Transponder error

[Degrawmd]

Two of my last three flights (IFR and FF) - I have been getting the message "transponder error" I set the transponder to the assigned code and ATC says it's indenting OK. But throughout the flight the error message remains

What could be causing this erroneous error message?

 

[GKC Aviation]

I have had this twice.
You didn't mention if you are running a HDX and a Transponder 261/262, but assuming you are:
On the ground, if you are getting the transponder error, go to set up screen (hold down the right hand two buttons.
Go to SETUP MENU > TRANSPONDER SETUP
It has a field there which will display what the error is.
There is a table on transponder warnings in the installation manual (it is Table 85 in my copy)
Both times for me it was transponder TX power low.
This was solved in the first case by replacing the antenna which was missing (a cow ate it while it was parked up in a field), and the other occasion was a bad coax connection causing a short between the inner conductor and the shield braid..