I am getting a full decat and am ordering a boost guage and knock link today, how ever will i need an AFR?

 

I would. It's not vital, but gives a good indication of whether your lambda's OK, but more importantly, can show the early signs of a failing MAF.

 

and how would i determine whether it was a MAF or Lambda prob? or would i just replace Lambda first? Cheers

 

Lambda's only used for closed-loop fuelling, so you'd see disco lights on the AFR whatever the condition of the lambda. The easiest way of telling if your lambda is screwed is if your fuel economy drops (it should be quite possible on a long journey to get ~30mpg - any less, if you've been cruising, & it's quite possible your lambda is knackered - they only last ~3 years / ~30,000 miles).
MAF: only really used when running open-loop. When the MAF is ok, when under boost/high-load/WOT the AFR should show a very rich mixture. If the MAF is failing, the reading will start to drop towards the leaner end. However, a lambda also begins to read lean when it gets hot (read: repeated power runs/driving like a nut/like you shoud drive the car ) so this can sometimes give a bit of a dicky reading.
But it's a very good indicator.

 

so its good to have one, even if it looks like a christmas tree but will indicate if it runs lean, which is a problem right? also do you know where i can put the knock link sensor on my 97 WRX?

 

Err, not quite, me old china if the lambda's knackered, it will only register a low voltage, that doesn't move around as a healthy one would. Therefore, the AFR gauge will only display whatever LED segment that corresponds to the low voltage. Well, at least in the case of gauges that show LEDs all the way down to 0v (like Autometer).

A BIG problem. Lean running = det, which over a period of time = melted piston. Just don't forget that this only applies to open-loop fuelling. An AFR gauge can be confusing if you've never seen one before, as the LEDs flicker all over the place (including lean) when idling, part throttle, low revs/boost WOT, and sometimes will not register at all, for instance when slowing down in gear. A good rule of thumb is when you start the engine, after the sensor has warmed up, if the LEDs flicker all over the place, your lambda's healthy. When open-loop (full boost WOT), if the LED's indicate a constant rich, your MAF's OK (probably!).

 

Do i need one then or is it gonna confuse the hell out of me, and give me no indication of any problems?

 

It won't confuse the hell out of you if you know what you're looking for...!

Idle or part throttle = lots of LEDs between 0 & 0.75v = healthy lambda.
Idle or part throttle = 1 lean LED, not moving = fcuked lambda.
Full boost = 1 or 2 rich LEDs = healthy MAF.
Full boost = anything other than 1 or 2 rich LEDs = failing MAF.

 

...unfortunately, I think the lambda AFR reading/display may well depend on the lambda probe in use, the particular ECU you are using, the particular AFR meter & how fooked the lambda probe is!
Easiest thing to do is to make sure everything works, see what happens in various circumstances, and then if the AFR meter starts doing something different, something's up

 

True, but that little lot, although for a UK MY00, can be used as a general rule of thumb for other MYs. Won't be a million miles away...