My training app can't find the trainer when it scans, or I can't connect to the trainer

General tips

Make sure the trainer is on.
Check that the speed sensor is aligned with the spoke magnet, about a finger's width apart. The red light should blink (Halcyon) or toggle on-off (STAC Powermeter) each time the magnet passes the sensor.
Try pedalling. Some apps may not recognize the trainer unless it is broadcasting power.
Re-scan for devices.
Close and restart the app.
Your trainer's battery level affects its radio signal strength and range. Try charging the trainer.
Try moving your device closer to your trainer. ANT+ and Bluetooth are low power protocols, with a maxium range of about 10 feet (3 meters) and 15 feet (4.5m) respectively.
Make sure the trainer is not connected to another app or device. On the Halcyon, if the green light is blinking, the trainer is already paired. Close other training apps and put devices briefly into airplane mode to ensure they're disconnected.
Make sure the app is compatible with your trainer. The Halcyon connects over Bluetooth FTMS and ANT+ FE-C (standard protocols for smart trainers) while the Powermeter uses Bluetooth Cycling Power and ANT+ PWR.

ANT+

Note that you'll need one ANT+ dongle per training app. They don't like to share. Other apps such as Garmin Connect can also monopolize an ANT+ dongle.
Try removing and reinstalling the ANT+ USB stick a few times. Do this with the app open so you can see if the ANT+ icon is turning on. If you don't see the ANT+ icon, see I can't I get my device and app to recognize my ANT+ USB stick
See also: How do I enable ANT+ for my device?

Bluetooth

Make sure you've enabled Bluetooth and location services on your device, and airplane mode is off.
On Android, enable Bluetooth and location permissions for the app. If you have denied location permissions in the past, go to Android Settings to enable permissions.
Do not attempt to connect to the trainer in your phone’s main Bluetooth connection menu. It is a different connection protocol. If you have previously tried connecting in the phone’s Bluetooth menu, put the device briefly into and out of airplane mode and connect from within the app.

My trainer loses connection intermittently during rides

Symptoms of interference

Your training app repeatedly disconnects, or disconnects and reconnects with the trainer.
The Halcyon stops responding to changing ERG targets or hill slopes partway through your ride.
Power and other readings stop populating, drop to 0, or "stick" at a certain value.
In a simulation, your avatar keeps stopping even though your wheel is spinning.
You may also notice other ANT+ and Bluetooth devices intermittently losing signal as well.

Solutions

1. Trainer radio range

Charge your trainer. Low batteries can affect the strength and range of the radio signal.
Check that the speed sensor is aligned with the spoke magnet, about a finger's width apart. The red light should blink (Halcyon) or toggle on-off (STAC Powermeter) each time the magnet passes the sensor.
Move the controlling device closer to the trainer. ANT+ and Bluetooth are both short-range, low-power protocols. In our experience, ANT+ generally tops out at about 10ft (3m), and Bluetooth about 15ft (4.5m).
For ANT+, you can use a USB Type A extensions cord (male to female) to get the dongle closer to the trainer.

2. Interference in the environment

Both the Bluetooth Smart and ANT+ protocols use radio waves in the 2.4 ghz range to communicate. Unfortunately, many other common household items, such as Wifi routers, microwaves, and cordless landline phones also communicate over the 2.4 ghz band.

Move the controlling device closer to the trainer. Much like speaking directly into someones ear can help them hear you in a loud room, moving the receiving radio close to the trainer's broadcasts will help it send and receive commands more reliably.
Streamline the number of devices using Bluetooth and ANT+ in the area. For example, there's no need to use a separate powermeter, and using "power matching" features available in some apps will cause ERG commands to lag.
Change your Wifi Router's channel. Changing to a different channel slightly changes the exact radio frequency your router uses. Wifi Routers typically ship using 2.4 ghz channel 1, 6, or 11. Changing channels may actually improve your household's wifi reception (by using a channel your neighbour's equipment isn't using), on top of helping with interference with your trainer.
If you have an older microwave, check if members of your household are using it during the incidents of poor connection your trainer. Microwaves work by blasting food with hundreds of watts of 2.4 ghz radio waves, exciting water molecules. This means that any shielding leaks from a microwave will typically overwhelm any 2.4 ghz communication in your house.
Switch to using only the 5 ghz band of your wifi router. This will completely eliminate your wifi as a source of interference with your trainer, since the trainer only uses 2.4 ghz. Switching to 5 ghz may also improve your general Wifi reception. Note: Older wifi-enabled devices in your house may not work on the 5 ghz band.
Move any large metal objects between your device and your trainer (not including your bike of course!).
Try moving your training set-up to a different area, if that's available to you.

3. Devices

Switch protocols if it's available. ANT+ has a shorter range but can be more reliable within that range in cases where Bluetooth is unreliable.
Try a different phone or device, if one is available. Not all Bluetooth radios are created equal.

I'm connected to a training app but my Halcyon doesn't change resistance

Some apps have the option to connect with the trainer as a powermeter or source of power only. Make sure that you are connected as a smart trainer. Apps may also use the terms “controllable” “FTMS" or "ANT+ FE-C trainer" instead of smart trainer.
Some apps are capable of receiving power data, but not of sending smart trainer commands. Apps created for or partnered with a specific trainer brand may not control other trainer brands, control only a limited number of trainer models, or only send commands to other trainers over ANT+ but not Bluetooth. Check with your app's support resources. Compatible apps must send commands over one of two standard smart trainer protocols:
- ANT+: FE-C
- Bluetooth Smart (BLE): FTMS
Zwift is capable of controlling the smart trainer on Windows, Mac, iOS, and Apple TV. However as of 2019, Zwift on Android is not.
Troubleshoot Bluetooth and ANT+ interference in the environment if the Halcyon stops responding to new commands partway through your ride, or if it stops and starts responding.
Your trainer may have shut down because it's low on batteries. Charge the trainer.
Make sure the trainer is not receiving commands from another app:
- Check the status light on the trainer to see if it is paired with more than one app or device over Bluetooth.
- Close all other training apps.
- Put other devices you have previously connected with into airplane mode briefly to force them to disconnect.
Check if the magnet arrays are able to respond to a manual commands in STAC Control Panel. Contact Support for assistance if one or both sides are not moving well.

STAC ZERO: Troubleshoot Bluetooth or ANT+ connectivity

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