U0156 on 2019-2020 Mazda MAZDA3: CMU Communication Error Causes and Fixes

What's Unique About the 2019-2020 Mazda MAZDA3

For the fourth-generation Mazda3, this code is very specifically tied to the Mazda Connect infotainment system's main computer, the CMU. Mazda has issued multiple Technical Service Bulletins (TSBs), most notably SA-029-20 and 16-004/22, acknowledging this issue. The problem is frequently caused by either software glitches within the CMU or a known hardware failure of an internal circuit board element (DRAM), making a trip to the dealer for software updates or a complete CMU replacement a common resolution.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• "IN-VEHICLE NETWORK MALFUNCTION" warning message on the instrument cluster.
🎬 Watch: See what this network malfunction warning looks like on your dash.
• Infotainment screen is blank, freezes, or reboots continuously.
• Audio system has no sound.
• Navigation system is inoperative.
• Loss of control over infotainment functions via buttons or commander knob.
• Backup camera fails to display.
• Bluetooth connectivity is lost or fails to pair.

Most Likely Causes

1. Failed Connectivity Master Unit (CMU) 🔴 High Probability A TSB from Mazda (16-004/22) explicitly states that CMU failures (blank screen, rebooting) are caused by a defective internal circuit board element (DRAM) in units installed in 2019-2022 Mazda3s before Dec 1, 2021. An improved part was subsequently released.
   How to confirm: If a software update doesn't work, the CMU is the likely culprit. A dealer can confirm with MDARS. You can also access a hidden service menu by pressing and holding the Volume, Entertainment, and Favorites buttons for 3-5 seconds to view the CMU part number and check against the TSB.
   Typical fix: Replace the Connectivity Master Unit (CMU). A new unit from a dealer requires programming. A used unit with a compatible part number can often be installed DIY without programming.
   Est. part cost: $800-$1500 (New OEM), $100-$250 (Used/Salvage)
2. Connectivity Master Unit (CMU) Software Glitch 🟡 Medium Probability Mazda has released multiple versions of TSB SA-029-20 to address communication errors, which points to software conflicts or bugs as a primary root cause for this generation. An update is the first step dealers are instructed to take.
   How to confirm: A Mazda dealer must check the current software version of the CMU against the latest available version using their MDARS diagnostic tool.
   Typical fix: Update the CMU software to the latest version. This must be done by a dealership or a qualified shop with Mazda-specific tools.
   Est. part cost: $0-$200
3. Wiring or Connector Issue ⚪ Low Probability While less common for U0156 than CMU failure, other infotainment issues on this platform (like U0186) are caused by loose pins in the main harness connector behind the CMU, suggesting a potential weak point.
   How to confirm: Visually inspect the wiring and connectors at the back of the CMU. Check for power, ground, and CAN bus signals at the CMU connector using a multimeter or oscilloscope.
   Typical fix: Repair the damaged section of the wiring harness or clean/reseat the connector.
   Est. part cost: $5-$50

Rare But Worth Checking

• Low Battery Voltage: While not the root cause, a weak or failing battery can cause various electronic modules to malfunction and set communication codes. Always ensure the battery is healthy (above 12.4V at rest) before proceeding with more complex diagnostics.
• Faulty Navigation SD Card: On some Mazda models, a corrupted or failing navigation SD card has been reported by owners to cause the CMU to repeatedly reboot, which could potentially lead to communication codes being set. This is an easy check: simply remove the SD card from the center console and see if the symptoms improve.

Diagnosis Steps

1. Connect a professional scan tool (preferably the Mazda MDARS) and check for DTCs in all modules.
2. Note all codes present. Pay close attention to codes like B1252, B119F, and U0198 appearing alongside U0156, as they confirm a CMU-centric fault.
3. Check for applicable Technical Service Bulletins, specifically SA-029-20 (for software) and 16-004/22 (for hardware failure).
4. Access the hidden service menu: With the car on, press and hold the Volume knob, Entertainment button, and Favorites button simultaneously for 3-5 seconds. Navigate to Service Information > Connected Device Information > CMU to view the part number and serial number. Compare this to the TSB information.
5. Follow the diagnostic procedure outlined in the TSB. This will likely start with a software check and update for the CMU.
6. If software is up to date or the update fails to fix the issue, and the CMU part number falls within the range of the hardware TSB, the CMU is the most likely failed component and should be replaced.
7. As a simple check, remove the navigation SD card from the center console to see if it affects the rebooting behavior.
8. If replacing, disconnect the negative battery terminal, remove the passenger-side dash trim, and unbolt the CMU. A used unit with a compatible part number can be a cost-effective DIY repair.
9. After repair, clear all DTCs and verify that the infotainment system functions correctly and no codes return.

