I have a Schindler VF drive that was removed from an elevator due to a communication failure. Unfortunately, I don't know the exact error code because the technician only told me that the drive was "not communicating" with the controller.

I have started troubleshooting the board and, so far, I haven't found any obvious hardware faults.

Here are my observations:

All power supply rails are present and appear to be normal.
Both PCA82C251 CAN transceivers are powered correctly.
VREF = 2.5 V
RS pin = 0 V (High-Speed mode)
CANH and CANL are both around 2.5 V in the idle state.
TXD and RXD lines show activity during startup when checked with an oscilloscope.

From these measurements, the CAN transceivers seem to be functioning normally.

The reason I'm asking is that this is not the first time I've encountered this problem.

Previously, I worked on another Schindler VF drive with the same symptom. After repairing it (or so I thought), the drive would communicate and operate normally for a short period, but after some time it would lose communication again.

Now I have another VF drive with a similar issue, and I'd like to identify the root cause instead of replacing parts randomly.

Has anyone experienced this type of failure on Schindler/Vacon VF drives?

I'm especially interested in knowing:

Common hardware failures causing CAN communication problems.
Whether there are any known issues with the PCA82C251 interface, CPU, EEPROM/Flash, or CAN isolation circuitry.
Any troubleshooting tips or known weak points on these boards.

Any advice or shared experience would be greatly appreciated.

Thank you!