I have a 1998 X/90 and I installed a Thorley Header and complete exhaust system with muffler and cat.About two months after I installed it the Service Engine Soon Light came on and stayed on.What is it telling me and how do I fix the problem.Thanks

I would double check the o2 sensors, make sure they are plugged and harness is not damaged.  But with out knowing what the code is...

How many miles are on the odometer?

The mileage is on ODBC I systems, not the ODBC II.  You should be able to borrow an ODBC II reader from the parts store, or pick up a Bluetooth one from Amazon for $20 and use an app on your phone.

The mileage is on ODBC I systems, not the ODBC II.  You should be able to borrow an ODBC II reader from the parts store, or pick up a Bluetooth one from Amazon for $20 and use an app on your phone.

I know I had a different 1996 x-90 that had a locking gas cap that would mess with the tank sensor, not all of the cheap code readers would work with my 1996 suzuki maybe because it was the first year of obdII but that is always a pain with mine.

You can guesswhats wrong  and maybe get lucky. Or you can start by pulling some codes and do the troubleshooting process .

There should'nt be any reason the code reader would'nt work on your X-90.
If the catalytic converter has been removed you will have to much air flow past the #2 o2 sensor

There is absolutely no way any of us can solve your issue without having a CEL code, sure we can toss answers out there, but it is like playing blindmans bluff, or blindfolded darts.....You need to have the codes read before ANY troubleshooting can be performed. All x90's are OBDII

Some additional set-up info regarding the exhaust may be required as well, dia, brand cat and muffler etc.