HP 3586 Error Message

[RCHRDM]

I have an HP 3586 Selective Level Meter.  On startup, it shows an error message "CE - p."  I can override the message by pressing the "MEAS CONT" key, but the message returns. 

The service manual says that this may indicate that board A20, 20 Hz Filter, may be the problem.  I pulled the board out and checked it visually and cleaned the edge connector, but it made no difference.

Can someone suggest what might cause this error?

Richard