Chart C ECCP Display Problem (Loss of ECCP Communication)

CIRCUIT DESCRIPTION
When the ECCP communications are lost service diagnostics usually cannot be entered but a quick scan of the panel will usually help isolate the source of the fault. Loss of serial data will result in three dashes being shown on the panel VF display. This indicates power and ground are OK. If the panel displays normal exterior or interior set temperature, power, ground, and serial communications are being received then panel replacement is indicated.

TEST DESCRIPTION
Numbers below refer to circled numbers on the diagnostic chart.
1. This step checks for power to the panel. Voltage should be HI for power to ground check.
2. Step checks for serial data voltage. Normal voltage should be above 1 volt and varying. A steady voltage indicates a second open exists, or the ALDL cover is off.