FMI 399 CAT Fault Code

Cause :

When the ignition switch is turned to ON, voltage from the ECM's MREL terminal is applied to the EFI MAIN relay. This causes the contacts of the EFI MAIN relay to close, which supplies power to terminal +B of the TCM.Thhis indicates a malfunction in the intake air flow. The cause of the problem may be a loss of tightness or a malfunction of the mass air flow sensor. If the contaminants are not critical, you can clean the flowmeter using a special carburetor.