MD4 Log shows "Instantaneous Fuel Economy j1939 error" after system start.

The status "J1939 error" is logged for the JPIN channel, you will see it when the JFIN is received, the value of the JPIN is in the J1939 range for errors, and the JPIN is configured to use J1939 error check. 

The log record will show the channel name, in this case "Instantaneous Fuel Economy", and the status "J1939 error". 

Without knowing the application it is hard to say, but I would guess you have a channel reading the Engine Instantaneous Fuel Economy (SPN 184), this would be connected to a JFIN with PGN 65266

It would look something like this, and have the property J1939 Error check set to Yes:

For a 2-byte JPIN, values above 64255 (0xFAFF) are seen as errors. 

Assuming that it is this Engine Instantaneous Fuel Economy (SPN 184), it would be better to set this property to No. 

The SAE J1939DA states the following about this parameter: 

"Current fuel economy at current vehicle velocity.

0xFB00 = Infinite (for example, during engine motoring)"