I got a 'Error 504' on my Bosch powered Trek electric bike recently.
From what I've read, this is down to overly sensitive tamper detection software.
In order to clear the error I think you have to ride for 90 minutes or so in 'limp' mode. The weather is particularly bad at the moment so I decided to set up this cordless drill rig to do my pedalling for me.
After about 9 miles or so the error had cleared. I believe I need to upgrade my firmware asap.
