AJA17881

Job Aid
September 2021
Version 2
MOST Bus Network: Overview, Troubleshooting, and Repairs
APPLIES TO
Year
Model
Trim Level
2018 and later
Odyssey
ALL
2019 and later
Passport
ALL
2019 and later
Pilot
ALL
BACKGROUND / INTRODUCTION
This job aid contains information on the MOST Bus Network and troubleshooting information for common problems.
NOTE: This job aid replaces the following publications and Tech2Tech video segments:
• Service Bulletin 19-097, Service Manual Update: MOST Bus Network Failure Log
• Service News, Popping or Crackling from the Speakers? Check the MOST Bus Network
• Job Aid, Using the MOST Connector Tools
• Tech2Tech Segment, Get to Know the MOST Connector Tools
• Tech2Tech Segment, A Look at MOST Bus Network Connectors
• Tech2Tech Segment, Let’s Talk MOST Bus Network and ECL Diagnostics
• Tech2Tech Segment, Check Out the MOST Bus Network Failure Log
TOPICS COVERED
MOST Bus Network Overview
Page 2
MOST Electronic Control Line (ECL) Diagnostic Mode
Page 5
MOST Bus Network Failure Log
Page 14
Proper Connector Inspection & Harness Routing Procedures
Page 18
Using the Most Connector Tools
Page 25
© 2021 American Honda Motor Co., Inc. – All Rights Reserved
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MOST BUS NETWORK OVERVIEW
The Media Oriented Systems Transport or MOST Bus Network is used for high-speed bandwidth communication between
the audio/visual system components on certain Honda models. This is a serial network and is configured in a ring structure
that links one component to the next like a daisy chain that passes data in a single direction. A bus is used for
communicating audio, video, and control signals to each unit in the network.
Below are examples of the MOST Bus Network.
Odyssey
Pilot
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Passport
The gauge control module acts as the gateway in the MOST Bus Network. Each control unit on the network has
transmitting and receiving coaxial cables and the electronic control line (ECL). The transmitting, known as a TX connector,
is green in color, and the receiving, known as a RX connector, is red in color. Any open connections or shorts in the
network wiring will cause audio and visual system functions to be inoperative.
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Within the TX and RX connections, there are the TX+ / TX- and the RX+ / RX- terminals. The “+” indicating the information
line and the “-“ indicating the shielding ground.
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MOST ELECTRONIC CONTROL LINE (ECL) DIAGNOSTIC MODE
A typical failure in the MOST Bus Network will cause a no-response condition from the infotainment system with the center
display unit going blank. Any component or connection failures within the network can cause this condition. If you see any
audio functions or information in the gauge control module, the blank screen may just be a display issue and not a network
failure.
When a failure occurs, it will be necessary to locate where in the network the failure occurred. To help you do this, a MOST
Bus Condition, commonly known as the ECL diagnostic mode, is available in the gauge control module.
NOTE: The ECL diagnostic mode is only helpful if the network has a hard failure. Do not enter this mode if the network has
an intermittent failure. For intermittent failures, proceed to the MOST Bus Network Failure Log on page 14.
To start the ECL diagnostic mode, do the following:
1. Close all doors and the tailgate.
2. Turn the vehicle to the ON mode.
3. Using the steering wheel controls, scroll and select Settings on the multi-information display.
4. While displaying the settings screen, press and hold the HANG-UP button and then the ENTER button until the MOST
bus diagnosis screen below is displayed.
How to read the display
The table below shows the meaning of the indicators.
Symbol
Status
OK
• The power or MOST signal status is OK at this time.
NG
• The component lost power or cannot receive a MOST signal.
• The component is not connected to the module using the bus.
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The Pwr (Power) column shows if each component is online or not.
The Sig (Signal) column shows if the component is receiving a signal.
When using the ECL diagnostic mode, it is important to understand the network structure for the model you are working on.
Always refer to the network chart found in the service manual to identify the network’s communication flow in direction.
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This network chart can be found in service information. First, select the model and year. Then, enter “MOST Bus
Diagnostics Mode” in the keyword search. From the search results, click on Audio and Visual System Most
Diagnostics Mode.
Scroll down towards the bottom for the chart.
Once you identify the communication flow, use the EWD to identify what components are equipped on the particular model
you are working on. To do this, go to the EWD. After choosing the trim or entering the VIN, go to Schematics, Body,
Multiplex Integrated Control System, then click on MOST Communication Line.
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Since the EWD does not show a ring structure, use the network chart from the service manual to visualize the network
communication flow.
NOTE: Use this same procedure to find the network chart and the MOST communication line schematic in the EWD for the
MOST Bus Network Failure Log procedure on page 14.
Based on the network chart, the network communication flow can be visualized like below.
NOTE: The component names between the service manual and the display will not match. Refer to the following table.
ECL Diagnostics Name
Service Manual Network Chart Name
Meter
Gauge Control Module
Infotainment
Infotainment Control Unit
Amplifier
Stereo Amplifier
Remote Tuner
Tuner Unit
Optical Disc Player
BD (Blue Ray) or CD Player
Telematics
Telematics Control Unit
Rear Infotainment
RES Control Unit
Tuner Amplifier
Tuner-Amplifier Unit
Camera
Rearview Camera
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The table below shows the different combinations of indicator patterns.
