I have a 02 chevy impala that sometimes wont turn on.

And you have load tested the battery? You say it is fine, and you know this how?

You should have a primary and secondary O2 sensor, upstream and downstream. Pre-Catalytic converter and Post-Catalytic converter.
Which one did you replace?

I think you could use this manual method instead.. My nephew used
it to repair my car and it's totally perfectly fine now. Here's the website he bought it from. www.reliable-store.com

change your signal light switch. i just had the same problem. cost me a 150.00 for an after market switch, comes with a full harness that plugs in under the dash.

need to make sure the wiper motor is getting power and has a ground from the wiper switch if so then replace wiper motor

This is a problem with chevy vehicles that have the system theft light. after a couple of years of using the key, the lock tumblers inside the ignition switch tend to wear out and so causing the system to think it's being stolen. Now you have to put the key in and turn the ignition to the on position, without cranking the car and leave it for 10 minutes until the theft light goes out. Very annoying especially if you're in a hurry. The dealer cost to repair the ignition switch goes for about $600-$800. that includes re-teaching the key.

Check the radio fuse? Box is drivers side end of dash(drivers door open)or pass side end of dash(passenger door open)
police models have a connection for the speakers (passenger floorboard under floor mat) to connect the speakers to the police radios.
If they had the speakers connected to their radios,they may have unhooked them and not connected them back to the radio.
The wires are brown,grey,blue and light blue.All into a blue connector.

heater core is leaking into floor

This sounds like a bad battery or a bad connection at the battery, the guages are the key, check the cables for a white power, this is battery acid corrosion and must be cleaned off, auto zone or kragen auto will test your battery for free.

replace trunk pop button