Here's a good writeup on how to decode Apple Hardware Test error codes: CNET - How to invoke and interpret the Apple hardware tests
Looking at your error: 4SNS/1/C0000008: TsOP--124
4SNS: System sensor
T: Temperature
s: Palm rests for laptops
O: Optical drives
P: Power bus
You have a few sensors firing off here ;-{ Lets see if you can isolate this down a bit.
Lets try restarting under Safe Boot mode. Follow this Apple TN" OS X: What is Safe Boot, Safe Mode?. Does the system appear to run longer without into a grey screen? If it does from another Mac system download this Temperature Gauge Pro and copy it over to your system and install it. It should help show you what your sensors are telling you. You may need to stick your system into a tight plastic bag (taking as much air out as possible) in your refrigerator for a few hours (not to long and don't freeze it!).
I suspect your heat sink has failed. The clue will be seeing all of the heat sensors running high and your fans running at full speed.
Paste in a screen shot or two so we can see the sensors of Temperature Gauge Pro.
OK, the screen shot of Temperature Gauge shows the fans are not working at all. Thats not good. Do you hear them? If you do the sensor on the fan units are not registering within SMC. So the SMC is not controlling them.
The screen shot of the display shows us the GPU is failing likely from the heat if the fans are not working correctly.
The function of cooling the system was to buy some time if the system didn't run very long. At this point I don't think it offers anything useful.
You stated you got the system back from service what did they do? And what was the reason you brought it in?
Just to be clear here, a defective temp sensor would not cause your display to react like it has.
Looking at Temperature Gauge shows us all of the temp sensors are working! So there is nothing here to replace on the sensors them selves.
The only sensor that appears not working is the tach's for the fans, they don't appear to be running high enough (better than 2k RPM) given the heat levels. If they are working and the screen shot is not telling me correctly then the GPU chip is failing here. You'll need to get the full version so you can track the temp & fan condition when the screen goes on you to verify this.
So far I don't see replacing the heat sink (or redoing the thermal paste) is the answer until we can deal with the fan issue. You are sure the system has not gotten wet some how? Correct
If the fans do ramp up as the system gets warmer and the temp of the GPU does not go red line then the GPU chip it's self has a problem.
It could be a cold solder joint or a defective chip. Other than swapping out the logic board there is not much one can do here with a BGA chip setup with out the proper gear.
You may want to get your system to a Apple Store as they check to see if there was a recall for this unit and they often offer a cheaper price for logic board swap outs.
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