What AFRs do you see with the original MAF, and where the driving conditions the same IE loadRPM?
AFRs should be very close, but if the replacement MAF is outputting a slightly different voltage to the ECU it will show different AFRs, as its been mapped to the original MAF voltage.

 

I see around 14 when driving with the original one, where I was seeing 16 17 with the spare one, on idle I get around 16 with the original one, but with the spare it wouldn't even measure as it's to high, just displays 3 dashes

 

When I used r33 from z32 , my car couldn't idle properly , couldn't hit boost etc. Don't have an afr gauge so yeah. ..

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Is it the exact same model MAF (part #'s match)? The MAF may have differences even within the same model cars.

norea - The R33/Z32 MAF's cat just be swapped out, they have a different flow ratio that the MAF puts out in the form of a voltage. As the MAF has a different voltage ratio to amount of air, your ECU has to be reprogrammed or a piggy back AFR system added for the new MAF to work properly.

 

U mean can't right? But yeah I've already sorted out out [emoji3]

Running an r33 now. .. detune [emoji17]
Till i get another afm

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While both the O45 MAFs may have the same numbers etc, they where used on lots of different cars/engines so you could have one used on a 3lt engine and the other that's used on a 2.5lt engine, so its voltage scale could be out.