Ac off comes on code po128 was on chg temp coolant sensor everything went off now ac off is back on

SOURCE: po128 code on 2001 corvette

replace the thermostat. computer sees engine is running too cool thermostat stuck open.

SOURCE: cooling fans on 2001 chevy malibu will not come on unless ac on

the relays control circuit is grounded by ECM (computer) . to check this make sure the is hot enough for the fans to be commanded on by computer and check to see if you have voltage at relay and it has ground from computer . you may need ECM.
good luck

SOURCE: 95 camaro engine runs hot but will stop if I turn

Sounds like the thermostat!

SOURCE: Silverado Coolant not getting hot enough?

it has to be either a broken gauge, faulty wiring, or poor coolant flow (although in the summer i doubt the coolant is staying 'too cold' in reality).
First, we will check the temp sensor 1. Pull the temp sensor out and let it hang. 2. Boil water -- it will be at 212* at its boiling point but cooling rapidly, so speed is key. Or better yet, if you have a thermometer and vegetable oil and steady hands, heat that up to 250* 3. Take the water or oil to the sensor and pop the sensor in it. Watch the gauge to see if it reads accurately (should show around 200 if you moved quickly with the water, oil will still be ~240ish and will work better, but is more dangerous). If the gauge is off, then we've found the problem. For better measure, if you have a thermometer you can place that next to the temp sensor to compare results.
if the temp reading is too low (lower than what your thermometer reads), it is also possible that you have a short in the wiring or simply corroded wires connected to it, adding resistance to the wire which would result in a lower reading. (people sometimes use this to their advantage with the air temp sensor -- add a 100ohm or so resistor in-line with the sensor to trick the cpu into thinking the temp is colder, prompting it to adjust the air/gas mix and yielding a couple hp increase, although damaging to your system.)
the more i think about it, the more a faulty wiring feels like it could be the problem. if the temperature is reading accurately, i'd take a serious look at the flow of coolant through the system. if it isn't heating up then it must not be contacting enough heat. there may be blockage somewhere; crimped hose or maybe just mystery gunk.
hopefully this helps, if anything it's a start. most of the time it seems like solutions are found unexpectedly when testing completely different things. good luck!

SOURCE: sensor keep coming in for

DTC P0128: Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature
Do you have any issues with heat in the vehicle? If so the thermostat may need replaced If not the engine coolant temperature sensor is most likely reading incorrectly due to a bad sensor or connection to the sensor.