I have a B&M Cyo Premium front light with a three position switch: On-Senso-Off. I bought it in 2018 for a bike with a Shimano Nexus dynamo front hub, and found the Senso function so good that I left it permanently on that setting.
This summer I moved the light to another bike with a Shimano DH-3D32 hub, and now I find that the Senso function is no longer working. If left in the Senso setting the light simply does not come on. If I switch it to the On setting the light comes on, and if I subsequently switch to the Senso setting the light dims and gradually fades out, which I guess indicates that the light is drawing power from the standlight capacitor instead of the hub.
I have checked the polarity of the wires in the connector block. I have also tried powering the light from the hub on the bike on which the light was previously fitted and worked, but the Senso function still did not work.
Obviously I can simply use the On/Off switch in future, but I would be interested to know what others make of this.
Senso function on B&M dynamo light not working
Re: Senso function on B&M dynamo light not working
I have the 60 lux version and have had the following issues.
1. After a few years (3?) of 5 days a week, all year round commuting on a muddy towpath the switch at the back got a bit dodgy leading me to leave it set to on permanently (or was it senso? I'll check).
2. Early on I turned the bike upside down to clean with a hosepipe and the water ran off the tyre and filled up the light!!! After drying out the light was fine but the standlight never worked again.
Although I replaced this Cyo on my old commuter/tourer with a Luxos/USB, the old Cyo is still running fine on my tandem..(where I have separate battery lights for the standlight function)
My new dedicated commuter bike has the 80 lux Cyo set permanently to senso and I have had zero issues in about 3 years use. Although its been in the rain a lot I haven't abused it like I did with the old Cyo.
I would twiddle with the switch and see if it's connecting properly. That was a weak point with my original Cyo.
1. After a few years (3?) of 5 days a week, all year round commuting on a muddy towpath the switch at the back got a bit dodgy leading me to leave it set to on permanently (or was it senso? I'll check).
2. Early on I turned the bike upside down to clean with a hosepipe and the water ran off the tyre and filled up the light!!! After drying out the light was fine but the standlight never worked again.
Although I replaced this Cyo on my old commuter/tourer with a Luxos/USB, the old Cyo is still running fine on my tandem..(where I have separate battery lights for the standlight function)
My new dedicated commuter bike has the 80 lux Cyo set permanently to senso and I have had zero issues in about 3 years use. Although its been in the rain a lot I haven't abused it like I did with the old Cyo.
I would twiddle with the switch and see if it's connecting properly. That was a weak point with my original Cyo.
Re: Senso function on B&M dynamo light not working
I've just been and put the bike in the stand to do that, and the light came on straight away in the Senso setting when I spun the wheel. I am bemused, because I played around with the switch on my ride home last week when I first realised that the Senso setting was not working, and also at home a couple of days ago when I did my trial and error experimentation to see if I could get it to work.
I now wonder if the Senso feature requires power from the standlight capacitor to function, and the capacitor was not previously working properly having become totally discharged over the summer months. The standlight was dim compared with what I normally see when I experimented with switching the light on recently, whereas just now it was much brighter. In other words, I wonder if after a long period of disuse, the light needs to be switched to the On setting and ridden for while to recharge the capacitor, before switching over to the Senso setting.
I've noticed before that the standlight capacitor in newly fitted B&M rear lights can require many minutes of riding before they have enough power for the standlight to work. That has seemed to me to be especially so for the Secula Plus model, which I think draws more power than most other B&M rear lights.
Re: Senso function on B&M dynamo light not working
I would suggest squirting switch cleaning lubricant into the switch and operating the switch to clean the contacts within. 
At the last count:- Peugeot 531 pro, Dawes Discovery Tandem, Dawes Kingpin X3, Raleigh 20 stowaway X2, 1965 Moulton deluxe, Falcon K2 MTB dropped bar tourer, Rudge Bi frame folder, Longstaff trike conversion on a Giant XTC 840 
Re: Senso function on B&M dynamo light not working
My cyo has had the opposite issue since I bought it. Whether in the senso or on position the light just stays on with no difference in lighting. So I just leave it on all the time. It's been going like that for 8 years.
Bill
“Ride as much or as little, or as long or as short as you feel. But ride.” ~ Eddy Merckx
It's a rich man whos children run to him when his pockets are empty.
“Ride as much or as little, or as long or as short as you feel. But ride.” ~ Eddy Merckx
It's a rich man whos children run to him when his pockets are empty.
Re: Senso function on B&M dynamo light not working
FWIW,
We have several variants on the Cyo/Premium with sensor in our household. We have found that the switch can be a bit fiddly if you move it from one setting to another or it gets knocked, it can end up not quite exactly lined up with the marker especially for the sensor (middle) position. If this happens, the light just doesn’t work. Generally, just carefully lining it up to exactly the right place deals with the problem.
Doesn’t seem like this was the problem for the OP but just to let others know that it can happen.
PeterH
We have several variants on the Cyo/Premium with sensor in our household. We have found that the switch can be a bit fiddly if you move it from one setting to another or it gets knocked, it can end up not quite exactly lined up with the marker especially for the sensor (middle) position. If this happens, the light just doesn’t work. Generally, just carefully lining it up to exactly the right place deals with the problem.
Doesn’t seem like this was the problem for the OP but just to let others know that it can happen.
PeterH