Rep Power: 263

okay . i have had a t25 turbo in my car w/ a testpipe ... but now i just put on a stock cat and a magnoflow exhaust to pass ..... but why is my engine light on after i had put on a cat ....the o2 sensors still in tact next to the cat .... what would be the problem ..... i believe 1: i might have to re-tune it ..because i tuned it the first time w/ no cat ...and now it might be sensoring why that piece is there meanin my cat ..... 2) went to my friends shop and he scanned it and it said the 2nd sensor ...he turn it off ..but it came back on whne he turned on the engine ..... what is the problem .....should't there not be a engine light since i put on a cat ...

 
Rep Power: 265

I was gona say tuning. If they tuned it with a certain A/F ratio in mind, the cat will throw it off and o2 sensors will set the light off...think of it as the same thing as when you remove a cat and dont add a defouler/simbox...

 
Rep Power: 0

Whether or not you have a cat has nothing to do with your your primary O2 sensor readings, and as such the AFR you are tuned for. Your primary O2 (and the wideband you hopefully have) provide the readings your car is using to determine how much fuel to dump and when to dump it, and these are both pre-cat sensors. Unless your tuner used a tail-pipe sniffer to tune the car (hope not), the cat will have nothing to do with the tune, and adding or subtracting one only changes your emissions output, and adds a little backpressure. Neither of which will cause a CEL from the addition of the cat.

Which code did it throw exactly? O2 sensors can be really fickle, and sometimes a simple jostle when you're doing the test-pipe/cat swap can be enough to make one go bad. Was it a P0402 catalyst inefficiency code or a different one? What method of AFR reading did the tuner use - pre-cat wideband or tailpipe sniffer?

 
Rep Power: 310

Actually, the secondary o2 sensor is after the cat.

It could be that you just have some bad sensors..

 
Rep Power: 237

yea, somethin sounds messed up?

Primary sensor - Before catalyst and reads dirty
Secondary sensor - After catalyst and reads clean

So like foos said....as long as the o2's were in place before & after should make no difference and not set of a CEL, and the Tuning should have nothing to do with that nor the magnaflow exhaust.

All i can think of is when a CEL goes off is it has something to do with emmissons somewhere down the line.....

Check timing, sparkplugs, etc also?

 
Rep Power: 0

Not to nitpick, but the secondary sensor on a civic is actually right smack in the middle of the catalyst element, and I did say the only sensors that are pre-cat would be the primary O2 and the wideband.

Back at the OP, what's the code it's throwing?

 
Rep Power: 310

You are correct...the secondary is in the middle of the catalytic converter, but there is only one sensor before it...not 2. There is no such thing as an o2 sensor and a wideband sensor before the cat. there is however a such thing as a wideband o2 sensor, or just a regular o2 sensor..but not both.

 
Rep Power: 0

i dunno, i thought that the d17a6 had 3 o2 sensor, 2 before, and 1 after?

 
Rep Power: 0

Sorry, I was going on the fact that he was boosted, so if he was running a wideband, he would then have 2 pre-cat sensors - the stocker and the wideband.

I think we're both talking at the same thing here. My downpipe has two O2 bungs, one for the stocker (narrowband in my case) and one for the PLX wideband I bought from Andy. Add that to the in-cat secondary sensor (takes off shoes and socks) and you get three sensors, two pre and one post cat.

Back at the OP, where are you and what code are you throwing?

 
Rep Power: 263

my mechanic did a diagnostic test and it showed only one cell and it the bank 1 o2 sensor ...i believe its the sensor after the cat if ur looking at it front the front .....when he plugged in his system and stepped on the gas it wasn't acting like the other sensor did .....he said the sensor might be shot or burnt out when the guy from the muffler shop was welding the cat back on ...... and yes the sensor was still intact when he was welding ...so hopefully that is the problem .... so he ordered me an o2 sensor from honda and hope that resolves the problem ..

as for the tuning ...would i have to retune it again ..NO RIGHT ?


thanks everyone for the input

 
Rep Power: 0

There are different CELs it can throw for that sensor. Do yourself a favor and go to Autozone and have them scan it for free to pull the code.

No you shouldn't have to re-tune.

 
Rep Power: 263

my mechanic already did a diagnostic test..and its the o2 sensor next to the cat ...go on aim i have you in my buddy list