Chosen Solution
The computer had water damage, some parts have been replaced. All the Apple resets known to man have been run. Now the fan spins full speed and the Apple hardware test reports 4SNS/1/C0000008: TPOP–124. I’ve found a few posts of people with the same error. Everyone assumes it’s the logic but no one seems to be certain. The issue I have is that the logic board has already been replaced. My next guess is that it’s in or near the track pad. Can anyone determine which component is actually failing from the hardware test results? Or point me in the right direction.
I believe it is the temperature sensor on the power supply the number is how far out of spec it is. If you replaced the logic board try a new DC-in board. I’d also notify the vendor for the logic board in case the DC-in board does not fix the problem. If this answer is acceptable please remember to return and mark it.
First heres the best guide on decoding AHT error codes: CNET - How to invoke and interpret the Apple hardware tests. So lets decipher yours here: 4SNS/1/C0000008: T POP–124 T 4SNS - System sensor T - Temperature The next one looks like a multiple P - Power BusOP - Optical Drive So what does this all mean? Sorry to say not much ;-{ The problem is you can’t depend on AHT alone here. You need to so a little more diagnoses to figure out whats up. Some of your actions also help here in removing possibilities, but only to a point. Water damaged systems can still have issues that one would not think of like wiring harnesses or sensors that didn’t get swapped. I think your on the right track looking at the sensors but before you start taking the system a part again lets do a little more testing. OK what is the next steps here: I would get a temperature monitoring app installed to see what it tells you. I like Temperature Gauge which you can download from the App Store (others will also work). The other thing here is running Activity Monitor and look at what process is running. Often this can offer a clue or it could lead to a bad app or malware app that is causing your system to overheat. Tell us what you see (post a screen capture or two) then we can know for sure what to dig into. Good Luck!
Late reply, but I’m adding this mostly for others finding this via search engines. I had exactly the same symptoms and same AHT error code with a machine of this model (A1278 i7 2.7) that experienced some water damage (the only other problem caused by this was the death of the SSD). I’ve been able to temporarily work around it by unplugging the AirPort card (it’s the one running from the logic board to the board with the three small round gold antenna connectors). If you’re going to try this, unplug it at the logic board end, and tuck the free end of the cable with the connector behind the logic board so that it doesn’t just re-mate with the logic board connector when you put the back case on again. I’ll update here when I’ve replaced the AirPort card.
