First, are you sure the engine is fully warming up?
Second, is to make sure it's actually plugged in.
Picture I stole from another thread. It's the sensor at the very bottom right of this picture with the brass terminal. It's unfortunately quite difficult to get to in the vehicle. If you remove the smog pump and take the belt off (which should only take about 10 minutes), it will give you much better access.
To test, unplug the sensor and stick a long piece of wire in the harness plug. Take the wire with you into the cab, turn the key on, and briefly tap the wire to a good ground inside the cab. The chrome ring on the door light switch is probably a good spot. The reason you want to be in the cab, is you can't leave the wire connected to ground - it will damage the gauge. You just want to briefly hold it - just long enough to see the gauge move.
