I have a 2000 Z28 and the check engine light came on.. Sometimes when i drive all my lights on the instrument panel turn on Check engne,Service engine, Brake etc... They all turn on and my all my gauges stop working except for Mph.. Had it diagnosed and said it could either be the O2 sensor or a bad Cat.. Whats the easiest way to tell if its even that??

 

A defective O2 sensor (or related system) or a faulty cat can turn on the SES light. That would not cause the instruments to fail, or the other dash lights to come on. Did they pull the codes and that's how they determined you may have a problem with the O2 sensor and/or cat? If so, what were the EXACT numerical codes? There are multiple codes for the O2 sensors, and some of them are not sensor problems, but problems that show up at the O2 sensors - e.g. system lean, etc.

Appears you have a couple of problems that may of may not be related. The instrument panel problems - all warning lights on, gauges not working - may be related to a problem with the power supply to the dash cluster, and/or a faulty ground. It is remotely possible a ground fault could trigger the trouble codes, but not likely.

 

I dont remember the codes but they told me it was either a o2 sensor or a bad cat.. I had 1 of those ceep free diagnostics done from autozone.. But i do have subs in the trunk and i grounded it on top of the trunk ground.. Do you think that could be the bad ground??

 

02 sensor or bad cat sounds like P0420 or P0430. Shouldn't have anything to do with your gauges issue. Disconnect your ground and see what happens.

 

Go back to Autozone and have them pull the codes again. Write them down. Autozone has the capability to give you a printout of what the codes might mean, if you ask for it.