2002 Protege LX reading P0421.

[Jglenn]

I've already replaced the Upstream O2 sensor, and what I assume is the downstream (although it was only on the under side of the engine, not post-cat like I thought). Still getting code after an Erase, so my only thought would that it would be the Cat. Really didn't want to have to replace the cat, so if someone else can think of something that I might be forgetting, then please let me know.

 

[thuzil]

Either your warm up cat is shot or your secondary o2 sensor is shot. If you do not care about emissions then do not worry about this code. It has no effect on your cars performance at all. If you want the code to go away, replace your cat or o2 sensor, or pm me for a permanent electronic spoof fix.

 

[Jglenn]

I've replaced both O2 sensors, so it has to be the cat. And unfortunately I have to worry about emissions since here in Georgia we have to be tested every year.

 

[thuzil]

Do they still do the tailpipe test there, or have they switched to the OBDII test?

 

[erik84]

Are you sure there are no holes/cracks in the air intake hose?

 

[03protolx]

take the cat off, gut it out, put it back on and get a spark plug defouler from autozone. Ream out (drill out) far enough that the 02 sensor will fit, and screw that into the downstream sensor location with the 02 sensor attached.

with the 02 sensor spaced out, the computer gets "tricked" with the emissions and the code will turn off.

it's half way legal and half not. there are two cats on the car.

i've done it on two on my proteges in the past. doesn't smell and it runs as it should.

my 03 is the lucky one..it's been replaced before so I would think it may need to be replaced in a couple years. common failure point.

 

[thuzil]

^ this will work if they do the obdII emissions and not the tailpipe emissions testing.

 

[915_P5]

Are you on stock headers or aftermarket

 

[TheMista]

Jglenn, any updates? I have a 2002 Protege LX with a 2.0 liter engine. I have codes P1250 (Pressure regulator control solenoid sensor) and the same P0421 that you have. I ordered the PRC to take care of the P1250 code and have my fingers crossed that it clears up the P0421. Two mechanics so far haven't been able to tell me exactly how to fix it and get it ready for inspection.

 

[pcb]

If you do not care about emissions then do not worry about this code. It has no effect on your cars performance at all. If you want the code to go away, replace your cat or o2 sensor, or pm me for a permanent electronic spoof fix.

I'm pretty sure that's not true,... If the car throws a P0421 code, it goes into limp mode (safe mode). That affects timing and a bunch of other things including dumping way more fuel into the engine which wastes gas and can foul things up.

The non-fouler trick doesn't just clear the code,... it prevents limp mode.