RV Thermostat Troubleshooting Guide

Today I’m going to walk you through how these RV thermostat’s work, what each of things dang wires are doing (cause its difference than a house thermostat), and how to test them with a multimeter. This is RV Repair Woman. You got this.

How thermostats work

Are you one of those people that cranks the thermostat to 55 to “cool down the RV faster?”

I find A LOT of people misunderstand how thermostats work whether that be in an RV or in a house. You see a thermostat is just a series of ON/OFF switches. Just like a regular light switch in your house.

I think because a lot of thermostats have a slider on them, people think that thermostat has some sort of dimming function. This isn’t the case.

All the thermostat is doing is turning on your AC, waiting until the temperature in your RV equals the temperature on the slider, and then turning the AC off. (The same goes for your furnace.)

All you’re doing when you when you crank the thermostat as low as it goes is increasing the time that the AC or furnace is on.

So just set the thermostat to the temperature you want the RV to be and walk away. You got this.

Overview

Are you having problems heating or cooling your RV?

Then the thermostat is THE place to start your troubleshooting.

If you’re someone who already has their multimeter out a ready here is the quick guide to what thermostat wires are doing.

You can also get a cheat sheet for this at here.

But If you need more of a walkthrough tutorial let’s jump in.

Voltage In

If you think the thermostat is your problem then the first thing I want us to check is if voltage is even making it TO the thermostat.

Open up your thermostat or turn it around to get access to the wires.

The first step to test the thermostat is to identify our KGG or our Known Good Ground. For the thermostat that ground is going to be the blue wire.

SO set your meter to Dc volts.

Put your black probe on the blue wire and your red probe on the red wire and you are looking for something in the realm of 12V.

If you do NOT see 12V on the red wire when you do this then your problem is not with your thermostat and instead is upstream. Check your fuses. Check you batteries. Check the wire connections of your thermostat.

I didn’t design it, I’m just the messenger.

If your furnace has an access panel and if your furnace has an on/off switch directly on the unit, go make sure that switch is in the ON position. People sometimes turn this switch OFF for the summer thinking they don’t need their furnace anymore and accidentally cut power to their thermostat. See if that doesn’t solve your problem.

Cheat Sheet

If you are sitting at home with your mimosa watching this video and taking notes you are

an RVer after my own heart

and

in luck

Because that cheat sheet I mentioned early also has little pictures on it to remind you where to put the probes for each test.

Just a reminder that you can get that cheat sheet at RVRepairWoman.com/thermostat.

YOU GOT THIS!

Compressor

If your thermostat is getting power to it, but your RV’s AC isn’t cooling you may want to test and see if the thermostat is sending signal to the compressor on the air conditioner. (The compressor is the part on the AC that does the cooling.)

To test this set your thermostat to cool. You may have to hold the cover that you took off of the thermostat back over it to see where you need the switches to go to do this. And set then set the temperature on the thermostat to as low as it will go, to insure that the AC should be kicking on.

Then put your black probe on the blue wire and your red probe on the yellow wire. You are looking for something in the realm of 12V.

If you see 12V then congratulations your thermostat is living its best life and doing its job just like it's supposed to.

If you had power going into the thermostat, but do not see 12V on the yellow wire check to see if the thermostat has an internal fuse.

If you do, pull it out and test that bad boy for continuity.

But if there is no fuse or the fuse is good then the thermostat is bad and should be changed out.

Good news is thermostats are way cheaper to exchange than an entire air conditioner.

And you solved the entire problem yourself! You got This!

High Fan or Low Fan

When a customer tells me their RV’s Air conditioner isn’t turning on at all, then what that usually tells me is that the AC’s fan isn’t turning on. Here’s how you can test if the thermostat is the problem.

Set the thermostat to either HIGH FAN or LOW FAN. Avoid the AUTO setting while testing because we are in less control of what’s going on with we choose that setting.

Again you may have to hold the cover that you took off of the thermostat back over it to see where you need the switches to go to do this.

