apple hardware test error code 4sns/1/C0000008 TPCD--124?

the “4SNS” in the example above means there is a problem with a system sensor, which can be a temperature sensor, a current sensor, or a voltage sensor. Now you can determine which of these is the case by looking at the final detail of the reading (TPCD--124). In this case “T” stands for temperature, but it could also be “I” for current, or “V” for voltage.

The letter following the “T,” “I,” or “V,” lets you know what component of the system the sensor is in. The following list is a breakdown of what each of these components is:

A: Ambient air sensor

B: Battery

C: Central processors (CPU)

D: DC (direct current)

e: PCI-express slot

F: FireWire port

G: Graphics processor (GPU)

H: Hard disk

h: Heat pipe (heat sink)

L: LCD display

M: Memory or memory riser boards

m: Misc. (i.e., battery chargers)

N: North bridge (motherboard controller)

O: Optical drives

P: Power bus

p: Power supply

s: Palm rests for laptops

W: Airport Wi-Fi card