P0420 Code: Catalyst Efficiency Below Threshold, What to Check

Last updated October 28, 2026 · By OBD Scanner Guide Editorial Team

P0420 means your engine computer ran its catalytic converter test on bank 1 and the converter failed it. The short answer on what to do: do not buy a converter first. Confirm there are no misfire or fuel trim codes, check the exhaust for leaks ahead of the rear oxygen sensor, and look at both oxygen sensors’ data. If those are clean and the code returns, the converter is the likely cause, and federal and state rules decide what replacement you can legally fit. This is an editorial, spec-based guide built from federal regulations, a factory service manual and manufacturer pages; no testing was done.

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What P0420 actually measures

Your converter stores oxygen when the mixture runs slightly lean and releases it when the mixture swings rich, which is how it burns off hydrocarbons and carbon monoxide. The oxygen sensor ahead of the converter (upstream) switches back and forth as the engine computer trims fuel. A healthy converter smooths that out, so the sensor behind it (downstream) reads fairly steady. As the converter ages and loses storage capacity, the rear sensor starts switching along with the front one.

That comparison is the test. Mazda’s workshop manual for the Mazda 6, for example, describes the computer counting how often the rear sensor switches while the front one switches a set number of times, and judging the catalyst deteriorated when that ratio crosses a threshold. The test only runs inside set conditions, in that manual roughly 1,300 to 3,000 rpm and 15 to 50 percent engine load, which is why it often needs steady cruising to complete.

The federal rule behind it, 40 CFR 86.1806-05, requires the system to flag a gasoline catalyst before hydrocarbon emissions exceed 1.5 times the standard compared with a representative 4,000-mile converter. So P0420 is a “your converter is past its limit” verdict, not “your converter is gone”.

Exact wording of sensor codes varies by maker; read the definition your scanner shows for your vehicle.

Before you replace the converter: the check order

1. Clear up other codes first

Read every stored and pending code, and save the freeze frame. If you see misfire codes, fix those first and read P0300 random misfire. Unburned fuel from a misfire overheats the converter; the federal rule singles out “any misfire capable of damaging the catalytic converter”, and EPA’s owner brochure says a flashing check engine light means exactly that kind of severe problem. A lean or rich code changes what the oxygen sensors read and can trigger P0420 by itself; see P0171 system too lean.

A new converter on an engine that is still misfiring or burning oil is a converter you will buy twice.

2. Check for exhaust leaks ahead of the rear sensor

A leak at the manifold, flex pipe or a gasket between the engine and the rear sensor can pull outside air into the exhaust on each pulse. That extra oxygen confuses the rear sensor and can make a healthy converter look bad. Listen for ticking at cold start, look for black soot streaks at joints, and check the hangers. Do this with the engine cool; exhaust parts get hot enough to burn skin quickly.

3. Look at both oxygen sensors in live data

This is where a scanner with graphing earns its keep. With the engine fully warm and held at a steady 2,000 to 2,500 rpm, or on a steady cruise with a helper watching, the upstream sensor should switch rapidly while the downstream one stays comparatively flat. If both lines move together, the converter is probably not storing oxygen. If the rear sensor is flat but stuck at one extreme, or slow to respond when you snap the throttle, suspect the sensor.

Live data on an OBD2 scanner explains how to set up a two-sensor graph and what normal looks like.

4. Check Mode 6 if your tool supports it

Mode 6 shows the raw results of the car’s self-tests, each with a measured value and minimum and maximum limits. A Vehicle Service Pros explainer on Mode 6 uses P0420 as its example: the catalyst monitor result for bank 1 tells you how far past the limit the converter is. A result just over the line after a repair tells a different story from one far outside it. Be aware that many generic tools list Mode 6 without translating the test IDs, so you may need the vehicle’s service information to read them.

5. Look for a software update

Carmakers sometimes revise the converter test calibration. A dealer or a technical service bulletin search for your exact vehicle can tell you whether an engine computer update applies before you buy hardware.

Other tools that cover the same checks: the BlueDriver Bluetooth scanner lists live data and Mode 6 in its app, and the FOXWELL NT301 graphs live data on a corded handheld. Best OBD2 scanner compares them.

Common causes, in rough order of what to rule out

These are common patterns, not a diagnosis for your vehicle. The service manual’s diagnostic tree for your engine governs.

If the converter is the problem: the rules on replacement

Check the emissions warranty first

EPA’s emission warranty fact sheet says that on 1995 and newer light-duty vehicles, specified major emission control components, which include the catalytic converter, are covered for 8 years or 80,000 miles, whichever comes first, when the failure is a defect in materials or workmanship. California vehicles may have extra coverage. If your car is inside those limits, take it to a dealer before you spend anything.

Federal rules for aftermarket converters

EPA’s aftermarket converter fact sheet says only converters that meet its policy may be sold and installed, and generally only in three cases: the converter is missing, a state or local inspection program found it needs replacing, or the vehicle is past 5 years or 50,000 miles (8 years or 80,000 miles for 1995 and newer vehicles) and a legitimate need has been documented. It must go in the same location and be the same type as the original, and a used converter from a salvage yard may only be installed if it has been tested and labeled. Shops must keep a signed statement of why the converter was replaced.

California and states that follow it

In California, an aftermarket converter must carry a CARB Executive Order exemption for your exact vehicle, and CARB’s aftermarket converter page says no used converter may be sold or installed there after July 10, 2008. You match a legal part by make, model, model year, engine size and the test group or engine family on the underhood emissions label. Some other states have adopted the California rules for converters, so check your state’s environmental agency before ordering.

After the repair: confirm it, do not just clear it

1. Clear the codes only after the repair, and note the date.
2. Drive normally, including steady cruising, so the catalyst monitor can run. It is one of the slower non-continuous monitors.
3. Recheck readiness and pending codes after a few days of mixed driving. A pending P0420 means the test ran and failed again.
4. Only go for an emissions test once the catalyst monitor shows complete. Readiness monitors and smog checks explains the allowances by model year.

Mistakes to avoid

• Replacing the converter while a misfire is still present. It is the fastest way to ruin the new one.
• Fitting the wrong part for your state. A federal-only converter is not legal on a California-certified car in California.
• Assuming an “O2 sensor spacer” fix is legal. Devices meant to fool the catalyst monitor can count as tampering with an emission control under the Clean Air Act, and they do not fix the converter. Do not rely on one.
• Clearing the code before an inspection. It resets readiness and usually earns a “not ready” result instead of a pass.
• Skipping the warranty check. Converters on cars inside 8 years or 80,000 miles may be covered.

Verdict

P0420 is a test result, not a part number. Rule out misfires, fuel trim faults, exhaust leaks and a bad rear sensor with a scanner that graphs both oxygen sensors and, ideally, shows Mode 6. If the converter is truly worn out, check the federal emissions warranty, then buy a converter that is legal in your state for your engine family, and let the catalyst monitor complete before you test.