Temperature gauge not working and cooling fans run all the time?

Nope. You stepped on the problem already. Thank you for including the voltage test results. You're a few steps ahead already.

For a temperature sensor circuit, it's fed with 5.0 volts, and part of the sensing circuitry is inside the Engine Computer. Some of that 5.0 volts is "dropped" inside the computer. What remains is the signal voltage that you can measure at the sensor. For this type of circuit, the acceptable range of signal voltage is 0.5 to 4.5 volts, give or take a little. The 5.0 volts you found is outside that range and is what triggers diagnostic fault codes, and, in this case, puts it into default mode for safety. The computer can't know actual coolant temperature, so it turns on the radiator fan just in case it's running hot.

There's one critical point you must be aware of. Voltage readings in these circuits are only valid when taken with the sensor plugged in. If you unplug it, then measure on the yellow wire, you will find 5.0 volts. In fact, doing that is a fast way to verify the cooling fan system is working.

You must take the readings by back-probing through the rubber weather seals alongside the wires. If you still find 5.0 volts on the yellow wire, there is a break in the circuit after that point. See what you find on the black wire at that connector. If you find 0 volts, one of the two pairs of mating terminals in that connector are not making contact. If you find 5.0 volts there, the break is in that black wire going back to the computer, pin 12. A good suspect is the mating terminals in connector C126, (red arrow). The second drawing shows what C126 looks like, but they don't show where it is. It's just a two-wire connector, inline on the way back to the computer.

To identify a bad connection in the sensor's connector, watch the voltage reading on the yellow wire as you wiggle that connector or slowly unplug it. If you see the voltage intermittently drop to around 2.0 to maybe 4.0 volts, it usually doesn't pay to try to pick the terminals to make a better connection. One in the connector is either corroded or spread. One alternative is to find a good one in a salvage yard. Considering the time involved, a better alternative is to get a new connector from an auto parts store. They have huge books showing almost every connector for every brand and model.

On older models, the sensor for the dash gauge had just one wire. That was totally independent of the Engine Computer sensor circuits. Depending on the car model, the gauge needle would go fully one way when you unplug the sensor, and fully the other way if you ground that wire.

Coolant temperature sensors, in fact all temperature sensors, for a computer, will always have two wires. That is so the ground return can be monitored. When the computer wants to set a diagnostic fault code, seeing what's on the ground wire helps it determine which code to set.

For a long time you'd find two coolant temperature sensors on the engine. Once the electronics became more complex, they went to just the one sensor with two wires. The Engine Computer learned coolant temperature from that, but then, all the computers talk back and forth to each other over a pair of twisted wires called the "data buss". Each computer takes a turn at sending out all of its information, then the other computers decide which pieces of that information to use and which to ignore. On some models the instrument cluster is most the intelligent, (complicated), computer on the car. It can see the engine data on the data buss, then set the gauges to where they should be. On some models, the Body Computer compiles a list of data it has learned from the other computers, then sends that list to the instrument cluster.

I don't see any reference to the single-wire sensor on your engine. If you did have that, it only runs the gauge. It's not involved with the radiator fan. With the one sensor, both problems will be solved when the sensor circuit is repaired.

You missed the most important point. Voltage readings in sensor circuits are only valid when taken with everything plugged in. While there can be rare exceptions, if you unplug something, then find 5.0 volts on a wire, the circuit has to be okay up to that point.

Place the voltmeter's negative probe on a paint and rust-free point on the engine or body, or on the battery's negative post. With the red positive probe, poke it into the back of the CTS connector alongside the yellow wire. You're supposed to find somewhere between roughly 2.0 and 4.0 volts. If you do, the circuit is working. The other part of the 5.0 volts is being "dropped" across circuitry inside the Engine Computer.

If you find 5.0 volts on the yellow wire, there has to be a break after that point. From this point on, follow the diagram to the black wire in the connector, (black arrow), then to the next connector, (red arrow), then, if necessary, to pin 12 at the computer. As you work your way down that circuit, if you keep on finding 5.0 volts, you haven't reached the break yet. As soon as you pass the break, you'll find 0 volts. A broken wire is always a possibility, but by far, corroded or spread connector terminals are more common.

If you're still confused, take the voltage reading on the black wire at the CTS connector and tell me what you find. The ignition switch must be in "run", but the engine doesn't have to be running. Remember, you have to take that reading with the connectors plugged in, and by sliding the probe in alongside the wire.

2.6 volts indicates the circuit is working properly at this time. That's why the fan wasn't running. A lot of GM models did run the fan when the engine was turned off. That was done to pull some heat out of the engine to reduce the chance of developing a head gasket leak.

As I was hoping, poking the test probe into the CTS connector put enough pressure on it for the terminals to make a better contact. Watch the voltage again on the yellow wire and see if it intermittently pops up to 5.0 volts when you wiggle that connector. If it does, you'll either need to pull the terminals out to squeeze them tighter, or the connector will need to be replaced. A lot of those terminals are too tiny to effectively repair them.

If you need to replace the connector, you can find one in a salvage yard, but you run the risk of getting one worse than yours. This might be a good time to buy a new one from an auto parts store.

There's over 2,000 defects the Engine Computer can detect and set fault codes for. Only about half of them refer to things that could adversely affect emissions. Those are the codes that turn on the Check Engine light. A code for the CTS will usually turn the light on because the computer won't be able to include coolant temperature in its fuel metering calculations. On most models the Check Engine light turns off while driving if the problem goes away, or it will turn off after the engine is stopped, then restarted. The code will still be stored in the computer.

At this point the CTS circuit is working. You'll have to wait for it to act up again, then do these tests over. A scanner is just going to allow you to see what the computer is seeing, in this case, 2.6 volts. That is just the starting point in finding the cause of this problem. Next comes the voltmeter tests. The scanner will just tell you there's a problem. You need the voltmeter to find the exact spot for the defect.

The gauge and fan are related. With a break anywhere in the CTS circuit, the computer will "see" 5.0 volts for the signal voltage. That's outside the acceptable limits of roughly 0.5 to 4.5 volts. It recognizes that as a defective condition, and sets the fault code for "CTS signal voltage too high. Since now it doesn't know actual coolant temperature, the computer turns on the radiator fan to be sure no overheating occurs. As I mentioned, whenever we have a car come in with an overheating complaint, the first thing we do is unplug the CTS. If the radiator fan starts running, we know we don't have to look in that system. That's a quick, ten-second test to start the diagnosis.

The fan running for one minute is further proof that system is working.

You're right about the old sensor likely being okay. Temperature sensors have just one component inside them, so failures are very uncommon. You're more likely to find one with one or both terminals corroded off before you find one with a bad sensing resistor.

When the problem occurs again, go right to the sensor and take the two voltage readings again. By poking the test probe alongside a wire, it's very possible the terminal moved or repositioned itself a little, just enough to make contact with its mate. There could also have been a light film of corrosion on the sensor's terminals. When that happens, simply unplugging the connector a few times can scratch that film off, then it can work fine after that for days or years.

I'll be back tomorrow to see how you're doing.

The gauge and the radiator fan relay are on two totally separate circuits. When the CTS reaches a predetermined voltage, usually closer to 1.0 volt or less, the computer turns on the radiator fan relay. That circuit is either on or off. The Engine Computer also sends out engine sensor data over the data buss for any other computer to see and use. The instrument cluster looks at that data, then decides where to place the gauge's pointer. You are correct, that is a stepper motor. Unlike motors that have brushes and a spinning armature, stepper motors have four coils of wire that are pulsed with varying voltages and polarities to place the armature to the desired position.

Do I take this to mean the CTS voltage is correct at roughly 2.6 volts, the radiator fan is not being turned on, but the temperature gauge is staying on "cold"? If so, suspect that stepper motor. GM has had a real lot of trouble with them. You can find repair kits on eBay that include four or six replacement motors.

Incorrect gauge operation will not affect anything else. Once the computer circuit in the instrument cluster has commanded the temp gauge to the desired position, it has no way of knowing if that actually took place. Same with the Engine Computer and the radiator fan circuit. They are not involved with anything to do with the gauge.

The next step is going to require a scanner to see what the Engine Computer is seeing. Under "live data" it will show the CTS voltage and the temperature it corresponds to. If the CTS voltage you measured right at the sensor, on the yellow wire, is different than what the computer is reporting, the computer itself would be the best suspect.

Unlike simple fault code readers, scanners are "bidirectional", meaning they allow you to talk back and forth to the computers and command them to do thigs. There is usually a gauge test function that typically places every gauge at 1/4, 1/2, 3/4, full, then back to 0. If the coolant temperature gauge follows the same steps as all the other gauges, we would know the stepper motor and instrument cluster's circuitry are okay.

Scanners also have tests that let you command the computer to turn relays on and off. It should also show when a relay has been commanded on by the computer.

There's only two other things I can think of that might cause the radiator fan to run. One is if the AC is turned on, or if the heater is in "defrost" mode. The AC will run in defrost mode to remove humidity from the heated air before it is blown onto the cold windshield.

The other thought is the computer may still be responding to the diagnostic fault code for a CTS problem if that code hasn't been erased yet. Fault codes in the Engine Computer can be erased by disconnecting the battery's negative cable for a minute. Fault codes can be erased too by using the function on the scanner.

By the way, do you have a single radiator fan with the AC condenser in front of the radiator, or do you have two separate fans, one for the radiator and one for the condenser, side by side? If you have two fans, double-check it's the radiator fan that is running.

Allow me to make another suggestion. Consider looking for an Automotive program at a nearby community college. We were always looking for live work to provide real-world learning experiences. There were about three dozen regular customers who knew the value of these projects, and would sit on a broken car until it fit what we were teaching. In this case I ran my Automotive Electrical class once a year for eight weeks. We didn't take in electrical work at other times as that would put us in competition with the local employers who hired our graduates.

In this case we don't need students to repair the problem, and they don't even have to diagnose it, if they could just connect the scanner to see what the Engine Computer is seeing and responding to.

Even if there is still a problem with the CTS circuit, that shouldn't turn on the AC condenser fan. There's something going on we're overlooking.