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· Registered
00 GC8 Silverthorn 2.5RS Coupe
29 Posts
Discussion Starter · #1 · (Edited)
Well this car has been through more than its fair share of o2 sensors and we have to pass an OBD scan emissions style test here in StL.

With 128k on the car we had a solid 0420 for the last 3 - 4 months.

I replaced the front o2 first.
this is probably the 8th or 9th front O2 to go in.
the dealer did 5 of them while it was still under warranty so for some reason it has always eaten o2s.

Next the stock header had developed a crack and so I picked up a slightly used Cobb racing EL header and Cobb high flow cat.
The previous owner said miraculously he never even had a 0420 with this setup.

When I installed the Cobb I figured it was good insurance to install a new rear o2 since it was still the original.

So with 2 brand new Bosch O2s and a fairly fresh Coob header and cat I get a CEL on the way to the emissions place.
We cleared the code at AZ and we able to squeak through the emissions scan.
Next week its back.
I installed the HELP spark plug fouler rear o2 spacer.
After much reading there are a large majority of the owners that this resolves the issue for.
Last night I took it out to fill up with gas and it was empty empty, like bucking as i pulled into the station empty.
Got on the highway after filling up and the CEL light came on.
Stopped by AZ again and 0420 is back.

So I have new header, new cat, both new o2 sensors and the mechanical fix.
car also has new plugs and wires, fresh oil change and is getting a new thermostat and coolant flush this weekend.

Someone else mentioned getting an i-speed reflash, but I wasn't sure if it was necessary with just the Cobb headers, cat, Borla cat back and a Ganzflow CAI.
I've got my inspection sticker that's good for 2 years but I am really disappointed that all the work I have put into resolving this is for not.

One thing that stands out in my mind was way back when we first got the car we tried an amsoil foam racing air filter and it had the stickiest gooiest filter oil Ive ever seen short of my ATV.

A fellow racer told me that the MAF on the 2.5RS was WAAAY too close to the filter to run an oil impregnated filter.
Could oil contamination of the MAF cause it to run rich enough to through a 0420 code?

I've read thread after thread about the dreaded 0420 and there really isn't much GOOD info out there regarding people that have conquered it, other than by the o2 spacer or by a reflash.

· Banned
'04 RS PBP w/ Black Tarmacs
2,225 Posts
OK lets try and diagnose this. Replacing O2's and the P0420 seem more like a symptom than cause.

If you ran an oil type air filter then you need to check to be sure your MAF is clean. I know there are threads on here that describe how to clean them without damaging them.

Next running the car out of gas is never a good thing. Besides burning out the fuel pump it will also clog the filter. Something to be aware of if the car starts to bog under heavy load (clogged filter) or you hear that dreaded squealing (scored pump).

Now if you run rich you could foul out the O2's and clog the cat with unburned oil (carbon). if you run lean the increased exhaust temps can melt the front O2 in extreme cases and damage the cat. In both cases damage or at the very least shortened life will be the result.

If your running lean chances are you have a clogged fuel filter or injector. I could also be a bad fuel pump or fuel pressure regulator. There are other causes, but these tend to be more common issues.

I'm guessing your running rich with what you've said. Have you checked the ignition system? Are the contacts going from the coil to wires and wires to plugs tight and free of oxidation or corrosion? What kind of shape are the wires in physically? Are they cracked or dried out?

How about the spark plugs. Are they sealing properly when installed? Are the gaps within spec? Are you using something besides NGK copper plugs?

See I would start with the basics and make sure everything is in good shape. I personally have found the ignition system to be a little fussy. I've found Iridium plugs don't work well on SOHC engines. Magnecor wires don't seal well enough to keep the brass tips on the coils sealed and free of oxidation. I also had one freakish plug that just didn't seal the cylinder which caused blow-by.

So I too was getting a P0420 CEL, but I haven't gotten one in a while since I fixed the ignition system with new wires and plugs.

You may also want to pull the PCV valve and make sure it's free. Our engines build up a lot of pressure in the block at high RPMs and a clogged PCV valve/system will force oil past the rings into the combustion chamber. This will cause excessive carbon as well as excessive oil consumption and in some cases engine knock from lowered octane caused by the oil.

Now I know this is just some basic stuff, but I hope it helps...Good Luck

· Registered
'00 RS SRP
469 Posts
I've had the 0420 code for about 3 years now. Everytime i need to get emissions tested, I clear the codes, wait a day for everything to reset, then test and pass with no problem. Light comes back on about 3 days later. Easy as that :p

· Registered
2001 Impreza STM Coupe
283 Posts
The rear o2 sensor (behind the cats) controls the p0420 code. did you replace that one? There is a fix for catless test pipes. weld on a 90 degree peice of metal. get one at lowes or home depot and weld that where the 02 sensor would go. Plug the sensor into the bend, this will remove it from the flow at not trigger the code. The heat from the pipe with make sure that it still is receiving signal to the ecu. I might just be lucky but I did that with mine and no cel.

· Premium Member
98 RS turbo
326 Posts
go onto ebay and look up o2 simulator. they only cost about $10. it basically pulls the sensor out of the stream of the exhaust about an inch or so. BTW, I've got a 98 RS with borlas, both cats hollowed out and an STI cat back, and I've never had any codes. I also have a stock legacy outback that the 0420 code always comes on! it doesn't make any sense.

· Registered
57 Posts
yeah, p0420 is "cat below effeciency," so the only thing that affects it is the rear O2 or the cat itself. all the MAF, MAP, fuel filters, and front O2's have nothing to do with the catalyst. those would be p0101-5, p0105-9, common sense, and p0130-p0167, respectively.

· Registered
01 Blue 2.5RS
144 Posts
I have had this problem as well. I bought a OBDII reader, in order to clear out the codes and check the engine. It will come on and go out every thousand miles on my car. I have thought about replacing the CAT on my car but with a military budget, I can not get a CAT thats good and cheap. I have tried various things myself and just said the heck with it. When the code is solved, I will quickly follow.

· the hard way
1972 240Z
11,474 Posts
does the 0420 code cause the ecu to change fuel/timing? ie affect fuel economy/performance? just wondering because i dont really feel like wrapping them, there's nobody local-ish that will do ceramic coating, MILs dont seem to be a reliable fix (and it doesnt make sense to me how they would anyway) so i'm thinking about just leaving it, throwing em on and clearing the code as it comes up, but not if it is going to affect performance/fuel economy having that CEL on most of the time

go onto ebay and look up o2 simulator. they only cost about $10. it basically pulls the sensor out of the stream of the exhaust about an inch or so. BTW, I've got a 98 RS with borlas, both cats hollowed out and an STI cat back, and I've never had any codes. I also have a stock legacy outback that the 0420 code always comes on! it doesn't make any sense.
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