Just bought a 99z with 195,000on the clock. 6 codes showing with ses light. P0410,p0412,p0418,p0410pd,p0412pd ,p0418pd. Car is m6 and all stock except aftermarket exhaust. what should I check first? Also have abs light on too. Any advice appreciated...

 

All those codes point to your AIR pump system being removed when the aftermarket exhaust was installed, very normal. They can be turned off with a tune, or you can reinstall the AIR system.
Not sure on the ABS light, unless that was also removed.