I've had mine about a year I guess, it's spent most of it's time in the glove box because I didn't have it set up right and it would always shut down after 15 seconds and the refresh rate was so slow.
This morning I read this: [LINK]
Attn Subaru owners: trouble with your ScanGauge? Here's the fix
I just got my ScanGauge II a few days ago. Nice unit, and I even read the first few pages of the manual before plugging it in. But after a little while with it I wasn't very satisfied with its performance. It would blank out periodically and sometimes wouldn't work at all after I starting the car. Worst of all, although it did show MPG and other useful stats, it only updated about every 30 seconds -- better feedback than waiting until the next fill maybe, but not exactly the real time feedback I was hoping for.
As it turns out, buried in the back of the SGII manual is a short instruction saying that if you have problems with the display not updating properly, you should select MORE -> MORE -> MODE -> PIDS and change the setting from ALL to SUPPORTED. Interestingly, it specifically mentions 1995-1999 Subarus as requiring this setting, and this is the only mention of a specific vehicle in the entire manual. I guess Subaru's first try at SGII must use a slightly nonstandard communication protocol or something.
Anyway, I entered that setting last night and it now works flawlessly, with updates every second or two, and the display is on whenever the engine is on. Took some baseline runs with it last night, and I'm pretty sure I can use the information from this thing to get my MPG up over 30.
Hope the mods don't mind this thread -- I thought this information might be helpful for any other 95-99 Subaru owners who might be lurking, since it is buried in the back in the ScanGuage manual, is not very obvious, and is absolutely essential to getting the SGII to work in your car.
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Well, now I'm in love with it !
'98 Outback: