Precision, Performance, Confidence. ™

Free a stuck relay in molbox, ACAL error, PRT error

A stuck relay might show on the molbox display as PRTERR, TIMEOUT, CALCERR, or ACALERR (Autocal):



Note that mechanical relays were previously used but we changed to solid state relays on all molbox models in July 2017. So, the newer molbox's would not produce these errors due to a stuck relay (see below for other potential causes). 


To help ensure the error message is not being caused by a connected molbloc, make sure you have first checked that the same error message is produced when connect to other molbloc's. If the error only occurs on a single molbloc you can check the molbloc using a separate the article at this link, Check molbloc PRT or ACAL error


See the attached "Stuck real,menus.PDF" document at the end of this article to diagnose and try to free a stuck relay in molbox1, molbox1+ or molbox RFM flow terminal. It basically describes how to use the front panel buttons to manually cycle between various the relays which might result in freeing up a stuck relay that produces an error message. Further suggestions to free a stuck relay are also detailed below.

*** Note that with molbox1+ firmware version 6.01, the keys to press to enter diagnostic mode have changed. New method is to press and hold the left and right arrow keys at the same time.


Beyond cycling the relays, there are a couple other ideas to try that we have had some good success with freeing up relays:

  1. Remove the molbox cover to access the internal electronics with power disconnected. Re-seat and/or press on any electrical connectors you can access to ensure they are fully seated.
  2. A known cause of these error messages on a molbox1 or molbox1+ would be that the driver board cable has come loose. See the attached "check back panel internal cable, molbox1+.pdf" for further details. mceclip3.png 
  3. Sometimes removing the cover alone creates enough temperature change to help free a stuck relay. Could also try placing the molbox in a room that is much cooler or warmer than current location for several hours to create a more extreme temperature change.
  4.  Use the blunt end of a plastic ball point pen to tap firmly on the relays installed on the molbox board. The relays chips have American Relay written on them and are at the top left on this photo of the molbox1 driver board. There are 5 of these relays on the driver board (yellow box below): mceclip0.png On a molbox RFM there are three of these relays and will have to remove surrounding boards to access them (red circles below):mceclip2.png
  5. Used a can of compressed air that is normally used to blow dust off electronics. Hold the dispensing tip close to each relay as you spray the compressed air as it will come out very cold to create a direct temperature change that will also then quickly return to ambient temperature to help free a stuck relay.


If none of the above resolves the issue the molbox will need to be sent in for repair as the relays are not field serviceable or easy to swap (tight solder installation). Would recommend considering upgrading your molbox to the newer style solid state relay boards when sending in. This would need to be specifically requested on the RMA/PO as otherwise the mechanic relay that is found to be at fault would be replaced with another mechanical relay. Once you send it in our Service group would then verify this upgrade could be done on your molbox. Contact Fluke Calibration Service for an RMA number via or +1.877.355.3225 or the below link.


If you have technical questions contact Flow Technical Support at or the same phone number above.


Have more questions? Submit a request


Please sign in to leave a comment.