty booklet states.

Does P0420 mean the same thing on a Vauxhall, Honda, Toyota or Ford?

Yes. P0420 is a standardised SAE code, the 0 after the P says so (Haynes), so a Vauxhall, a Honda, a Toyota and a Ford all use it for the same fault: low catalyst efficiency on bank 1.

What differs between makes is the fix. Manufacturers sometimes publish service bulletins that recommend a cheaper repair than a new converter, such as a software update, according to Samarins. Before paying for a converter, ask a dealer or shop to check the bulletins for the exact model and year.

Will P0420 fail an MOT emissions test, and should you clear it?

In the US, emissions inspection rules are set by each state, and with P0420 stored the vehicle may fail emissions testing, according to Identifix. Clearing the code does not get around it: clearing codes resets the readiness monitors, and EPA guidelines allow only one monitor to be not ready on a 2001 or newer vehicle, according to the New York DMV. The same logic applies to the MOT emissions test in the UK: do not clear codes just before a test.

The Ohio EPA (a US state agency) advises not clearing codes after a repair when possible: if the repair was right, the check engine light turns off by itself as the car completes its drive cycle. If the codes were cleared or the battery disconnected, it advises two or three days of motorway and town driving before the test. The full rules are in readiness monitors and the drive cycle.

How do you fix a P0420 fault code, step by step?

Fixing a P0420 code means finding why the converter looks weak, then repairing that cause. The order below follows the causes Samarins and Identifix list, cheapest checks first.

• Read every code and the freeze frame: other codes point at a misfire or a sensor, and the freeze frame shows the conditions the engine was in when the fault was saved. WrenchTime, an iPhone app for car and motorcycle maintenance, reads stored, pending and permanent codes and that freeze frame over a Bluetooth Low Energy OBD-II adapter or the OBDLink MX+, as part of WrenchTime Pro.
• Fix any misfire first: spark plugs, coils and injectors, because misfiring can damage a converter (US EPA), including a new one (Identifix).
• Look for exhaust leaks: cracked manifolds, loose joints and leaking gaskets near the sensors.
• Check the oxygen sensors: an ageing front air/fuel sensor or another defective oxygen sensor can set the same code.
• Ask for service bulletins: a software update or a cheaper repair may exist for the exact model.
• Replace the converter last: only once the rest is ruled out, and only after the cause that killed the old one is repaired.
• Let the light go out on its own: then confirm the readiness monitors are complete before an inspection, as the Ohio EPA advises.

Key takeaways: the P0420 code

The whole article in five lines.

• Meaning: Catalyst System Efficiency Below Threshold (Bank 1), a standardised code with the same meaning on every make (Identifix, Haynes).
• Cause: usually a failed converter, but exhaust leaks, oxygen sensors and misfires set it too, and the cause has to be fixed (Samarins).
• Driving: a steady light with no symptoms often allows a short drive to be diagnosed; a flashing light means reduce load and get service now (Identifix).
• Cost: $1,000 to $4,000 for a converter with labour (AAA), $454 to $550 for an oxygen sensor (Identifix).
• Warranty: 8 years or 80,000 miles on the converter for 1995 and newer US vehicles, unless the failure came from skipped maintenance (US EPA).

Read the P0420 fault code and its freeze frame on your iPhone

With a Bluetooth Low Energy ELM327 adapter or the OBDLink MX+, WrenchTime reads the stored, pending and permanent codes, describes generic codes like P0420 in plain language, and shows the freeze frame and the readiness monitors in the same scan. Every scan is saved with its date and odometer reading, and codes can be cleared once the repair is done, only with the engine off and after two confirmations. The OBD-II code reader for iPhone guide walks through it.

Trouble Codes is part of WrenchTime Pro. Log the repair in the vehicle's maintenance history in WrenchTime too: dated service records are what you can show if the manufacturer asks whether scheduled maintenance was done. Download WrenchTime on the App Store.