Worn-out or defective warm-up catalytic converter (both sources). Engine misfires that have damaged the catalytic converter (RepairPal). An oxygen sensor that is not reading properly (OBD-Codes.com). A fouled spark plug (OBD-Codes.com). Internal oil or coolant leaks that contaminate the converter (RepairPal, as a common reason converters fail).

Repair any misfire, oil burning or coolant leak first (RepairPal). Replace a faulty oxygen sensor if testing confirms it (OBD-Codes.com). Replace fouled spark plugs (OBD-Codes.com). Replace the bank 1 warm-up catalytic converter once other causes are ruled out (both sources).

P0421🌫 Emissions / catalytic converterDrive with caution, fix soon

P0421: Warm Up Catalyst Efficiency Below Threshold, Bank 1 (Meaning, Causes, Fixes)

Q: What does P0421 mean?

P0421 means the engine computer thinks the bank 1 warm-up catalytic converter (the small converter close to the engine that heats up first) is no longer cleaning the exhaust as well as it should. A worn-out or damaged converter is the usual cause, but a faulty oxygen sensor, a misfire or a fouled spark plug can also set it. Many engines have a small warm-up catalytic converter mounted close to the exhaust manifold so it reaches working temperature quickly after a cold start, plus a larger main converter further down the exhaust. RepairPal explains that the engine computer compares the front and rear oxygen sensors to judge converter efficiency: a working converter stores oxygen, so the rear sensor reading stays fairly steady, and if the rear sensor starts to match the front sensor, the computer sets P0421. OBD-Codes.com describes the same thing as the downstream oxygen sensor on bank 1 detecting that the converter is not working as efficiently as specified. Bank 1 is the side of the engine with cylinder 1 (a four-cylinder engine only has bank 1). OBD-Codes.com adds that the code may be more likely after many cold starts in the past day or two. P0420 is the more common general catalyst efficiency code for bank 1, P0422 is the main catalyst on bank 1, and P0431 is the warm-up catalyst on bank 2.

Q: Can I keep driving with P0421?

Drive with caution, fix soon. Usually drivable short-term, but don't ignore it. Sources differ on how serious it is: RepairPal rates P0421 Urgent, while OBD-Codes.com says you will likely not notice any drivability problems. RepairPal also notes some cars lose power. The bigger worry is what damaged the converter: RepairPal says catalytic converters usually fail because of something else, such as an engine misfire or an internal oil or coolant leak, so get that found and fixed 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 P0421 as fix soon, not as an emergency.

Reviewed 2026-10-22 · Based on 2 cited source(s)

What P0421 means

Many engines have a small warm-up catalytic converter mounted close to the exhaust manifold so it reaches working temperature quickly after a cold start, plus a larger main converter further down the exhaust. RepairPal explains that the engine computer compares the front and rear oxygen sensors to judge converter efficiency: a working converter stores oxygen, so the rear sensor reading stays fairly steady, and if the rear sensor starts to match the front sensor, the computer sets P0421. OBD-Codes.com describes the same thing as the downstream oxygen sensor on bank 1 detecting that the converter is not working as efficiently as specified. Bank 1 is the side of the engine with cylinder 1 (a four-cylinder engine only has bank 1). OBD-Codes.com adds that the code may be more likely after many cold starts in the past day or two. P0420 is the more common general catalyst efficiency code for bank 1, P0422 is the main catalyst on bank 1, and P0431 is the warm-up catalyst on bank 2.

Severity: can I keep driving?

Drive with caution, fix soon

Usually drivable short-term, but don't ignore it. Sources differ on how serious it is: RepairPal rates P0421 Urgent, while OBD-Codes.com says you will likely not notice any drivability problems. RepairPal also notes some cars lose power. The bigger worry is what damaged the converter: RepairPal says catalytic converters usually fail because of something else, such as an engine misfire or an internal oil or coolant leak, so get that found and fixed 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 P0421 as fix soon, not as an emergency.

Common symptoms

Check engine light on
Often no noticeable drivability problems (both sources)
Noticeable lack of engine power in some cases (RepairPal)
May show up more after many cold starts over a day or two (OBD-Codes.com)

Likely causes

Roughly most to least common, based on the sources below. Your vehicle may differ.

Worn-out or defective warm-up catalytic converter (both sources)
Engine misfires that have damaged the catalytic converter (RepairPal)
An oxygen sensor that is not reading properly (OBD-Codes.com)
A fouled spark plug (OBD-Codes.com)
Internal oil or coolant leaks that contaminate the converter (RepairPal, as a common reason converters fail)

Diagnostic steps you can do

⚠ Before you start

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 P0308) or fuel trim codes are also present, fix those first, because RepairPal says an unrepaired misfire is a common reason converters fail.
Check the spark plugs for fouling, which OBD-Codes.com lists as a cause.
Test the bank 1 oxygen sensors, especially the rear (downstream) sensor after the converter. OBD-Codes.com suggests measuring its voltage and testing every oxygen sensor while you're at it. With a scan tool on a warm engine, a rear sensor that switches up and down almost as fast as the front sensor suggests the converter is no longer storing oxygen.
Look for signs the engine is burning oil or coolant, such as blue or white exhaust smoke or falling fluid levels.
Check your emissions warranty before paying for a converter. RepairPal cites a federally mandated 8 year, 80,000 mile warranty on catalytic converters, while OBD-Codes.com says many manufacturers give a five year, unlimited mileage warranty on emissions parts. Terms vary, so check your owner's warranty booklet or ask a dealer.

Typical fixes

Repair any misfire, oil burning or coolant leak first (RepairPal)
Replace a faulty oxygen sensor if testing confirms it (OBD-Codes.com)
Replace fouled spark plugs (OBD-Codes.com)
Replace the bank 1 warm-up catalytic converter once other causes are ruled out (both sources)

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

RepairPal lists common misdiagnoses: replacing oxygen sensors when the warm-up converter is really damaged, and replacing the converter when the real problem is a misfire or internal engine damage raising emissions
The car may still be under its emissions warranty, so a dealer diagnosis could save you the cost of a converter
The check engine light is flashing, or the engine has lost power
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.

OBD2 scanners on Amazon Parts at Advance Auto Parts catalog at RockAuto

FAQs

Can I drive with P0421?

Usually for a short while. OBD-Codes.com says you will likely not notice drivability problems, but RepairPal rates it Urgent and notes some cars lose power. Get it diagnosed soon, and stop driving if the check engine light flashes.

What's the difference between P0420 and P0421?

Both mean bank 1 catalyst efficiency is below the allowed threshold. P0421 specifically names the warm-up catalyst, the small converter close to the engine that heats up first after a cold start, while P0420 is the general bank 1 catalyst code. P0422 names the main catalyst on bank 1.

Does P0421 always mean a new catalytic converter?

No. OBD-Codes.com lists a faulty oxygen sensor and a fouled spark plug as possible causes, and RepairPal warns that converters are often replaced when a misfire or internal engine problem is the real cause. Rule those out first.

Is the catalytic converter covered by warranty?

It may be. RepairPal cites a federally mandated 8 year, 80,000 mile warranty on catalytic converters, and notes the problem that caused the failure is usually not covered. OBD-Codes.com mentions five year, unlimited mileage coverage from many manufacturers. Check your own warranty terms.

How much does it cost to fix P0421?

Neither source gives a repair cost for P0421. It ranges from a spark plug or oxygen sensor to a catalytic converter, so get it diagnosed first.

Sources

Last reviewed 2026-10-22. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.