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My car is throwing 2 codes, and the SES light turned on from the way back from aamco where i just had a 2k trans rebiuld. The first is P0404, "EGR system fault". the second is P0756 "2-3 shift solenoid valve performance". The second one i can get take care of for free at aamco. But for the first one, the guy at aamco says my cat is colgged and its giving me my 4 mpg, incapeability to go over 2k rpm and 20mph.

I guess my question would be, wouldnt a colgged cat give me codes? and is it possible that if my cat was clogged, that it could be holding the EGR valve open, throwing the code that i am getting?

I did the sulfer smell test at operating tempatures, and i didnt smell anything. BUT, the cat does rattle like a bitch when im in the pedal over 1500rpm.

any sugestions or experiences shared would be greatly apperciated.
 

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If the cat is clogged you will notice very little gasses comming out the muffler, and if you hit 2 or 3 times the cat with the handle of a big screwdriver and accelerate your car you will notice dust comming out of the muffler. Anyway its a good idea to get a magnaflow.

Good Luck,

Ivan
 

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2001 pontiac firebird 3800 GT w68 sports appearance package
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So.... what was the ending result? I have the same problem with my 4th gen v6. Code for
1. p0102, mass air flow.
2. P0404 egr
3. P0756.

The past 4 years that I had this car had a p0420 code. I bought it with the code and the car has a muffler delete. I also think the cat was gutted. I had a shop put a catch and muff on and still had the p0420 code. After having that code there for years, it went away right as these codes came up. Do I have a clogged catalytic converter. I'll gladly gut it if that's my problem but how would the catalytic converter cause the trany to slip?
 

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1994 Firebird Formula 381ci LT1 / TH400+GV O/D
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14,258 Posts
It helps if you provide the year of the vehicle. There can be subtle variations from year-to-year.

If you plan to post frequently, go to your “Profile”, click the “About“ tab, and enter year/engine/model in the “Vehicle Details” field. That info, at least the first part of it, will show up in each post under your avatar and screen name.
 

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1994 Firebird Formula 381ci LT1 / TH400+GV O/D
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14,258 Posts
P0420 went away when the codes for the MAF sensor (P0102) and EGR (P0404) valve set. The PCM can't run the P0420 diagnostic if those codes are set.

P0102 sets when the frequency signal from the MAF sensor corresponds to an unreasonably low air flow into the engine.

P0404 sets when the EGR valve feedback to the PCM indicates the pintle position oin the EGR valve oes not match the commanded position.

Low air flow into the engine and maloperation of the EGR MAY be related to a plugged cat. Very easy for a muffler shop or mechanic to check the back pressure in the exhaust system to see if the cat is clogged.

P0756 is a transmission code. - "2-3 shift solenoid valve performance". When that code sets, the PCM commands 3rd gear only. Highly unlikely that code is related to any of the others you mentioned.
 
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