Code P0421

first of all i searched and found lots of good info, but most of them went toward one same problem... i believe that mine might not be it


CODE P0421

Definition: Catalyst efficiency low-bank 1

Explenation: THe oxygen sensors monitor the catalytic converters ability to store oxygen.

Probable Causes:

1. Catalytic Converter defective (failure possibly dut to #2, 3, or 4)

2. Engine Misfire or running condition

3. Large Vacumm Leak

4. Engine oil Leakage into exhaust-valve guide seals, piston rings.


The PCM has determined that the warm up catalyst efficiency for bank 1 is below threshold for the current engine operating conditions. (Bank 1 identifies the location of cylinder #1, while bank 2 identifies the cylinders on the opposite bank)



I'm running catless and been so for a good 25,000+ miles and never any problems.

I do need to add a good quart or bit more of oil a month.


Mods i got:
K&N Cai
OBX HEADER JET coated
MAM catless midpipe
Racing Beat CB
RR Racing Crank Pulley
Magnacor ignition wires
Grounding wire kit
EGR pipe blocked on the header, open at the Intake mani


i'm thinkin that i might have a vacuum leak, but also for some time now i been also thinkin that it might be misfiring, dont know why but i just think, and for the Engine oil Leakage, i dunno, all i kno is that i need to to add some every now and than.... any help appriciated


i'm gonna start by
checkin ignition wires
and spark plugs
and than i will check for vacuum leak
i'm also thinkin of checkin my vlalve cover gasket?
i will also go ahead and do Spark Plug Non-fouler even tho i've been running good w/o it

besides that i dont kno wat else do???? any input will be helpful thx


peace out

i checked my ignition wires, changed my spark plugs, looked for vacuum leak, and also i did the Non-fouler, well sorta, i just used one piece instead of two, i thought that would be enuf,, i reset my ecu, and now just play the waitin game

you need to use too. if you use one, the o2 sensor doesnt sit far away enough

does'nt this code come up if you go catless? there is a mod for it if i'm not mistaken...what it is..i have no idea....i've had a check engine light on constantly for 3 years now. price for modifications

NCZ13 said:

you need to use too. if you use one, the o2 sensor doesnt sit far away enough

Click to expand...

i measured it and i think it should be good, if it comes on, i'll just throw on there the 2nd piece, but i ran catless for 25 000 miles and i never got the CEL cuz of it

i can catless for 2 days and my CEL came one. how many miles are on your car?

53 000 miles give or take few, i went catless at around 28, 000 miles and never any probs till now