just fitted an air fuel gauge as i was sure my car was running rich,exhaust was always black,sure enough it is on light throttle or idle but with any boost it jumps to lean which im sure isnt good,what could cause this,engine issue or the gauge itself

 

1st suggestion is check wiring, 2nd is are you running from the signal wire on the ecu or tapped at the o2 sensor, 3rd are you on the pre turbo o2 or post cat and 4th remember it will take more than 800degf before the sensor will actually be hot enough to make any kind of reading that is acceptable, finally a/f gauges that are norrowband really dont give an accurate reading and the colder the o2 sensor is the worse the reading is.... so you really only see a decent reading at full pelt and for a little while before and after depending on egt's

 

cheers will check that out,was fitted by garage as im too lazy to run wires lol