Low idle RPM/rough idle and SES

Alright, just had new pipes and a muffler put on, got it inspected, guy tells me my o2 sensor might be bad. the ses light comes and goes, gone more than not, mainly after I run it for a little while intown, turn it off for less than fifteen minutes and I start it again does the ses come on.

The idle is low all of the time, and w/o a tach I didn't know the rpm's, during inspection I saw it at idle and it read something like 538rpm, more or less but still under 600, that's not normal is it? What is the normal rpm at idle for a 4.3?

I've changed plugs, wires, pcv, cap and rotor, and tps. Put on a cherrybomb turbo with new piping behind it, original configuration on the pipe just went up from 2 1/4 to 2 1/2 inch pipe coming out of the muffler, no change going in.

I haven't messed with the o2 sensor, the egr or map sensor, not really familiar with any of those is why. What's my next move here to get it idling right?

BTW - the muffler is real mellow sounding, not loud at all, just smooth and deep, just like I wanted.

The guy passed the truck for emissions anyway but I need to get this thing running right finally, spent a lot of cash and time on it these last two weeks, ready to enjoy it now.

Thanks for your help.

Find the diagnostic link at the lower dash...basically above your right knee.
With a piece of wire or paperclip, jumper the 2 top righthand holes then turn the keyswitch on and count the flashes of the Check Engine light.
You should get 1 flash, pause, then 2 flashes...1-2 or code 12. This code means you're now in diagnostic mode. It'll flash the 1-2 three times then go on to the next code if any and it'll flash them 3 times too.
At the end of the test, it'll flash the 1-2 again...letting you know the test is complete. Turn the keyswitch off and remove the jumper.
We can discuss the codes here.