Black01Bird

Car is a 2001 trans am, ls1, a4, 110k miles


Ok, well ive been recently noticing that my gas mileage has been going down, the check engine light has been on for some time now, but i went on a trip yesterday and used a full tank and only got 270 miles before the light came on. I know for a fact that this is much worse than it should be, so i went and had the codes pulled and it came up with:

p0420 - Catalyst efficiency low bank - 1
p0128 - Coolant temp always low

The car ALWAYS runs right where it should as far as temp goes on the gauge (200-205)

So im thinking the p0420 code has something to do with the mileage being bad, am i correct in thinking this?

What does this code exactly mean? 02 sensors?

Any help would be greatly appreciated!

foxtrot87duval02SS

yeah 02 sensors easy fix just make sure you use the 02 sensor removal tool. i had the same problem with mine it was my no.2 o2 bank sensor. also 270 miles id kill for on a full tank im around 220 miles on a full tank but i got 3 inch headers lol

Black01Bird

Well i mean, im positive i was getting over 300 before, and being a 16 gal. tank 270 miles isnt even 20 miles to the gal... something is obviously not right.

Upon investigating this code more, looks like it usually means that the drivers side cat is no good?..

I just dont know where the gas mileage went..

foxtrot87duval02SS

yeah man, easy fix literally disconnect electrical conenction , remove the old sensor with 02 sensor wrench , install new one , and reconnect the electrical connection. The sensor itself helps determine air/ fuel mixture so your computer was probually tellin the car to dump more fuel into your engine and thats probually were your gas mileage is at.

Black01Bird

See thats what i figured, but most people say that code is RARELY 02 sensors, and almost always a bad cat. How do i know which 02 sensor needs replaced (if it is an 02 sensor)

foxtrot87duval02SS

http://www.p0420.com/ maybe that will help

2000PewterT/A

More o2s go bad that cats, (remember catalytic converters don't wear out they are damage from something whether it be a rich mixture etc. Since the code is for a catalyst efficiency problem it will be the rear o2 sensor and you said it was for bank 1 so there you go. I would still follow the diagnostic flow chart before condemning any part and just replacing it. It may be a bad cat or a bad o2 sensor or something wiring for the sensor. Only way to tell is to diagnose it, don't just through parts at it.

great421

iTrader: (1)

OP -

Had a very similar issue - poor mileage (but no codes) - replaced the 2 post-cat O2 sensors and (Ta-Da!) problem solved!

Good luck!

AmDWs6

i'm only bolt-on's and i'm doing 14 mpg, i ain't complaining lol 1 7/8 3" Full Exhaust.

although with the numbers you are saying.. man i WISH.

Jonota

Any relief on this one? I'm running down my own mileage problem so I ran across this.
I'm curious on the "coolant always low". I can't remember on the LS cars, but I know some F-bodies had one sensor for the dash, and another for the computer. If the computer thinks the engine is cold, regardless of what your dash gauge says, then it will never go to closed loop fueling, and you will be running open loop with added fuel to protect for cold starts. So if haven't figured it out yet, might want to start getting that coolant code worked on.

Red89GTA

Definately get that coolant code looked into, you're engine will be in 'warm up' mode all the time, excess fuel and probably lower power. The p0420 -could- be related, but all the ones I've see were dead convrters. Best (not easiest) way to ck is to get a scanner that can watch the o2 sensor in real time, graphing is the best. You then watch what the front and rear o2s are reading, once warmed up, front should go up and down, holding at about 2k rpm will get it to switch high to low about once every couple seconds. The rears should be flat, or nearly so. If it copys the front, your cat is not doing anything (or not enough) and thus its efficiency is too low.

basically diagnose the problem before throwing parts at it.