Diagnostic Aids

Use A11 Radio's scan tool data display to determine the Surrogate MOST Master by interpreting the "Surrogate MOST Master Node Upstream Position" parameter and determine the MOST bus configuration from "Last Working MOST ID of Node 1-9" parameters.
If the MOST bus configuration is A11 Radio > K74 Human Machine Interface Control Module > T3 Audio Amplifier > P16 Instrument Cluster > A33 Media Disc Player and back to A11 Radio. The A11 Radio's scan tool data display will be shown as followed:
- Last Working MOST ID of Node 1: Radio
- Last Working MOST ID of Node 2: Human Machine Interface Control Module
- Last Working MOST ID of Node 3: Audio Amplifier
- Last Working MOST ID of Node 4: Instrument Cluster
- Last Working MOST ID of Node 5: Media Disc Player
After a ring break diagnostic session, the A11 Radio, which is the MOST bus master, indicates a ring break was detected and sets DTC U0028. The "Surrogate MOST Master Node Upstream Position" parameter will now be available in the A11 Radio's scan tool data display. Starting from the bottom of the MOST device list, count upward to determine the surrogate MOST Master device. The MOST ring break will be found upstream from the surrogate MOST Master since this will be the node which timed out and initiated the MOST node assignments during the MOST ring break.
- If the Surrogate MOST Master Node Upstream Position value is 0, the Surrogate MOST Master is also the A11 Radio. The MOST ring break is either at the A33 Media Disc Player or between the A11 Radio and between the A33 Media Disc Player.
- If the Surrogate MOST Master Node Upstream Position value is 1, the Surrogate MOST Master is the A33 Media Disc Player which is 1 node upstream from the A11 Radio. The MOST ring break is either at the P16 Instrument Cluster or between the A33 Media Disc Player and the P16 Instrument Cluster.
- If the Surrogate MOST Master Node Upstream Position value is 2, the Surrogate MOST Master is the P16 Instrument Cluster which is 2 nodes upstream from the A11 Radio. The MOST ring break is either at the T3 Audio Amplifier or between the P16 Instrument Cluster and the T3 Audio Amplifier.
- If the Surrogate MOST Master Node Upstream Position value is 3, the Surrogate MOST Master is the T3 Audio Amplifier which is 3 nodes upstream from the A11 Radio. The MOST ring break is either at the K74 Human Machine Interface Control Module or between the T3 Audio Amplifier and the K74 Human Machine Interface Control Module.
- If the Surrogate MOST Master Node Upstream Position value is 4, the Surrogate MOST Master is the K74 Human Machine Interface Control Module which is 4 nodes upstream from the A11 Radio. The MOST ring break is between the K74 Human Machine Interface Control Module and the A11 Radio.
In the case of a valid optional device being installed, DTC U0028 may set because of the change in the node assignment list. Be sure to clear the DTC after the installation is finished.
An open in the MOST control line or a fault in the MOST transmit/receive lines will cause DTC U0028 to be set.
The Last Working MOST ID of Node 1-9 may be reset to None after an ignition cycle when there is a fault on the MOST bus. In this case, use the MOST bus configuration above or from the schematics.