Set your multimeter to Dc Volts. Then put your black probe on the blue wire and your red probe on the Green wire if you chose HIGH FAN or the gray wire if your chose LOW FAN.

Do you see 12V? Then yaaaaay. Your thermostat is good.

If you had power going into the thermostat, but do not see 12V on either the green wire or the gray wire check to see if the thermostat has an internal fuse. If you do, pull it out and test that bad boy for continuity. If its bad change it.

But if there is no fuse or the fuse is good then booooo. That’s a sign that the thermostat is bad and should be changed out.

You’re doing great.

Next steps

If you found this post because you’re struggling to get your AC working and madly combing through YouTube for the solution you’re in luck.

I’ve put together some step by step pdf guides that walk you through how to diagnose specific problems with your AC. Like “my AC isn’t cooling at all!!” or “my AC won’t turn on!!”

They are set up just like this post is, showing you where to put your meter’s probes for every step and telling you what the expected reading should be.

We go from the thermostat, to the control box, to the capacitor, and on to the compressor. We test it all.

These guides cost 1/10th of the cost of some mobile techs and most people can go from beginning to end through the entire guide in about an hour.

So stop combing through Youtube HOPING to find the solution to your AC woes and just figure out the solution now.

You can find these guides here or at rvrepairwoman.com/acguides.

Back to your thermostat!

Furnace

If you do see 12V then the thermostat is NOT your problem.

If you don’t see 12V, you know the drill, check for a removable fuse. If the fuse is bad change it.

If you don’t have a fuse or the fuse is good, then the thermostat is your problem. Switch that bad boy out and hopefully you should have your RV warmer than Hell and half of Georgia in no time.

How RV Thermostats Are Different

If your thermostat is bad you may be thinking about upgrading your RV thermostat to one of these fancy residential ones. The dream sounds awesome. “Harmonize Energy Savings and Home Comfort.”

“Advanced comfort tailored to your home.”

Welllll…. Unfortunately residential thermostats aren’t compatible with RV’s. Remember in other videos when I’ve talked about how RV’s have two different kinds of power going to them? AC and DC. AC comes from plugging in and DC comes from your batteries?

Residential thermostats work off of AC power. And RV thermostats work off of DC power. So you can’t switch back and forth between the two.

If you are looking to upgrade your thermostat to get some of of those modern amenities like bluetooth and wifi control, try looking into Micro-Air thermostats.

https://www.microair.net/collections/easytouch-rv-thermostat-dometic-replacements

They are the only SMART RV thermostat I know of that works. The interface isn’t as cool and modern looking as say a nest, but it works and gets the job done.

And the touch pad is waaaaay better than the touch controls on these Dometic CT thermostats. If you have one of these you know what I’m talking about.

“Cat 5” thermostats

If you opened up your thermostat and went I have no idea what this Emily woman is talking about, my thermostat only has three wires going into it - I see you. Don’t worry I’m not trying to gas light you here. Some Dometic thermostats like this one.

have only three wires going to them. 12V in, the ground, and a communication cable.

Other thermostats like this one,

have a cat 5 looking cable coming out of it that works very similarly. 12V, ground, and communication. (That cable isn’t actually cat 5 by the way. Its a proprietary cable made by Dometic, just to be annoying. Its wired differently than cat 5).

Unfortunately these type of thermostats aren’t very testable. You can see if power is going to them, but that’s usually obvious by whether or not they are turning on.

And that’s about it testing wise.

If some tech out there knows a good way to test these thermostats, please let me know in the comments. I would love to learn about it.

The only sure fire way I know to confirm if these guys are your problem is to change them out for a new one and see if that fixes it.

I know that sucks. Most tech drive around with extras of these on their trucks for this exact reason.

Its stupid. I hate it. Its the price of modernity.

THAT is how you test RV thermostats. If you found this video helpful, please hit subscribe. I put out new videos like this every week giving you the RV tips and tricks that most RV techs don’t want you to know.

THIS has been RV Repair Woman.