Hi all, This is my first posting/question.
I found that on checking the cooled air temperature, at the Dash level Vents, it is sometimes around the 8 - 12 degrees C, which is great, but often the aircon decides to cool to only 16 - 20 degrees C. This is not dependant on how high the outside temperature is, and I cannot hear any strange noises from the Compressor, or belt. I see there is an A/C sensor and a Thermostat, on the Workshop Manual Electrical Diagram, and assume that this is most likely the part/unit which is malfunctioning, and am thinking I should purchase and fit this, which which hopefully can be accessed from behind the panel, under the LH side of the Dash. Advice on what action I should take, would be appreciated.

i am assuming that you have non climate control a/c
is the compressor cycling out when it gets to those temps? if not then i'd look elsewhere ie charge level/TX function/evap drain pipe/ heater flap not fully closing
8-12 degrees is still a bit warm you should expect ~5 degrees

Thanks for your reply. It hasn't got the climate control. It cycles on and off, with about the same amount of time as it always has, when it is working well. It fairly quickly pulls the temperature down, then compressor clutch disengages. I used a temperature probe attached to a cheaper type of Multimeter, so it wouldn't be the most accurate, but serves the purpose well, in this instance. I am not familiar with that area, yet, but seems like I will need to replace whatever senses the temperature, as it is not stable, and perhaps then it will cool down nearer 5 degrees. Really gets hot in the Riverland, all too often, so it seems best if I contact a Mitsubishi Dealer, about price and availability for the Thermostat. Or is would there be a reliable non genuine thermostat available?

genuine only for the thermostat... i've only ever replaced 3 such sensors in 12 years, all of then on nissans.... not to say yours isn't naffed
could also be the pressure switch (switch mounted on receiver drier), to test with the system faulting firstly give it a wiggle, then disconnect it and bridge out the terminals on the harness plug. (if this is naffed then it will need to be replaced by an ARC liscenced mechanic/auto sparkie with the later being your best bet due to their extended knowledge of electrical stuff

also when it faults check that your condensor fan is running... if not that is where i'd look next

Thanks for the info, the fan seems to be working well, but will have to pull off the road and check, when cooling is poor. I will check what you have told me to, and I am fairly sure I will need to take it to an Auto Electrical Workshop, which is about 45 min drive away, that we have found to be quite professional (One staff member there mostly does A/C work, so I am told, so should be knowledgable and reliable).
Next thing I need to look at, is the Park Brake, as it often needs adjusting, and is almost at its maximum adjustment. I have read comments on this forum that this is a troublesome/weak area, of Third Generation Magna's. If you could point me in the right direction, it would be much appreciated? We are limited to dial-up Internet, so if you could reply with some links for postings about this, it would save frustration, and time. Thanks for your support.

The 3rd gen have a handbrake shoe (this sits inside the rear disc) for the handbrake, ie they don't use the existing rear disc pads. Last time I got a quote from mitsubishi they were $170 for a pair (part only) and around $350 to have it done (inc parts) and no one else seems to sell them.

Link (http://www.flickr.com/photos/18735806@N00/4246524993/) to exploded view of assembly

Thanks for that. Will do some checking around for prices, as seems like shoes must be well worn.

PBR brand
PBK124 (minor kit) ~$40.... Shoe and clip for when the actuator is fine but shoe is worn/disc rotor is replaced
PBK125 (major kit) $??..... Shoe,clip and actuator for when its fully stuffed

you will need 2 of either kit

http://www.pbr.com.au/media/documents/BanksiaRepairKitBrochure.pdf