Hi All, new member here so hope this post is okay. I've got a 1997 GEMs 4.6L that runs very rich. Using a Nanocom I can see my short-term trim is 38 and my long term trim is zero. Both O2 sensors appear to be working and with light driving they run at around 4.6V to 5V and both banks run in a similar way. The MAF seems fine - sensible voltage and mass flow numbers and so does the TPS. I've tried resetting the adaptive, but the P38 always returns to this high short-term trim number. I always run a minimum of 95Ron but 98Ron makes no difference. Any idea what I should try to fix this? So far I've cleaned the MAF (with MAF cleaner), changed all the plugs for NGK plus OEM leads - the original plugs were the richest I've ever seen on a fuel injected car! My next step would be to change the O2 sensors except they seem to both be working. Any help really appreciated!

The O2 sensors shouldn't read a constant voltage, they should flip flop between 5V and 0V about once a second. 5V means it is running lean so the ECU will put more and more fuel in to richen the mixture until they read 0V. If they stay reading around 5V, they are dead so never will drop to a lower voltage.

Thank you. Yes they stay at 4.6 to 5V - two new sensors needed then. I expected an O2 sensor error but it's early system I suppose. Guessing the max short term trim is 38 then? Anyway thanks very much for your help.

You won't get an error as the ECU doesn't know the difference between failed sensors giving 5V or a lean mixture giving 5V. Don't forget that GEMS is only just OBD compliant as it became mandatory from 1994 in the US market, it only changed when OBD2 was required from 2000 onwards (hence the change to Bosch Motronic).