Chart B Odometer Display "Error" (Loss of Serial Data)

CIRCUIT DESCRIPTION
The IPC indicates a loss of serial data communication by displaying ERROR in the odometer and illuminating the Electrical Problem telltale.

Loss of serial data communications can occur for the following reasons:
^ Short to ground anywhere in the 800 CKT.
^ Short to voltage anywhere in the 800 CKT.
^ Two opens in the 800 CKT.
^ Internal BCM or IPC fault.
^ Open in ECCP ground circuit.

TEST DESCRIPTION
Numbers below refer to circled numbers on the diagnostic chart.
1. Checks to see if the BCM (communications controller) is awake and functioning. If not, chart B-1 is used to isolate the fault.
2. Checks to see if the ECCP head has lost data communication along with the IPC.
3. Code B556 indicates an IPC EEPROM error. With this failure the odometer will display ERROR, but diagnostics will be accessible. See diagnosis in Diagnostic Trouble Code Charts. B Code Charts
4. Since the IPC has lost data communications, creating an open in the data line at the ALDL connector should remove data communications from the ECCP as well. If not, the IPC is receiving data but is unable to communicate due to a poor connection or an internal problem.
5. Indicates data line shorted to ground.
6. Indicates data line functioning properly, but with multiple opens isolating the IPC and ECCP from the BCM.
7. Indicates data line shorted to voltage.