Temperature sensors have a very long history of failure. Right back to the first temperature sensors of the D Jetronic and L Jetronic fame. Every fuel injection system seems to be accompanied by its failing temperature sensor. CIS temperature sensors in the VW Fox and Audi 5000 were mounted on the bottom of coolant flanges. The flanges and hoses would seep coolant, which would corrode the sensors and their wiring. The Audi expensive 4 prong sensor continued to fail even when they flipped it to upright on the 100 models, just not as frequently. Digifant sensors did not fail as much, the blue ones. The screw in type in Digijet and Vanagon Syncros fail commonly. All the black coolant temperature sensors from the late 90’s and early 2000’s failed like a cheap nuclear warhead. And so did certain series of the “quality brand” rubber seals.
At the core of a temperature sensor is a carbon pile. As the temperature sensor heats up, the stress on the carbon core lowers the resistance value. Room temperature resistance reading generally comes out at about 2000 ohms (2kΩ). When hot, the temperature sensor measures between 40 and 100 Ohms (40-100Ω). Most temp sensor circuits run on a 5 volt signal, measured at the signal wire with the sensor disconnected. When connected, the readings should approximate 1 volt when cold, and go down to .1 to .2 volts when hot. Given that the temperature gauge regulates the amperage current, as the resistance drops, so must the voltage. That’s the law. Ohm’s law.
Although sometimes obvious and permanent, most temperature sensors fail intermittently. An oscilloscope graph of the voltage curve while a temperature sensor is heating up may reveal voltage spikes that correspond to fractures in the carbon core that open up and cause resistance changes. Be aware that damaged and/or corroded wiring can cause the same issues and codes.
As the page title implies, these are always at least two coolant temperature sensors. One, or two, for the coolant gauge or instrument panel, the other sensor is for the fuel injection system. In many vehicles, both temperature sensors are in the same unit, and just have more wires. Replacing sensors does not necessarily involve draining all the coolant. Most are at fairly high level in the cooling system, and the coolant will not be completely changed. A Karmakanix technician may even pull a slight vacuum on the system to keep most of the coolant inside, if he knows the coolant to be very fresh. Although it is good practice to replace even some portion of the coolant to help maintain the percentages of the additives that slowly leave coolant during its service life.