A) Check the connection of the RPM sensor to the lower control board. Inspect RPM sensor and disc for build up of dirt, dust, or debris. Clean if necessary. Verify RPM sensor and disc are properly aligned. Units equipped with an "optical" RPM sensor (uses plastic RPM disc) should have the disc evenly spaced between the walls of the RPM sensor. Adjust if necessary. Units equipped with a magnetic "cherry" RPM sensor (uses a metal RPM disc) should have the sensor spaced 1/8" away from the flywheel. Adjust if necessary.
B) Inspect the display cable for any nicks or breaks. Replace if necessary. If there is no visible damage to the display cable, wiggle the display cable to verify intermittent connection. If by wiggling the display cable, the PWM LED on the lower control board does not flash or intermittently respond, replace the display cable.
C) Refer to the NO RPM Flowchart from the service manual for MCB testing.
D) Refer to the NO RPM Flowchart from the service manual for testing of Filter Network.