The short answer
A Rheem EcoNet thermostat in Northern Virginia shows each problem as a code: a letter, a number and a suffix. A is an alarm (red), T is an alert (yellow), and the suffix says which equipment: _O the outdoor unit, _F the furnace, _A the air handler, _C communication, _S the system and _Z a zone. Some are reminders you clear yourself, like S001_S for the filter. A021_A means the indoor coil is freezing. A001_C through A008_C mean the thermostat lost touch with a piece of equipment.
How to read it
- Read the whole code, suffix included. A002_C and A002_S are different problems.
- An L in front means that equipment locked out.
- Only alarms reach the EcoNet app. A yellow alert shows on the thermostat but not on your phone, so look at the screen when something feels off.
The $67 Heating Check
We check your furnace, heat pump or boiler before winter. Through December 31. See the offer
The codes we see most
Swipe to see all columns →
Swipe to see all columns →
| Code | What it means | What to do |
|---|---|---|
| A001_C | Lost the furnace | Cycle power once, then call |
| A002_C | Lost the outdoor unit | Cycle power once, then call |
| A005_C / A006_C | Lost the air handler / the outdoor unit | Cycle power once, then call |
| A004_C | No air source found | A wiring or firmware problem: call |
| A007_C / A008_C | A zone panel stopped talking | It runs as one zone until fixed: call |
| A002_S | Sensor failure | An installer menu fix: call |
| A004_S | Duct static over 1 inch | Check the filter and vents, then call |
| A021_A | Indoor coil freezing | Cooling off, fan on, change the filter |
| A026_C | Old firmware | Needs an update |
| A028_S / A029_S | More than 10 degrees off the setting for over 2 hours | Check the filter and the outdoor unit, then call |
| A041_C | Outdoor unit not supported | An installer fix |
| A900_S | Thermostat failure | Replace the thermostat |
| T009_S | Motion sensor | Cycle power |
| S004_S | Clock not advancing | Replace the thermostat |
Reminders you can clear
S001_S is the filter reminder: change the filter, then reset the timer from the alert. A003_S is the service reminder. S003_S means set the clock, which happens after the thermostat has been without power for about a week.
A021_A: the coil is freezing
The air handler saw its coil at 37F for 15 minutes while the airflow was getting choked off. Set the system to off and the fan to on so the ice melts, then change the filter and open any closed vents. If it comes back with a clean filter, it’s usually low refrigerant or a blower problem.
A028_S and A029_S: it can’t keep up
The house has been more than 10 degrees off the setting for over two hours. In a cold snap a heat pump can fall behind on its own, and our heat pump winter guide covers when that’s normal. With a dirty filter, a frozen outdoor coil or backup heat that isn’t coming on, it needs a look.
Communication codes
EcoNet equipment talks over the thermostat wire. When a code ends in _C, the thermostat stopped hearing from something. Turn the power to the equipment and the thermostat off for a minute and back on, once. If the code returns, the cause is usually a loose or damaged wire, a bent pin at the thermostat base, wires on the wrong terminals or old firmware, and that’s a technician’s job. Our C wire guide explains the wiring.
What not to do
Don’t go into the installer menu to clear codes or change settings, and don’t factory reset the thermostat. It holds your system’s setup, the equipment, the airflow and the zones, and a reset can leave the system running wrong or not at all. A002_S is fixed from that menu, which is why it’s a call.
What a technician does
We read the alarm history on the thermostat, check the communication wiring end to end and the firmware versions, then test the equipment the suffix points at. You get the price before we fix anything.
Our heating check is $67 per system, through December 31: furnace, heat pump or boiler.
Related: Rheem heat pump codes, Rheem furnace codes, Rheem AC not turning on, thermostat not working, thermostat messages and every brand in our error codes guide.
Every code meaning here comes from Rheem’s own documents: EcoNet thermostat troubleshooting, A17 troubleshooting and air handler troubleshooting.
Call or book
A repair visit starts with a flat $99 diagnostic, or $250 after hours, and you get the price of the fix before we touch anything. Call (703) 997-9757, day or night, and tell us the code you counted, or book online from goswiftpro.com/cookiesoffer by November 30 and a box of warm MOLTN cookies comes with the heating check. Every current deal is on our offers page. Rated 5.0 stars across 1,138 reviews on Google, Angi, Yelp, the BBB and Facebook, Virginia Class A license 2705192276.
Questions people ask
What does A021_A mean on a Rheem thermostat?
The indoor coil is freezing. Set the system to off and the fan to on to thaw it, change the filter and open closed vents. If it comes back, call for service.
What does S001_S mean on a Rheem EcoNet thermostat?
It’s the filter reminder. Change the filter, then reset the filter timer from the alert screen.
What does A004_S mean on a Rheem thermostat?
The duct static pressure is over 1 inch, so airflow is choked. Check the filter and the vents first; if it stays, the ductwork or the blower needs a look.
Why didn’t the EcoNet app show my thermostat alert?
Only alarms, the codes that start with A, go to the app. Yellow alerts that start with T show on the thermostat screen only.
