check engine light came on and engine started to run rough

My wife told me she was hearing a high pitched whine like the fuel pump, but coming from the engine bay. During an errand the engine started to run rough and the check engine light came on. I found a low level of coolant. I pulled the code (1299) and found that the cylinder high temperature protection feature had been activated. In this mode the engine runs only four cylinders in normal mode and uses the other four as heat pumps to cool the engine, alternating the four cylinders every 32 cycles. I replaced the cylinder high temperature sensor (NAPA $25) which is behind the alternator on the left cylinder head. I drained and refilled the coolant. The car runs fine.