Fixing Nest error e73 – No power to Rc wire detected

It’s been heating up in Texas recently, even more so when I had discovered that
our Nest unit was giving us grief about the “Rc wire” not being detected.

This is an existing unit, not one that I recently installed, so I was skeptical
that the wire wasn’t connected properly. I still removed the unit and checked
that the Rc wire was in fact still connected.

It was. Next stop, going outside to check that the air conditioning hadn’t
suffered some unforeseen tragedy.

Reads: was anything smoking?

Still all good, so I did a bit of research on the issues, one of which was that
unit’s battery could have been too low. You can check that easily enough via the “TECH INFO” section under “Power”.

Battery was within normal voltages, up to the attic we go!

Meanwhile, in the attic

While up in the attic, I cut the power to the unit and started to do some light
weight debugging.

The filter was fairly clean, but it was time to swap it out, so I went ahead and
did that. Next thing to check was the drip pan under the unit.

Bone dry. The safety switch couldn’t have been enabled without any water in the pan. That’s assuming that the switch didn’t take a shit or something like that.

Before blaming the switch, I took a look at the PVC pipes around that unit,
which are to/from where water condensates. There happened to be a tube with a second safety switch in it!

Upon pulling the second safety switch, I found that the tube was full of water.
Best as I could tell there was a clog somewhere in the line. My theory is that
it was probably in this one section of tubing that angled in such a way that
water could easily pool in there.

At this point, I went ahead and turned the A/C back on and within a minute or
two, things started back up again. Upon checking the Nest unit downstairs,
everything was back to working as expected.

Safety first

The safety switch that was being triggered is designed to be a kill switch to
the air conditioning unit. When it’s engaged, the A/C shuts off and more
importantly, your Nest unit will report that it’s not receiving any power, which
it’s not.

That’s not where I left things, using what we had available, the wife and I used
a baby nasal aspirator to clear out the water to get things back to a decent
state. I also used a snake from a Drano Snake Plus kit that we had.

Side note, I always keep at least one of those Drano Snake Plus kits in the
house. You never know you need it until it’s too late, so I like to always be

Without a wet/dry shop vac (and the adapter to hook up to your A/C) or the magic little CO2 air blaster thingy, we opted to call a professional to actually flush the line.

That said, a professional in our area runs about $125 to clear the line, which
is a bit more expensive than the aforementioned items.

Knowing what we know now, there are a few home remedies to help keep things from getting gunked up in the tubes. Like most things, it just takes regular maintenance, including but not limited to putting vinegar and/or hydrogen peroxide and/or bleach (YMMV on that one some swear by it, some swear it off) into the line’s T tube once a month to once a quarter.

There’s also some tablets on the market that seem a bit more up my alley
assuming they work (review to follow, perhaps?)

Hopefully this post helped you figure out what was up with your Nest and/or air conditioning unit!

Josh Sherman - The Man, The Myth, The Avatar

About Josh

Husband. Father. Pug dad. Musician. Founder of Holiday API, Head of Engineering and Emoji Specialist at Mailshake, and author of the best damn Lorem Ipsum Library for PHP.

If you found this article helpful, please consider buying me a coffee.