O2 Sensor Codes for smog?

I just bought a 2016 Ford Mustang GT that had 3 inch headers and an exhaust system already installed on it, it also came BAMA tuned with a couple of other smaller mods. When I tried to smog it here in Washoe County they gave me two o2 sensor codes saying that I just need to drive it to set the monitors for the codes. I took it to the mechanic a couple of weeks later after driving it constantly and hes telling me that the tune that the previous owner gave the car must have shut off the o2 sensor monitors? Because they both say N/A on his diagnostic tool. My car doesn't have a check engine light it just doesn't pass smog. I bought a O2 Sensor spacer hoping to get around the issue but I didn't realize I would need two, one for each rear o2 sensor, so I haven't installed them yet. I'm thinking about going to a tuning shop and talking to them about it but I'm not sure, if anyone has any ideas that would be great thank you

You will have to re flash the EPROM in the PCM to make it pass smog, about $250 for each PCM flash at a PCM shop. If you can get the proper factory programmed EPROM, it won't do you any good because they removed the O2 sensors when they installed the aftermarket custom 3" exhaust system. The up and down stream O2 sensors were deleted in the factory EPROM programming. Now all you can do is install dummy out resistors and hope it passes on a factory EPROM pulled from a junk yard PCM. If the last alpha digit is F on the PCM cavity, it must be flash programmed because the EPROM is soldered in the PCM board without a zif socket. Welcome to Reno, Washoe County NV.

I would get a factory programmed PCM and resistor out the O2 sensors for the smog testing.

Your trying to cheat the system will never work! Here's what happens at an emissions testing -

On-Board Diagnostic Tests - Nevada test personnel will administer an ODBII test on most motor vehicles made in the 1996 or newer model years. The OBDII system installed in US manufactured vehicles made in the 1996 and newer model years is a mechanical issue early-warning system. The system records the vehicle's performance by administering a specific kind of test referred to as a "Monitor". Monitors test a vehicle's primary emissions components and subsystems. While the key is in the ignition and the engine is off, a test technician will make sure the vehicle's DLC (Data Link Connector) works properly. Then, the test technician hooks an emissions diagnostic tool up to your vehicle's DLC using a cable to allow the technician's diagnostic tool to communicate with the vehicle's OBD II system. The Nevada test technician can check your vehicle's engine and emissions systems, and see if any monitors state a "Not Ready" status. If there are too many monitors stating a "Not Ready" status, then the vehicle will not pass the test. The test personnel can complete the test in around 2 minutes.

the best thing for you to do is to get the vehicle emissions equipment back to OEM standards