Engine light..

**oakrdrs187** · 08-25-2011, 06:04 AM

Bad Pre-Cat....Most likely anyway. Possibly o2 sensor but unlikely since it's "Below Threshold"

That code means the first Catalytic Converter, one right off the manifold isn't heating up enough to convert toxic exhaust emissions into non-toxic substances. The core or "substrate" has a special coating on it that when heated it reduces carbon monoxide(CO), unburned hydrocarbons(HC), and oxides of nitrogen(NO, NO2, & N2O) to produce carbon dioxide(CO2), nitrogen(N2), and water(H2O).

So basically the pre-cat is bad or one of the oxygen sensors is gone. Either way you should be under warranty. I wouldn't take your car to Mitsu with your new intake though. The may give you a hard time because of it.

**oakrdrs187** · 08-25-2011, 06:05 AM

Re-post of the post above. Not sure why. Edited.

**wetamup2k3g** · 08-25-2011, 07:04 AM

I got that code a few months ago, I reset the light and waited for it to come back on, I'm still waiting. While it may be on its way out, I want it to do it again so I know. If you still have less than 8 years on your car though, you should be covered, I think that's the mandatory warranty on emission items.

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**monnewbie3** · 08-25-2011, 07:27 AM

I also have this code. Resetting the code with the computer doesn't work. I'm betting you got the ralliart? Seems few of our cars have this. Same bank, same side.

My solution: Headers lol (mitsu wanted to much money since it's welded to the manifold)

**brent-ralliart08** · 08-25-2011, 09:55 AM

thats crap. yeah no i think the warranty is voided because i got the intake and res delete. so headers? how would this solve the problem. they flash the ecu or something?

**monnewbie3** · 08-25-2011, 12:58 PM

Originally Posted by brent-ralliart08

thats crap. yeah no i think the warranty is voided because i got the intake and res delete. so headers? how would this solve the problem. they flash the ecu or something?

They have to prove that your mods caused the problems... So ya probably voided if I know the dealers...

Putting headers on your car eliminates your pre cats... so there are no cats except the main... and you stack your O2's so they read properly.. Sooo basically there's no cats to mess up :P

And yes you'll need a tune afterwards since your afr's will be messy.

**Isaurio** · 08-25-2011, 01:12 PM

is usually the o2 sensor at that bank. the pre-cat o2 sensor.

**oakrdrs187** · 08-25-2011, 02:20 PM

^^^ I changed it and nothing problem still there.

A cheap fix would be to leave the Cat alone and use the fouler trick on the o2 after the first cat.

**monnewbie3** · 08-26-2011, 07:10 AM

Originally Posted by oakrdrs187

^^^ I changed it and nothing problem still there.

A cheap fix would be to leave the Cat alone and use the fouler trick on the o2 after the first cat.

Good idea

**Isaurio** · 08-26-2011, 05:09 PM

I will only recommend that on the main cat o2 "2nd" on the I4. The precat o2 sensor i will leave it as it is. Since is the one adjusting your AFR at close loop.

**brent-ralliart08** · 10-06-2011, 12:37 AM

i hate you engine light.. =[

**unknown_owner** · 10-13-2011, 09:28 AM

Originally Posted by brent-ralliart08

i finally got that code... po421 ( warm up catalyst) fefficiency bank 1 below threshold. can anyone tell me whats going on.

I have the same code. had it for months now and it seems to be keeping my TCL/anti-lock brakes lights on as well (at least that what the dealer told me). For me, it's the rear cat that has gone bad. Dealer wanted $1300 to replace it and the 2 O2 sensors. I may have to replace the sensors too since I had the light on since last year. (yeah I know bad). I went out and bought a rear direct fit rear cat for a little over $300 but it looks like hell to try and get it in. May have to find a mechanic who can do it.

**brent-ralliart08** · 10-25-2011, 05:09 PM

Just got mine serviced. with the code of po421 . they replaced the rear converter? not quite sure what that is. but it was on the back side of the engine apparently. 2 hours later NO CELs =] but that is with a stock intake. im scared to throw my injen back on now

**oakrdrs187** · 10-26-2011, 05:41 AM

Do it but check and re-check all of your hoses.

**brent-ralliart08** · 11-01-2011, 08:41 AM

im just gonna wait until spring now.. should have some other add ons like exhaust and headers by then.

**Isaurio** · 11-01-2011, 12:13 PM

That cool you got the light off. But if you go to the dealer they will change everything. Thats how they make money. It was no need to change the cats. But anyhow is done. I have headers and i'm cat-less no Check engine code or nothing fresh for 2 years. I going to put back a main magnaflow cat back since the exhaust gases are a little strong.

**oakrdrs187** · 11-02-2011, 01:58 AM

^ Tell me about it. My pre-cat is gutted, can't stand anywhere near the back of my car.

**stayer** · 11-03-2011, 01:52 PM

P0421: Warm Up Catalyst Efficiency Below Threshold (cylinder 1, 4). Most likely sensor died. Sensor must be replaced. Not necessarily buy the original sensor, it is expensive. You can buy a universal sensor Bosch LS-07 № 0258986507 - 36Y. Original Denso sensor, worth $300, Bosch is worth us $35(the Price in Russia, you might have another price).