Hi,
I just bought this car and the P1000 code prevented it from passing smog. I drove it a lot and it went away which I thought was great. The next day I did a scan and it was back. It’s done this 3 times. I’m also now getting a P0158 code but no check engine light is on. Would replacing the O2 sensor help with the P0158? Wondering if the P1000 coming and going so often is possibly a sign of a bigger problem.

 

P0158 Code: O2 Sensor Circuit High Voltage (Bank 2, Sensor 2) - In The Garage with CarParts.com

Note this is a circuit code so changing the O2 without evidence is a crap shoot..

P1000 indicates that the OBD-II diagnostic check has yet to be completed. So until the P0158 is resolved this will not clear..

 

Any time the battery is disconnected the P1000 comes up and that is how they know it was disconnected.
Some folks try doing a forced reset to get rid of codes and wind up with the P1000 code.

Considering the age of the vehicle and unknown history, it might pay to replace the sensor after the converter on the driver's side.

The code suggesting the voltage possibly being high could indicate an open oxygen heater circuit or no ground available.
If you do not have the equip to check and verify , replacing the O2 sensor just might save you some trouble.