Pilot reported a NMAC.

Narrative

On Date; at approximately XA:00 local time I departed ZZZ after picking up the aircraft from maintenance (new avionics install). This is the first flight post maintenance. Upon climbing to pattern altitude I engaged the autopilot with the intent of testing out the approach capabilities by using the RNAV XX approach into ZZZ1. VMC prevailed.Approximately 10 minutes into the flight my ADSB fail light indicated and my transponder was dark. I have a policy of not resetting breakers until I am safely on the ground unless there is no other option. I continued the flight since ZZZ1 was now approximately as close as ZZZ. I made adjustments to course and altitude as I was testing the new GPS unit. I stayed at climbed to approximately 2;300 ft and followed the course. At approximately XA:15 local time I looked down to scan my engine gauges and made minor adjustments. I resumed my traffic scan. A few moments later I spotted a [Cessna] 152/172 climbing directly towards my left wing and only approximately 15 feet away and approximately 10-15 feet below me.I was unable to react quickly enough to disengage the autopilot and take evasive action as the aircraft was under me in less than three seconds. The other pilot took no action as well. I estimate we passed each other at an extremely unsafe separation (sub 10 feet separation).