OK to keep driving short-term — the car normally runs fine and this is an emissions fault, not a safety one. Fix any misfire or rich-running problem quickly, though, so it doesn't cook the converter further, and repair before an emissions test. P0422 means the engine computer has judged the main (under-floor) catalytic converter on bank 1 to be worn out, based on how closely the sensor after the converter follows the sensor ahead of it. It is the same catalyst-efficiency test as P0420, but aimed specifically at the larger main converter rather than the small warm-up cat covered by P0421.
What P0422 means
On engines that use two converters per bank — a small close-coupled warm-up catalyst near the manifold and a larger main catalyst further downstream — the PCM runs a separate efficiency check on each one. P0422 targets the main under-floor converter on bank 1. A healthy three-way catalyst stores and releases oxygen, so the oxygen sensor mounted after the main converter should hold a fairly steady voltage even while the sensor ahead of it swings rapidly between rich and lean. The computer counts how often each sensor's signal inverts once the converter is hot — typically above roughly 400 °C (752 °F) at a steady moderate speed and light-to-moderate load. When the rear sensor begins switching nearly as often as the front one, the calculated oxygen-storage figure falls below the stored limit and the PCM records P0422. It usually takes two consecutive failed monitor runs before the check-engine light comes on.
Symptoms
- Check-engine light on, most often with no noticeable change in how the vehicle drives
- Vehicle fails a tailpipe or OBD-II emissions inspection despite running normally
- Occasional faint rotten-egg or sulfur smell from the exhaust as the converter breaks down
- Slightly reduced fuel economy in some cases, usually when an underlying fueling fault is also present
- A rattle or ticking from underneath if the main converter's internal honeycomb has cracked or crumbled
Common causes
- Aged or worn-out main (under-floor) catalyst on bank 1 that has simply lost its oxygen-storage capacity with mileage
- A lazy, contaminated or aged downstream oxygen sensor whose signal falsely tracks the upstream sensor
- An exhaust leak between the converters or near the rear oxygen sensor, drawing in outside air that skews the reading
- Upstream misfire or rich/lean fueling faults that overheat and chemically poison the main converter over time
- Engine oil or coolant entering the exhaust from an internal leak, coating and deactivating the catalyst substrate
- A leaking fuel injector, contaminated fuel or a silicone/lead-poisoned catalyst from bad additives or a coolant leak
- A loose or incorrectly installed oxygen sensor letting the monitor see false switching activity
Severity & driving advice
Severity: Low — Safe to drive short-term and usually no drivability change, but the vehicle will fail emissions; fix any underlying fuel or misfire fault promptly.
Can I drive? OK to keep driving short-term — the car normally runs fine and this is an emissions fault, not a safety one. Fix any misfire or rich-running problem quickly, though, so it doesn't cook the converter further, and repair before an emissions test.
Diagnostic approach
- Scan for companion codes and record freeze-frame first — Before clearing anything, save the freeze-frame conditions and monitor test results, then read all stored codes. Misfire, fuel-trim, oxygen-sensor or exhaust-leak codes will all fool the catalyst monitor and must be repaired first. Treat P0422 as the last suspect, not the first — condemning the converter while a fueling fault remains just kills the replacement too.
- Confirm the code targets the main cat, not the warm-up cat — On dual-converter systems, P0421 covers the small warm-up (close-coupled) catalyst and P0422 the larger main under-floor catalyst, both on bank 1. Identify which converter the rear oxygen sensor is measuring so you inspect and, if needed, replace the correct one rather than the wrong section of the exhaust.
- Compare front and rear oxygen sensor signals on live data — With the engine fully warm and lightly loaded, graph the upstream air-fuel (or front O2) sensor against the sensor after the main converter. A good main catalyst lets the front signal swing quickly while the rear stays comparatively flat and slow. If the rear sensor mirrors the front's rapid switching almost one-for-one, the converter has lost its oxygen-storage ability.
- Inspect the exhaust for leaks and loose sensors — A pinhole leak or a loose oxygen sensor anywhere ahead of the rear sensor pulls in outside air and makes even a good converter fail the monitor. Check the manifold, converter flanges, gaskets and both sensor bungs for leaks and looseness; retorque or reseal as needed and confirm the sensors are tight and correctly installed before spending money on a catalyst.
- Verify the monitor actually completed under the right conditions — The catalyst monitor only runs when the converter is hot (roughly above 400 °C / 752 °F) at a steady moderate rpm and light-to-moderate load. Take the vehicle for a sustained steady-speed drive so the readiness monitor can run to completion, rather than judging the result from a cold start or short trip where the test never finishes.
- Test or substitute the main catalyst, then re-verify after a drive cycle — If fueling is correct, there are no exhaust leaks, and the rear sensor tracks the front, the main converter is the likely fault. Confirm with an inlet-versus-outlet temperature comparison across the main cat or an oxygen-storage test, replace the converter if it fails, clear the code and complete the drive cycle. The light needs two consecutive failed cycles to relight, so a clean re-run confirms the repair held.
Make & model notes
Ford: On 5.0L and other Ford V8/V6 applications with a close-coupled plus main converter layout, the PCM runs the catalyst monitor per converter using front air-fuel and rear HO2S switch-ratio data once the cat is hot. Ford diagnosis checks for exhaust leaks, a lazy downstream sensor and any misfire or fuel-trim fault before condemning the main catalyst.
Chrysler / Jeep / Dodge: On Chrysler-group engines with two converters per bank, P0421 flags the upstream warm-up cat and P0422 the main downstream cat on bank 1 (P0430-series covers bank 2). Rule out an intake or exhaust leak and a slow rear O2 sensor first, since both commonly trigger a false efficiency failure.
Toyota: Most Toyotas evaluate the whole converter with P0420/P0430 using an oxygen-storage (OSC) calculation rather than a separate main-cat code, but where P0422 is reported the same logic applies. Toyota advises replacing the front and rear catalyst sections together where they are combined in one assembly.
FAQ
What is the difference between P0422 and P0420?
Both come from the same catalyst-efficiency monitor, which compares how closely the rear oxygen sensor mimics the front one. P0420 is the generic 'catalyst system efficiency below threshold' code for bank 1, while P0422 specifically names the main (under-floor) converter on bank 1 on vehicles that also have a separate warm-up cat covered by P0421. In practice they point to the same worn-converter or false-reading causes.
How is P0422 different from P0421?
P0421 refers to the small warm-up (close-coupled) catalyst mounted right at the exhaust manifold, which heats up fast to control cold-start emissions. P0422 refers to the larger main catalyst further downstream on the same bank. On a system with both converters you can set one code without the other, so it's important to identify which converter the code is measuring before replacing anything.
Can I keep driving with a P0422 code?
Usually yes in the short term — the vehicle typically drives normally and the fault is about emissions rather than safety. The problem is that it will fail an emissions test, and if the real cause is a misfire or rich-running condition, continuing to drive can overheat and further destroy the converter. Fix any underlying fuel or ignition fault promptly.
Does P0422 always mean I need a new catalytic converter?
No. A downstream oxygen sensor that has gone lazy, an exhaust leak near the sensor, or an upstream fueling fault can all make a healthy main converter look bad to the monitor. Check for other codes, verify the sensors and exhaust are sound, and compare the front and rear sensor signals on live data before replacing the catalyst.
Why did the check-engine light come back after I cleared P0422?
The catalyst monitor only runs under specific conditions — a hot converter, steady moderate speed and light load — and normally needs two consecutive failed drive cycles before it relights the light. If the actual fault wasn't repaired, the monitor simply fails again on the next qualifying drive and resets the code.