hi pal really you want to buy a fault code reader off ebay something like a u480 will do unless you know someone who can scan it for you otherwise you could end up spending £££ on the car and it still not fix it ..
tip121 is a transistor thats inside the inlet manifold runner control box this some times burns out and can be fixed but thats if your lucky really a lot of the time the cable breaks and destroys the imrc box these are prone to going the make the car run very rough no power below 4000 rpm have a look at this link it shows you a how too change tip121
http://sites.google.com/site/mrhightower/imrcfix
but you will need some soldering skills to do it the box is bolted to the firewall just behind the engine check the cable is not broke first just follow it and have a look
in the link the guy it working on a st24 mondy i think but the imrc box is the same to repair so it still a good link ..
your lambda sensors are under the front of the car ontop of the cat you will need a 21mm ring spanner or a lambda socket to remove them if these have gone the car runs in a fail safe mode thats very ritch so between this & the imrc box not working your car will drink fuel bigtime and loose a lot of power
but like i said if your engine management (EML) light is lit get the codes read before you touch it its well worth it
good luck
cheers mark