Starting dropping my ANT+ power meter last week.

Usually the problem is always at the side of these kind of cheap power meter manufacturers brand like this half-dead Zwatt or InPeak, i wonder why (i know, it is cheaper with 50-100eur than others...) people buy pmeters from such a company if there are a few reliable brands since 10-15 years with extreme good support. i dont think that the Zwatt has/will have new firmware to eliminate this issue (the InPeak could do it so-so because that is "still" alive) so just use the BT, that is also good for power.

Tested the PM with my old 820 and it still works fine via ANT+.   It has worked perfectly for 4 years, and still does. It's only last week that the 840 stopped working with it.  Granted it was a calculated risk to go with Zwatt  but I don't think this issues is with Zwatt.  FWIW:  Zwatt has OMEd their PM and tech to other major companies.  Their original business model of having to pay a yearly fee for the PM didn't do them any favors.  I never had to.

i wrote it is the situation with (some) other brands too. yes, with older Edges it is working but since the new Edge series (x40) the Garmin changed some things in the ANT communication and certain power meters with bad or non-existent support cant handle it (because they developed their firmwares 4-5years ago for the actual head units and their actual ANT communication, and probably they made a few mistakes in the programming, they didnt follow every rules in the ANT specification just developed the firmware for the actual models, and thats all but not for the future, i mean the future possible changes but within ANT protocol specs)  until they modifiy their firmwares for the new communication method. i dont know why just last week stopped in your case, there is a hidden menu (on the main 1st datapage, touch the display, there will be on the bottom a left and right arrow and a house icon, press and hold for 6secs the right arrow and the menu will pop up) in the Edge where You can activate the ANT RX/TX logging (in the beta fw it is default as i remember) and if You share/send this data (it is somewhere in the Garmin folder, i have to check my 840's drive. UPD: It is in the Garmin\Debug folder, every files which start with "ANT" prefix, csv, fit, you can open the .csv files with any fileviewer, check what is written there with the date/time of the 15s dropping/reconnecting happen, maybe that info helps a bit for us/Garmin here ) to Garmin probably they will be able to see where  is the exact problem. or You can ask the Zwatt also about the "ANT comm issue with new x40 Edge series"

btw, did You try to press and hold the power button for ~20-30sec until the unit rebooting by itself? sometimes it helps.

meanwhile i checked my previous logs and i found that there is a huge signal quality change/decrease ONLY with power sensor from 9.29->18.15 (, the usually 97-98% to below 90%, 80-88% range or lower, the latest outdoor signal quality was 79% with 19.09 last monday, which is terrible low, i never had such a bad RX/TX ratio with my pmeters (Power2Max NG), and since the summer i noticed a few 3s dropouts in my logs and/ or weird cadence values in almost every rides (i never ever had it before), probably it is the reason, i dont know . If i will have more time, i will do a deeper analysis with exact fw numbers and signal ratios ( i dont know yet is it only beta fw issue/problem or all stable fw involved, maybe the ANT logging is decreasing the signal ration , i still dont know). But for sure , there are much more rxfail messages in the files as before...  And probably in your case the signal quality is already so low that You almost always have data drops , probably much below as 75-80% , which is still the lower limit imho for continous power reading (with 1-2 3secs drops per activity)

short list:
1st aug 9.29 96%
2nd aug 9.29 97%
3rd aug 18.15 85%
4th aug 18.15 88%
16th aug 18.17 85%
2nd sept 18.23 85%
23rd sept 18.25 80%
1st oct 18.25 85%
14th oct 19.08 78%
30th oct 19.09 79%


aweatherall do You have any idea about it? 

I have comparable problems with an 840 solar and a Garmin Rally PM since a few weeks. The unit looses connections to all sensor within the first few minutes of a ride or doesn't connect at all until I reboot it. The setup is unchanged and worked fine since the release.

Just noticed a new firmware and try tomorrow...

I have been having the same issues with my xcadey single sided crank power meter. I thought it was fixed with 19.19 but now it has resurfaced again with 19.22

Had a few more rides and still no disconnects/reconnects with the ANT+ connection to my PM.  IDK what the problem was in the first place, but it seems fixed now, at least for me.

I don't use a PM but several times I have gone for a ride and my ANT devices (HR, speed, and cadence sensors) don't connect with my edge. I have to restart the unit to have them connect. It may be a fault of the ANT software on the device.