I had a shop replace my cat and put on dual exhaust. They put an O2 sensor before the cat (actually right on the inlet to the cat) and one behind it... it ran great BUT... horrible fuel economy and the CE came on yesterday. It seemed to drive okay but when I kicked it down to pass a car, it felt like another cylinder kicked in and it ran great... I took it to a mechanic who put it on the computer and he told me that bank 1 was showing lean and that the 02 sensor had no reading at all... he said it should read some volts but doesnt. He recommended replacing the 02 sensor.... A quick check of my Chiltons showed me that there was an 02 sensor on each of the down pipes from the exhaust... I dont have that.... i have 1 before and 1 after....

My question is... which one do I replace and is that in fact the problem?

Thanks for the help.....

Jerry

 

o2 sensors often cause lots of problems in dual exhaust setups with trucks that are built after 1995.

It's because of that (and the fact that there is rarely a performance gain) that I dislike and advise against dual exhaust systems.

Without knowing what you had before though for sensors, it's hard to tell what you SHOULD have (as there are many different emissions packages).

Maybe someone will chime in with how their dual exhaust is setup.

It would help to know the year of your truck.

 

I don't have dual exhaust, but I had the exact same problem. Same reading from the OBDII, and only two 02 sensors. It took my own time and multi meter and tracked down the one that wasn't sending any info. The pre-cat, or in the cat 02 should be replaced first. Change that one and all you motor will wake back up again. I'll bet your throwing black smoke looking like a diesel right now too. Mine was.

 

Its a 96 and yes, it was throwing black smoke so bad that it plugged the cat... had the cat/dual exhaust put in and asked them if the 02 sensor was bad... their answer was "they looked good" little did I know... anyway.... I got the new sensor but I'll be damned if I can get the pre cat sensor to break loose. I cant get the slotted socket on it but I can very easily get a box end wrench on but cant break it loose. I'm not a small guy (6-4 300) and I put my weight into it but cant move it. Any suggestions?

thanks for all the help... this site is an absolute godsend...

Jerry

 

Try heating it up first, don't heat the sensor directly, but heat the area around it. The metal shoudl expand a bit and break the corrosion loose. It may also be heat welded in there because it got so hot, if you cannot get it loose from with that, try some penetrating oil overnight, otherwise you could replace the cat with a high flow one, they are pretty cheap on the ebay.

 

should have a 02 before and after the cat.. thats the "norm" emmisions package.. the cali boys have some 3 cats.. anyway.. your foward 02 measures misture and adjusts accordingly.. and the aft one measures cat efficiency.

i had a same problem and the wiring from the 02 was frayed really bad and was shorting out.. the 02's also have their own heating element in them .. all the wires have to be in perfect condition for it to work right.. also pulling it out and giving it a really good cleaning witha specific electrical parts spray cleaner would be a good idea.. let air dry for a few hours.. to get the o2's out.. let the exhaust cool all the way down and spray the hell out of the bungs with some good penetrating oil.. like PB blaster..