Sensi Thermostat Not Turning On AC [Fixed]

Is your Sensi thermostat not turning on? If so then you will find this guide useful.

There are a few things that will prevent your thermostat from turning on  the AC and we are going to be looking at them shortly in this guide together with their respective fixes.

Usually, when your Sensi won’t turn on your AC that is possibly due to wrong thermostat settings, condenser unit power issues, incorrect thermostat settings, bad ac contactor, a defective capacitor or a defective compressor unit or simply a thermostat that may need a reset.

While most of these issues can easily be fixed without the need for calling HVAC professionals for help, for some issues you will require help from licensed HVAC professionals to come through.

Let’s look at the table below which shows the main reasons why a  Sensi  will not  turn on an ac and possible fixes.

Read also: Ecobee thermostat not turning ac

8 Reasons for a Sensi thermostat not turning on A/C

Possible causeFix
Wrong thermostat settingsSet thermostat to cool and temp point below current room temperature
Wrong thermostat wiringEnsure wires for the compressor relays are connected
Power issuesCheck breakers and reset them where necessary
Thermostat needs a resetReset your thermostat
Low thermostat batteryIf your thermostat uses battery, replace them
Defective AC contactorReplace AC contactor
Defective AC capacitorReplace AC capacitor
Defective compressor Replace compressor

Now let’s dive into the details and the procedure for troubleshooting these issues.

Wrong thermostat settings

Is your thermostat correctly set?  Check your thermostat settings and verify that it is set to cooling mode.

 If it is, ensure that you set the temperature of your thermostat  to a point that is lower than the current room temperature. Otherwise if the temperature point is set to a point above the current room temperature, your condenser unit won’t kick on.

But hey, if everything seems to be properly set, you may want to check if the outdoor condenser unit is connected to the power.

Power issues

After you have taken a second look at  your thermostat and everything seems great, you might want to also verify that the ac compressor unit has access to power.

Power issue could be due to tripped circuit breakers.

If the condenser unit is not turning on with your Sensi thermostat, go to the electrical box and check if the  switches have not tripped. Switch has different labels in the breaker box but for the ac unit it is typically labeled A/C. 

If this switch is tripped, reset it and if it is not tripped and your condenser unit doesn’t seem to come on when you turn on your thermostat, there could be another power issue which could be a bad ac contactor, located in the condenser unit itself.

At this stage however, I would advise that you check with your thermostat wiring. If the wiring is good with your thermostat, I would recommend you to process with the  ac contactor and capacitor.

Read also: Outside condenser unit running but no air blowing inside

Wrong thermostat wiring

Wrong thermostat wiring prevents your sensi from turning on your ac. If this is a new thermostat installation, ensure you double check the wires in your thermostat to rule out ‘silly mistakes’ which I myself sometimes make.

If you have a manual, refer to the manual for the speak model but here is the general wiring for cooling on a sensi thermostat.

You will need to make sure the following wires are connected in the respective terminals

RC terminal: this is the power for cooling

Y wire: this is the wire for 1 stage air conditioner(conventional cooling) or heat pump systems

Y2  wire: this is the wire for the 2nd stage air conditioner(conventional cooling) or heat pump systems.

If your system doesn’t use batteries ensure also that there is a C- wire connected, this is typically a blue wire wire. Supplies consistent power for the thermostat to properly operate.

Read also: Sensi Thermostat Blowing Cold Air On Heat [Fixed]

Thermostat needs a reset

If the wiring of your sensi appears to be good but your ac won’t turn on you might want to give your thermostat a quick restart or reset. But be careful before resetting your thermostat, you might lose the programming on your thermostats. Start by first restarting it because restarting does not  erase the schedules and programs as a reset does. If none of the two works and your thermostat uses batteries to operapare, you might just have a low thermostat battery

Low thermostat battery

Did you know that when batteries are low, they can cause all sorts of problems with your cooling or heating systems.  Batteries can live up to 12 months but you need to specify with the specific manufacturer how long they can live. 

If batteries are low or about to die, you may also notice a faint display on your thermostat. When thermostat batteries are low your system will have a hard time responding to any adjustments on your thermostat and in some cases your system will not respond.

In other words, if you believe your thermostat batteries are low, consider replacing them and this can fix the whole problem. 

Defective AC contactor

A defective contactor could be another reason your sensi or any other thermostat model is not turning your ac.  A contactor is a device that completes or ‘powers’ up your ac system when there is calling for cooling.

With time, the AC contactor could  become defective and get stuck on the open position, when this happens, it wouldn’t  power the codeser unit when there is calling for cooling on your thermostat.

A contactor can easily and cheaply be replaced and this will fix the problem.

Also read: ac outdoor unit blowing cold air outside

Defective AC capacitor

If the contactor is not defective, you might also want to take a look at the condenser capacitor. A capacitor is a device that stores energy  for starting the compressor. A defective compressor could prevent your AC from turning regardless of the model of thermostat you are using.

However, it is easy to tell if you have a defective capacitor.

One of the ways is just by looking at it, does it have a bulged top if that is a good sign your capacitor needs a replacement.

Other Times, the signs won’t be visible and you’ll need to use a multimeter to test it for continuity.

 An AC capacitor, just like an AC contactor can easily and cheaply be replaced. You can even order online, but just before taking necessary precautions as capacitors can store high voltage which must be handled with caution.

Defective compressor

A defective compressor will top the AC from functioning completely. This is the core component of the  AC system. When the compressor dies, the only solution is to buy a new one. And if the costs of the compressor is more than 50% of the total cost of getting a new system, it is generally recommended fo for for a whole new system

A compressor dying is rare but just like any other machine it is susceptible to wear.

Final Thoughts

In closing, these above issues discussed in this guide are the most common causes for a sensi thermostat not not turning on an ac. We hope you have resolved the problem at this stage. If not, we highly recommend you call in local professionals for help.


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