Pwr
Sig
Description or Probable Cause
The power and MOST signal status is OK at this time.
The component is online but is not receiving signal on the network.
• Check the TX+ / RX+ and TX- / RX- lines between the previous control unit in the MOST
network ring.
• Check the wires and connector terminals. If they are OK, install a known-good component
and retest.
• Check the ground circuit of the indicted component.
• Check the wires and connector terminals. If they are OK, the component’s internal power
circuit may be faulty. Substitute a known-good component and retest.
The component is either offline or not equipped.
NOTE: If all components indicate double dashes, the MOST ECL line may be shorted.
• Check the network chart to confirm if the component is part of the network. The model and
trim you are working on may not be equipped with that component or is not part of the
network.
• Check the power and ground circuit for the indicated component.
• Check the wires and connector terminals. If they are OK, the component’s internal power
circuit may be faulty. Substitute a known-good component and retest.
NOTE: The infotainment control unit must never be substituted with a known-good unit as it will cause telematic
network problems.
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In this first example, the following components reported a power problem and signal problem or just a signal problem.
First, focus on the components that reported both a power and signal problem. Go to the applicable EWD and check to see
if these three components are equipped on the model you are working on.
By checking the EWD, the amplifer (stereo amplifier), remote tuner (tuner unit) and telematics (telematics control unit) are
not equipped for the model and trim level you are working on and can be ignored. That leaves the meter (gauge control
module) and the infotainment (infortainment control unit).
When a problem occurs in the network, it means that there is a problem at both the reporting component and the previous
one in the network.
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First, the gauge control module reported a problem. Looking at the circuit, the previous component is the infotainment
control unit.
Second, the infotainment control module also reported a problem. Looking at the circuit, the previous component is the BD
player. This means that the circuit from the BD player to the gauge control module needs to be checked.
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Troubleshooting this circuit revealed a bad connection at the MOST connector in the infotainment control unit.
In this next example, the ECL diagnostics results shows the meter, telematics and tuner amplifer reporting problems. After
checking the EWD, the telematics (telematic control unit) and the tuner amplifier (tuner-amplifier unit) is not equipped on
the vehicle, leaving only the meter (gauge control module).
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Looking at the EWD, the previous component to the gauge control module is the stereo amplifer.
Troubleshooting this circuit revealed a bad connection at inline connector C202.
Video
For a video on ECL diagnostics, click on this link.
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MOST BUS NETWORK FAILURE LOG
Whenever a problem occurs in the network, the information of the failure is captured in a log. This failure log is primarily
meant to be used during troubleshooting intermittent network failures.
These failures may range from:
• Complete signal loss and a blank screen on the center display unit.
• Popping or crackling noise from the speakers.
The failure log lists each failure by date, time, and type.
There are two failure types:
• SUDDEN_SIGNAL_OFF: This failure is for a temporary or complete loss of signal on the network.
• CRITICAL_UNLOCK: This failure is for electrical noise on the network.
This log also identifies which control unit on the MOST Bus Network recorded the failure. The recording control unit tells
you the failure occurred between that control unit and the previous one on the network. This log can hold up to 10,000
failures, with the most recent events at the top.
Follow these steps to access the logs.
1. Turn the vehicle to the ON mode.
2. Press and hold the VOL/Power AUDIO knob, Day/Night icon, and HOME icon at the same time. Hold these buttons
until the Dealer Diagnostics menu screen appears.
ODYSSEY
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PASSPORT AND PILOT
3. Select DETAIL INFORMATION & SETTING.
4. Scroll down the menu and select SHUTDOWN REASON then the failure log will appear.
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Look for the most recent event on the list. In the example below, the cause of the intermittent failure can be identified
as Meter:SUDDEN_SIGNAL_OFF, dated 2019-07-14 with a time stamped 07:02:12. There was a complete loss of
signal between the gauge control module and the previous transmitting component on the network.
Similar to the MOST Electronic Control Line (ECL) Diagnostic Mode procedure, you must determine the network flow
and the previous component to the reported failed component. In this case, the previous component to the gauge
control module is the stereo amplifier, and troubleshooting will focus on the circuit between the two components.
System Limitations
Below are things to remember when reviewing the logs:
• Will only show reported failures, not current ones.
• May not report speaker popping noises.
• Will not record during a permanent network failure and cannot be accessed during that time. In the event of a current
network failure, use the MOST Electronic Control Line (ECL) Diagnostic Mode on page 5.
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After Repairs
After repairs are completed to the network, be sure to clear the failure logs. This can only be done through the failure log
screen by pressing the CLEAR button.
Video
For a video of how to view the logs and how to interpret them, click on this link.
NOTE: The video references the service bulletin, 19-097 Service Manual Update: MOST Bus Network Failure Log
which has been replaced with this job aid.
