Unknown Diode

[ManUtd]

Hi,
Hope this forum can help me find out of this Diode.
On PCB it says D18 and with ordinary diode label on PCB with anode/kathode.
When i measure it with a multimeter it says Open both ways.

If anyone can recognise it i would be vary glad for any help.
Diode is suited in an sub amplifier(Class D) near a 2SK2698 Mosfet.

 

[Elvee]

My first guess would be a diac, but there are other possibilities: for example, I have seen thermistors and surge arrestors under the same guise.

If you draw the circuitry around, the function will probably become obvious.

 

[JonSnell Electronic]

I would say a Diac.
BR100 or BR101

 

[davidsrsb]

Possible, a DIAC would not conduct either way on a test meter.
What about the DB6, which would explain the blue stripe?