Volvo Forums - Volvo Enthusiasts Forum - Is this a engine temp sensor problem?

I'm in LA but this comes to me from my son who lives in San Diego. His wife was driving their 2006 S40 and noticed that the temperature reading on the dash would suddenly and erratically jump to the red 'hot' zone and then just as quickly shoot back down to the cold zone. And it did this repeatedly. However, the CEL (check engine light) had also come on but I think that occurred before this erratic temp signal. We agreed that it sounded like an electrical problem, perhaps a temp sensor or some such thing. When he and his wife were out later that day it happened again. Worried that maybe it was more than just the sensor he went to add fluid to the coolant reservoir and noticed it VERY LOW. When he tried to add coolant to the reservoir he said it looked like it wasn't filling and then noticed a crack in the reservoir near the top. He cautiously drove it home but thinks it may have overheated in the short time they drove it. We are NOT DIY'ers. I'm too old and he doesn't have the time nor the tools nor the expertise. However, any suggestions as to what may be occurring and a recommendation for an independent volvo mechanic in the North Park area of San Diego would be very much appreciated. Thank you.