Toyota Repair: 1997 landcruiser, closed loop control, oxygen sensors

Question
i was driving and i suddenly heard what i thought was my muffler it was a muffled loud sound i thought had a hole in it. diagnostic test read insufficient coolant temperature for closed loop control code PO125  soo i was told it needed a sensor for that and perhaps it is the thermostat. it is not overheating though. what should i try first  the sensor or the thermostat and would that make my car loose some compression on take off and make that muffled sound like its the muffler and i smell exhaust a lot now. bear in mind before this it purred like a kitten not loud at all  thanks

Answer
One of the oxygen sensors may have come loose or there is a break in the exhaust system, the O2 sensor may not be the problem if there is an exhaust leak upstream of the sensor which will draw in oxygen, this will turn the check engine light on and give the P0125 code, the thermostat has nothing to do with this.