P0422: Main Catalyst Efficiency Below Threshold, Bank 1 (Meaning, Causes, Fixes)
What P0422 means
The catalytic converter turns harmful exhaust gases (hydrocarbons, carbon monoxide and nitrogen oxides) into water vapor, carbon dioxide and nitrogen. OBD-Codes.com explains that the engine computer watches converter efficiency with the oxygen sensor after the converter, which should switch much more slowly than the front sensor; if the rear sensor switches too quickly, there is no difference between what goes into and comes out of the converter, so the computer decides it isn't working. RepairPal describes the same check as the rear sensor reading starting to match the front sensor, and says the code means the bank 1 main converter's efficiency has fallen below the minimum allowed limit. Bank 1 is the side of the engine that has cylinder 1; on a four-cylinder engine there is only bank 1. P0420 is the general bank 1 catalyst efficiency code, P0421 is the bank 1 warm-up (close-mounted) converter, and P0432 is the main converter on bank 2.
Severity: can I keep driving?
Drive with caution, fix soon
Usually drivable short-term, but get it checked soon. Sources differ on how serious it is: RepairPal rates P0422 Urgent, yet also says that in many cases you won't notice anything except the check engine light, and OBD-Codes.com lists only higher tailpipe emissions and the light. RepairPal warns that converters usually fail because of something else, such as a misfire or an internal oil or coolant leak, so find that cause before it ruins a new converter. If the check engine light is flashing, the engine is misfiring and you should stop driving soon. We treat P0422 as fix soon, not as an emergency.
Common symptoms
- Check engine light on (both sources)
- Often no other symptoms you can notice (RepairPal)
- Higher tailpipe emissions, which can fail an emissions test (OBD-Codes.com)
Likely causes
Roughly most to least common, based on the sources below. Your vehicle may differ.
- Worn-out, damaged or non-functional catalytic converter (both sources)
- Engine not running at the right air-fuel ratio (OBD-Codes.com)
- Engine misfires that damaged the converter (RepairPal)
- Internal engine damage causing high oil consumption or a leaking head gasket that damaged the converter (RepairPal)
- A low-quality aftermarket replacement converter the car's computer won't accept (OBD-Codes.com, if the code returns after a converter replacement)
Diagnostic steps you can do
Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.
- Read all stored codes. If misfire codes (P0300 to P0304) or fuel trim codes such as P0171 or P0172 are also present, fix those first, since OBD-Codes.com says an engine running outside the right air-fuel ratio can set this code and RepairPal says misfires commonly destroy converters.
- Watch for signs the engine is burning oil or coolant, such as blue or white exhaust smoke or a dropping coolant level. RepairPal says replacing the converter when internal engine damage is the real problem is a common misdiagnosis.
- Check the bank 1 oxygen sensors with a scan tool on a warm engine. The rear sensor should switch much more slowly than the front one; if it switches almost as fast, the converter is likely not storing oxygen. RepairPal cautions that oxygen sensors are often replaced when the real problem is a damaged converter, so test before buying sensors.
- OBD-Codes.com notes that a tune-up or engine repair may clear the code if the engine was running too rich or too lean.
- Check your emissions warranty before paying for a converter. RepairPal cites a federally mandated 8 year, 80,000 mile warranty on catalytic converters, though the problem that caused the failure is usually not covered.
Typical fixes
- Repair any misfire, fuel mixture problem, oil burning or coolant leak first (both sources)
- Replace the bank 1 main catalytic converter once other causes are ruled out (both sources). OBD-Codes.com warns that some makes, especially Toyota and Subaru, are fussy about converter quality, and Subarus need a CARB-certified converter to keep the light off
Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.
🛠 When to call a pro
- The check engine light is flashing, or the engine runs rough or misfires
- The engine is burning oil or losing coolant, which RepairPal says can destroy the converter
- The code came back after a converter was replaced, which OBD-Codes.com says can mean the replacement converter isn't good enough for that car
- The car may still be under its emissions warranty, so a dealer diagnosis could save you the cost of a converter
- You need to pass an emissions or state inspection test
Where to buy
Plain search links. Always match parts to your exact year, make, model and engine.
FAQs
Can I drive with P0422?
Usually for a short while. RepairPal rates it Urgent but says that in many cases nothing feels wrong besides the check engine light, and OBD-Codes.com lists only higher emissions and the light. Get it diagnosed soon, and stop driving if the check engine light flashes.
What's the difference between P0420, P0421 and P0422?
All three mean catalyst efficiency on bank 1 is below the allowed threshold. P0422 names the main converter, P0421 names the smaller warm-up converter close to the engine, and P0420 is the general bank 1 catalyst code. P0430 to P0432 are the same codes for bank 2.
Should I replace the oxygen sensor for P0422?
Only if testing shows it's bad. RepairPal lists replacing oxygen sensors when the real problem is a damaged converter as a common misdiagnosis.
Why did P0422 come back after I replaced the catalytic converter?
OBD-Codes.com says some cars, especially Toyotas and Subarus, won't accept cheaper aftermarket converters, and Subarus need a CARB-certified converter. It's also possible the original cause, such as a misfire, oil burning or a fuel mixture problem, was never fixed.
How much does it cost to fix P0422?
Neither source gives a repair cost for P0422. It can range from a tune-up or misfire repair to a catalytic converter, so get it diagnosed first.
Sources
Last reviewed 2026-10-25. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.