I switched from a cat to a test pipe, and the check engine light came on (P0137 or P0138) it said to switch out the second o2 sensor so, i switched out the 2nd o2 sensor and the light came back on with the same code. i took it to my tech and he said that without the cat it will always come up...is this true...and if not what can i do to fix it??

 

do a search for "cel fix" , that should bring up hours worth of info on your situation.
basically, the secondary 02 sensor is there to check the operation of the catalyst converter. you just relpaced the converter with a straight pipe and now the sensor thinks that the cat is junk and is not doing it's job.
there is a common fix to "trick" the 02 sensor into thinking that the cat is still there and is still working properly.

 

I'll be on the same boat as you when my test pipe comes in. I believe there is an 02 simulator or anti-fouler that will basically space the sensor further from the test pipe. This should get rid of the CEL. I'm still doing research on it too.

 

Two antifouler trick

https://www.s2ki.com/forums/index.php?showtopic=229959

 

 

you need 2 spark plug spacer if your tp didnt come with a cel fix

 

Yea, see your ECU thinks your car is broken now because the O2 sensor (it's not broken) is telling the ECU that the exhaust is not getting cleaned by the catalytic converter.
So either you have to buy a testpipe with a CEL fix already made into it, or you will need to buy an O2 emulator....

 

Interesting, yours is throwing the code for the "Secondary Heated Oxygen Sensor Circuit Low (or High) Voltage." My 07 throws the P0420 "Catalytic System Below Efficiency." But that's 00 vs. 07, so maybe that's just how it handles that.