Is engine actually not warming up or is the gauge saying it is not warming up?
There are two temp sensing devices located just above thermostat housing on lower intake .
One is a TWO wire ECT(engine coolant temp) sensor, this is for the computer only, when temp is cold computer runs Choke mode, rich fuel mix, higher idle and advanced spark timing, O2 sensors are ignored.
As coolant/engine warms up idle will go down and computer will start to use O2 sensors to fine tune fuel mix.
ECT sender is a ONE wire sender used only for the dash board temp gauge.
These look alike but are not the same
Thermostat shouldn't open for 4 to 8 minutes after cold start, this means upper rad hose should stay cold for 4 to 8 minutes until thermostat starts to open and sends warm coolant thru upper hose.
If upper hose is warming up with the engine then thermostat is leaking or stuck open.
TPS(throttle position sensor) check the wiring, 3 wires, make sure connection is good and wires are not loose in connector.