A/F/R gauge for tuning

Hope the op doesn't mind. I have two o2 related experiences to share. Both on fuel injection systems.
1. Had a friend who brought a big block chevy to the dyno. We broke in and baselined on a known good carb and dyno headers. Then we switched to fuel injection and customers headers so no egts. I had had the dyno o2's set up one on each side just behind the collector and a set of bungs next to those for the fuel injection (FI). The FI used a single sensor we placed on the pass side.
The motor wasn't happy and didn’t run as well as with the carb. It was a dual plane manifold and it had some split between the o2 readings side to side. Long story short we ended up moving the o2 to the driver side and the motor ran great.
Customer took the motor home and installed sometime later. Motor wouldn't run right and discovered he had a leak at the collector upstream of the sensor. Fixed that and it ran better but still not great. Then discovered he put the o2 sensor in the wrong side. Moved the o2 sensor and it ran great.

I'll post the other story later.