P0420 Explained: Is Your Catalytic Converter Bad?

P0420 stands for "Catalyst System Efficiency Below Threshold (Bank 1)." It means the engine computer ran its catalytic converter test and the result came in under the limit it was programmed with. That often does mean the converter is worn out, but not always, and a converter is one of the more expensive parts on a car. A careful hour of checking can save you from replacing a good one.

This guide covers what the code actually measures, the causes in rough order of how often they turn out to be the problem, how to test them, and what you are and are not allowed to do about it.

A catalytic converter stores and releases oxygen as it cleans the exhaust. The computer watches two oxygen sensors to judge how well it is doing that:

• The upstream sensor (before the converter) switches rapidly between rich and lean as the computer adjusts the fuel mixture.
• The downstream sensor (after the converter) should stay fairly steady, because a healthy converter evens out those swings.

When the downstream signal starts to copy the upstream signal, the converter is no longer storing much oxygen. After enough failed tests, usually across more than one drive, the computer sets P0420 and turns on the check engine light.

"Bank 1" is the side of the engine with cylinder 1. On a four-cylinder engine there is only one bank. On a V6 or V8, the same fault on the other side is P0430. Seeing both at once points toward something that affects the whole engine, such as misfires, oil consumption or a sensor or software issue, rather than two converters failing on the same day.

Usually yes, in the short term, as long as the check engine light is steady, the car runs normally and there are no other codes. The code itself is an emissions fault.

Two exceptions matter:

• Rattling, a sulfur smell or loss of power. A rattle from under the car can mean the converter's inside has broken up. A blocked converter causes power loss that gets worse as you accelerate.

The practical problems with ignoring P0420 are that you will likely fail an emissions inspection, and a lit check engine light hides any new fault that comes along.

Costs are typical estimates and vary widely by vehicle and region. Converters on some vehicles are part of the exhaust manifold or come in pairs, which raises the price.

The converter itself

Converters wear out from age and heat, but they are also killed by what the engine sends them. Oil burning (from worn rings or valve seals), coolant entering the combustion chamber (often a head gasket), and long periods of misfiring all coat or overheat the catalyst. If you replace a converter without fixing those, the new one will fail too.

Exhaust leaks

A leak at a flange, flex pipe or cracked manifold can pull in outside air between exhaust pulses. Extra oxygen reaching the downstream sensor can make a decent converter look bad. Listen for ticking at cold start that fades as the engine warms, and look for black soot around joints.

The downstream oxygen sensor

A sensor that responds slowly or reads incorrectly can trigger the code, but this is less common than people hope. It is worth testing, not assuming.

Software updates

Some manufacturers have released technical service bulletins (TSBs) that update the catalyst monitor's software for specific models. Ask a dealer service department, or search the NHTSA recalls and investigations database (opens in a new tab), which also lists manufacturer communications for many vehicles.

You need a scanner that shows live data, ideally with graphs. See our guide to cheap OBD-II scanners for what to look for.

1. 1Read all codes and freeze frame. Write them down before clearing anything. Any misfire, fuel trim, oxygen sensor or coolant temperature code gets fixed first.
2. 2Check fuel trims. With the engine fully warm, add short-term and long-term trims. Totals far from zero (beyond roughly plus or minus 10%) suggest a mixture problem that can stress the converter. Lean codes are covered in our P0171 guide.
3. 3Watch both oxygen sensors at a steady 2,000 to 2,500 rpm, warm engine. On a typical narrowband setup the upstream sensor should swing quickly, roughly 0.1 to 0.9 volts. The downstream should sit fairly steady, often somewhere around 0.5 to 0.8 volts. If the downstream line is switching almost as fast as the upstream, the converter's oxygen storage is low. Some newer cars use a wideband (air-fuel ratio) upstream sensor that reads differently, so check your service information for expected values.
4. 4Check Mode 06 results if your scanner supports them. The catalyst monitor test shows the measured value against the limit. A result just barely over the line behaves differently from one that fails by a wide margin.
5. 5Inspect for exhaust leaks from the manifold back to the downstream sensor. A shop can do a smoke test on the exhaust if you cannot find one by sight and sound.
6. 6Confirm the engine is not burning oil or coolant. Track oil level between changes and look for unexplained coolant loss or white, sweet-smelling exhaust.

If the converter really is the problem, buy one that is legal for your car and your state.

• OEM converters fit exactly and carry the manufacturer's emissions design. They usually cost the most.
• Aftermarket converters must be EPA-compliant for your vehicle, and in California and states that follow its rules, they must be CARB-approved for your specific vehicle. A cheap universal converter that is not approved for your car can fail inspection or come back with P0420 quickly.
• Install the converter where the original was and fix leaks at the same time. Replace gaskets and hardware rather than reusing them.

After the repair, clear the codes and drive normally. The catalyst monitor needs specific conditions to run, which can take several days of mixed driving.

• Do not install an oxygen sensor "spacer" or "defouler" to fool the downstream sensor, and do not remove the converter or install a "test pipe" on a road car. Under the federal Clean Air Act, removing or disabling emissions equipment on a road vehicle is tampering. The EPA (opens in a new tab) enforces it, and states can also refuse registration or inspection.
• Do not keep clearing the code to pass inspection. Clearing resets readiness monitors, and most inspection programs reject a car whose monitors are not ready.
• Do not replace the converter while misfire or oil-burning problems remain.

Federal rules require manufacturers to warranty specified major emissions components, including the catalytic converter and the engine computer, for 8 years or 80,000 miles on most light-duty vehicles, whichever comes first. California and states that adopt its rules can add longer coverage for some parts. The exact terms are in the warranty booklet that came with the car. Check before you pay for a repair.

If the car is within that window and the converter failed under normal use, the dealer should repair it under warranty. If they refuse, ask for the decision in writing and contact the manufacturer's customer service line. The FTC's guide to auto warranties (opens in a new tab) explains your rights under federal warranty law.

Log the code, the diagnosis and any repair in your free CarManager garage. If the code returns months later, that history tells you whether the converter or the cause behind it was fixed.