How does the RPG MONITOR operation code work

We are on V5R1  and I cant get MONITOR to work.
I saw references in the archives but I cant see an answer to my problem.

Program CHKALPFLD is a MOD and uses procedures.
INPUTR is a program described file overridden to a DBF.
 This is the code
 //  Open the file
     MONITOR;
     OPEN  INPUTR;
       If not %error;
         Exsr ChkData;
       Endif;
     ON-ERROR;
     ENDMON;

I get these messages in the log, I expected MONITOR to suppress them.

 File AVM01V in library UAUBPCF with member *N not found.
 Error message CPF4102 appeared during OPEN.
 Error message CPF4102 appeared during OPEN (C G D F).
 C
 Error message CPF4102 appeared during OPEN (C G D F).
 C
 Application error.  RNX1217 unmonitored by CHKALPFLD at statement
   0000008500, instruction X'0000'.

I have also put in a rudimentary PSSR to see if it  works.

CSR   *PSSR         BEGSR
 *
CSR                 ENDSR
The program still crashes,  any suggestions. TIA
Frank kolmann