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PROPER CONNECTOR INSPECTION & HARNESS ROUTING PROCEDURES
As previously described, some of the symptoms the customer may experience are popping or crackling noises from the
speakers, a blank center display unit, or the sound cutting off. After performing the ECL diagnostics or the MOST Bus
Failure Log and narrowing down the area to inspect, you need to inspect for proper connections of the MOST coaxial
connectors and proper harness routing.
Connectors
There are three types of connectors you will find in the network. These connectors are commonly known as FAKRA
connectors and will be referenced as such in various Honda publications and training materials.
Connector Image
Connector Style
Color
Purpose
Straight
Red
Receiving
Straight
Blue
Inline
90° Straight
Green
Transmitting
90° Offset
Green
Transmitting
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Due to their design, it is important that the connectors are installed properly and any external forces, such as from the wire
harness, do not contribute any tension. The tension created on the connector terminal such as a bad wire harness routing
will create a condition known as side loading. Here are some examples:
No tension on the harness, no side load
Harness tension at 45 and 90 degrees creating side load. The terminal will spread at the gap.
NOTE: The various service bulletin repair procedures available on SIS related to repair MOST-related problems will have
you install a new terminal to help avoid terminal spreading. However, it is important to still make sure that the harness wire
and connectors are installed back to their original condition.
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The following are some examples of mis-installed connectors and incorrect wire harness routing.
Example 1
The green connector sits correctly, perpendicular to the control unit, while the red connector sits at an angle caused by
tension on the wire harness.
Below is another example of tension on a connector causing the green connector to sit an angle.
In both examples, the connector is considered damaged, and the wire harness must be replaced. These connectors
are not repairable.
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Example 2
One wire is mis-routed causing tension to the green connector. To repair this, disconnect the MOST connector and reroute
the harness wire to remove the tension.
Below are more examples of incorrect harness mis-routing.
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Example 3
The MOST service cord was not installed as instructed in service bulletin 21-012, Warranty Extension: 2019−20
Passport/Pilot MOST Bus Network Connectors. If you encounter this, install the service cord as described in the service
bulletin, then install the wires so that they are not pinched or tensioned.
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Example 4
The updated terminal is correctly installed onto the green FAKRA housing but not on the red one. The connector was
installed 90 degrees off and was not engaged to the locking tabs.
Example 5
The wire is not fully seated into the new terminal. The edge of the wire must sit with the edge of the updated terminal.
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When inserting the wire into the new terminal, make sure you hear a click to ensure it is fully seated.
The previous examples shown can cause failures in the network. For additional examples of good harness routing, refer to
the following service bulletins.
Service Bulletin Number
Service Bulletin Title
20-049
2021 Odyssey: Popping or Crackling from the Speakers; No Sound from the Audio System;
Network Loss Message and/or Display Issues
20-058
2021 Passport/Pilot: Popping or Crackling from the Speakers; No Sound from the Audio
System; Network Loss Message and/or Display Issues
21-012
Warranty Extension: 2019−20 Passport/Pilot MOST Bus Network Connectors
21-013
Warranty Extension: 2018−20 Odyssey MOST Bus Network Connectors
Video
For additional information on poor connections, click on this link.
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USING THE MOST CONNECTOR TOOLS
This section contains information on tools to help you remove the connectors on various units within the MOST bus
network. Honda highly recommends using these tools when removing the connectors to prevent damage to both the
connector and the terminal. If either are damaged, the harness must be replaced.
Tool Information
Tool Image
Tool Name
Tool Number
STRAIGHT FAKRA PULLER
07AAC-THRA200
90 DEGREE FAKRA A PULLER
07AAC-THRA100
90 DEGREE FAKRA B PULLER
07AAC-TJBA100
Tool Usage
Before unplugging a MOST connector, note the orientation of the harness leading to it and make sure to install it the same
way. A mis-routed harness can cause the connector to not seat properly, resulting in network problems.
STRAIGHT FAKRA PULLER (T/N 07AAC-THRA200)
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1. Slide the tool over the rear of the connector. Make sure the ramps inside of the tool push on the locking tab of
the connector.
2. Once you feel it click into place, the tool is fully seated. Pull straight back to release the connector.
3. Remove the connector from the tool. Rotate the bottom of the connector (opposite the lock tab) out first. This will avoid
the lock tab catching on the tool.
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90 DEGREE FAKRA B PULLER (T/N 07AAC-TJBA100)
1. Install the tool from the side, allowing the lower part of the tool to reach behind the connector.
2. Squeeze both sides of the tool to unlock the connector, then pull straight back.
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90 DEGREE FAKRA A PULLER (T/N 07AAC-THRA100)
NOTE: Use this tool for removing 90° connectors in tight areas such as the Odyssey RES Control Unit behind the
glove box.
1. Install the puller.
2. Rotate the puller.
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3. Firmly hold the base of the tool and press down on the ball to disconnect the locking tabs on the connector.
4. While holding down the ball, slide off the connector.
Video
For a video of how to use these tools, click on this link.
NOTE: The video references the job aid, Using the MOST Connector Tools, have been replaced with this job aid.
END
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