P0421: Catalyst 1 Efficiency Below Threshold Bank 1
A generic powertrain (engine and transmission) trouble code, same meaning on every brand.
Yes - no driveability effect, but the car will fail an emissions test.
Symptoms
- Check engine light
- Sulphur or rotten egg smell in some cases
- Emissions test failure
Likely causes, most common first
- Warm-up catalytic converter aged or damaged
- Exhaust leak upstream of the converter
- Oxygen sensor inaccurate
- Engine burning oil or coolant, poisoning the converter
Read P0421 yourself before paying for diagnosis
A basic Bluetooth OBD-II scanner ($20-40) plugs into the port under the dashboard and shows this code with live sensor data on your phone - useful both for confirming the fix and for talking to a shop from an informed position.
What is going on and how it gets diagnosed
P0421 concerns the warm-up catalytic converter on bank 1 - the small converter mounted close to the engine so that it reaches working temperature within seconds of a cold start. It sees the harshest conditions in the exhaust system and ages faster than the main converter.
Before agreeing to a converter, rule out the two cheaper explanations. An exhaust leak ahead of the sensors upsets the comparison the test relies on, and a lazy oxygen sensor makes a healthy converter look failed. Both cost far less than the part this code appears to blame.
Repair options and cost
Typical US independent-shop ranges, parts and labor. Dealers run higher; regional labor rates vary.
Owner reports mentioning P0421
“2 years ago I brought a new catalytic converter for my car from de dealership. That cost me 496.76$ few months ago the check engine turn in and when I check I have the code p0421. That is the code for the catalytic converter again. I couldn't believe it. I when to mitsubishi 2 days ago and they told me that they couldn't believe it because I'm the first client that comes back for the same part. One of the employees check my car and he said that my car is burning oil. He told me that I have to change the valve lifter. Do I really need to do that then buy a new catalytic converter?”2003 Mitsubishi Lancer · Aug 2017
“Numerous times the diesel has stuttered and acted like it was going to die. This has happened several times. Was taken to sealer and was told there is nothing wrong with it. It is making weird noises while shifting and is now throwing a cel with the code p0421.”2016 Ram 2500 · Jan 2017 · 8,000 mi
“This car is a 2008 and I don't know if it is a factory error but is already asking for the catalyze converter to be changed and it only has 83,000 miles. I know because it didn't pass the emissions test. They asked me to fix code p0421.”2008 Mitsubishi Eclipse · Aug 2016 · 82,000 mi
Show 2 more owner reports
“The car began to shutter and dropped from 50 to 25 in seconds. The check engine light (cil) starting flashing and the car began to stutter like it was going to die, I pulled to the side of the road and the car died. Started car again the cil continued to flash, it only would accelerate to 25 miles an hours even with my foot all the way down on the gas. I took it to the a mechanic and the codes were p0421, 171, 300. The mechanic did everything from engine flush, spark plugs, oil, air filters and installed a new catalytic converter but the problem persists. Its now a year later and the car is doing the exact same thing. *tr”2002 Mazda Protege · Dec 2010 · 156,000 mi
“Catalytic converter failure due to excessive fuel burning in exhaust manifold. Currently throwing p0421 code. Replaced 1st cat and 2nd cat failure on 8/23/05. See reference tsb 89. Lewisville mitsubishi in lewisville tx claims so such tsb exists. *jb”2002 Mitsubishi Lancer · Aug 2005 · 98,000 mi
Cars that report P0421 most often
Based on mentions of this code in NHTSA owner complaints.
Frequently asked questions
Can I drive with P0421?
Yes - no driveability effect, but the car will fail an emissions test.
How much does it cost to fix P0421?
Typical US repair costs range from $150 for the simplest fix (exhaust leak repair) to $2,500 in the worst case. The right number depends on which cause your car has - see the causes list.
Related codes
The codes closest to P0421. They often appear together, and telling them apart is usually what narrows the diagnosis.
Definition per SAE J2012 standard nomenclature. Symptoms, causes and costs are editorial guidance - always confirm with a diagnosis of your specific vehicle.