I'm stalled working on my 01 - new driveshaft coming in from California.

The heater does not work. Turning the temp knob does nothing.

Would somebody tell me what that's likely to be, please?

A number of things could be going on. A couple guesses:

1. The cable that connects the heater control to the heater valve can break or come loose. Could take apart dash and look. See one of my prior posts for a photo.

2. Hot coolant isn't flowing through your heater core for some other reason. Could be very low on coolant or have very old or improperly mixed coolant that clogged up your heater core. Take a look at your little coolant expansion tank near radiator and in the rad itself. OEM is red, green probably ok, but heavy residues, particulate, or weird color is suspicious.

Cable is ok but does not move. I removed the clip and opened the valve manually. Piped got very hot, very quickly. Still no heat from heater - maybe a little warmer, but for sure not hot

What style of climate control do you have? There's three different kinds that control the "blend door" that switches from cold air to hot air passing through the heater core: one is the pushrod style (you'll know if you have this since you can feel the sliders pulling the rods), one had dials but it's all controlled electronically, and the third is a digital display.

The digital display kind are known to break and there are repair services online to fix them. If you've got the electronic dial style, you'll need to find the blend door servo on the driver's side of the center dash, it's a black box with silver brackets and controls the opening of the blend door. If it doesn't budge, one or more parts have gone bad. If it does move when you go from cold to hot on the dial, it's the heater valve that's clogged up or not moving.

If you have the pushrod style one, it's pretty easy to diagnose what is wrong. Either it moves or it doesn't lol.

Thanks 4 the reply.

I have the electronic dial. So I remove the plastic that surrounds the heater controls and the black box is behind that - yes?

It just might be a fuse... Mine makes no noise at all.

In the meantime, found this video:

https://www.youtube.com/watch?v=xxUJam4Km6k

Save 'yall some bucks.

If it was a fuse more than just 1 ac actuator wouldn't be working.

Maybe there is. Lights on the head unit, clock...

If my new driveshaft ever gets here I can proceed

No, actually the faceplate is not removable. The blend door actuator/servo is near the gas pedal, just to the right on the side of the dash. There might be trim pieces or wires obstructing the view but it's there behind it all. With the car running, turn the dial from cold to hot and you should see/hear it operate. If not, something wrong either with the climate control unit or the blend door actuator/servo.

Lights are burnt out and the springs in your clock need to be stretched to get better contact to work.

You guys missed the easier diagnosis. Start at the beginning. He has the digital readout, so it'll say on the LCD screen what the temp SHOULD be. If the temp doesn't change when the dial twists, then it's the climate control unit itself. This is extremely common, and has/will affect near EVERY automatic climate control out there.


OP, this can either be repaired DIY (usually bad solder joints; I fixed mine myself, easy peasy) or you can use a repair service like this:
99 00 01 02 Toyota 4 Runner Limited Digital Climate Control Repair Service | eBay

I must have missed that because he said he had the electronic dial design but didn't say he had the digital display. There are 3 different hvac control unit designs like previously stated.

I don't have digital display. Sorry for the confusion. Just 3 big circular dials - fan temp and air flow.