TheOtherEric

Why's that?

I was told by a 993 chip tuner that some of the 1996 ECUs can be reflashed.

g_murray

If the CEL is on (all the time) then, yes, of course if yet another 'error' (of significance) is, in the meantime, detected by the ECU then you'll be unaware - coz you'll just be 'used' to the CEL being on because of the previous 'insignificant' error.
Dangerous territory IMHO -- though driveability issues might give you a clue that 'something else' is amiss.

I personally check (once a week) the 'pending codes' of both my DD and the blue-bomber. In that way, you get a 'preview' of something that might be about to 'crop up'. Having said that, I've found that most 'pending codes' "go away" because they are, in fact, 'transient'. Still, I like the 'heads up' aspect.

Gerry

993inNC

If you run the secondary O2's in the bypass pipe just as they are, then the system reads no difference between pre/post O2's and thinks the system is running rich. The Ecu will adjust for rich conditions by leaning out the mixture. I would be willing to guess that it wouldn't be long before holes get burned into pistons

TheOtherEric

I'm surprised the ECU's logic would be: "so both O2 sensors are reading the same. Let's lean it out. Oops now they're both reading lean. Let's lean it out some more." That seems like really poor programming.

993inNC

No, I think what it reads (and of course these are electric valuations) is that it continues to run rich both before and after what should be the cats. It would probably also throw bad cat codes, maybe even bad O2 codes. None the less it would want to lean to some degree, could even go into a "limp" mode. Again Steve W has light years more program experience and should be the one to consult, this is just my basic understandings