Parts You'll Likely Need

• Connectivity Master Unit (CMU) (OEM #BDGF-66-9C0 (and subsequent revisions)) — This is the main computer for the infotainment system. TSB 16-004/22 identifies a hardware defect (DRAM) in units installed on 2019-2022 Mazda3s, making replacement the definitive fix.
  Trusted brands: Mazda (OEM)
  OEM price range: $800-$1500
  Aftermarket price range: $100-$250 (Used/Salvage from sources like eBay/LKQ).

Related Codes That Often Appear With This One

• B1252:87 — This code indicates a 'Communication error with connectivity master unit (CMU)' logged by the Telematics Control Unit (TCU). It is frequently listed in the same TSBs (SA-029-20) as U0156 and points to the same core issue.
• B119F:01 / B119F:2B — These codes relate to GPS or telephone antenna malfunctions. Since the antenna signals are processed by the CMU and TCU, they often appear when the CMU is malfunctioning and are cited in TSB SA-029-20.
• U0198:00 — This indicates a 'Communication error with emergency call unit' (TCU). It is often logged by the BCM when the entire CMU/TCU system has a fault, as described in Service Alert SA-029/20.

Technical Service Bulletins (TSBs) & Recalls

• SA-029-20g (Sept 10, 2020): Communication error with connectivity master unit (CMU).
• SA-029-20e (June 29, 2020): Communication error with connectivity master unit (CMU).
• SA-029-20 (Feb 25, 2020): "IN-VEHICLE NETWORK MALFUNCTION" INSTRUMENT CLUSTER WARNING MESSAGE.
• 16-004/22 (Aug 11, 2022, revised Mar 20, 2023): MAZDA CONNECT DOES NOT START OR REBOOTS. Identifies defective DRAM in the CMU as the cause.

Platform-Specific Known Issues

• Mazda has issued multiple versions of Technical Service Bulletin SA-029-20 for the 2019-2020 Mazda3, which directly addresses the "IN-VEHICLE NETWORK MALFUNCTION" message and DTC U0156, among others. The bulletins point towards software issues and provide diagnostic steps for dealers.
• TSB 16-004/22, issued March 20, 2023, is highly relevant. It notes that CMU failure (blank screen, rebooting) on 2019-2022 Mazda3s built before Dec 1, 2021, can be caused by a defective internal circuit board element (DRAM), and that the part was later improved.

Mechanic-Grade Diagnostic Values

• CAN Bus Network Integrity Resistance — expected: Approximately 60 Ω. Failure: A reading of 120 Ω indicates a missing terminating resistor or open circuit; a reading near 0 Ω indicates a short between CAN high and low wires.
• Module Power Supply Voltage — expected: 12.0V - 12.6V. Failure: Voltage significantly below 12.0V can cause erratic module behavior and communication faults.
• Module Ground Connection Resistance — expected: Below 0.2 Ω. Failure: Resistance higher than 0.2 Ω indicates a poor ground connection which can disrupt module operation.

Hidden / Shadow Codes Worth Checking

• U0156:00-2E: This is a more detailed version of the U0156 code logged by the Powertrain Gateway Unit (PT_GWU). The ':2E' is a failure type byte indicating a specific type of communication error, often related to software logic. (see via This extended code is typically only visible using the dealer-level Mazda Diagnostic and Repair Software (MDARS). A TSB indicates this specific code points to a software issue that can be resolved by reprogramming the PT_GWU.)

Scan Tool Commands That Help

• Mazda MDARS: Software Update — This function is the primary first step for resolving U0156, as outlined in multiple TSBs. It is used to reprogram the CMU, BCM, and TCU with updated firmware to fix known bugs that cause communication loss.
• Mazda MDARS: TCU and BCM Reprogramming (Conditional) — Specifically for 2019 Mazda3 models, if the CMU software is being updated from a version older than 7000C0A-NA01_10040, the TCU and BCM must also be reprogrammed in the same session. Failure to do so will result in a new communication error.
• Mazda MDARS: TCU Activation/Initialization — If the Telematics Control Unit (TCU) is replaced as part of diagnostics, the new unit must be activated using a specific procedure within MDARS to establish communication with the vehicle network.

Wiring & Ground Locations

• 0920-101F — This is the main instrument panel harness connector that plugs into the rear of the Connectivity Master Unit (CMU) in the dashboard.. While U0156 is a general CMU communication loss, related TSBs for other codes (U0186, U2145) on this platform point to pins (specifically 6F and 6H) backing out of this exact connector, causing network interruptions. Inspecting this connector for loose pins is a valid diagnostic step if software updates do not resolve the fault.

OEM Part Supersession History

• BDGF-66-9C0 (and revisions A, B, C, D, E, K, M, P, R, S, U, V) → BDGF-66-9C0W — Multiple revisions were released to address ongoing hardware and software issues.
  Heads up: According to TSB 16-004/22, the improved internal hardware (DRAM) did not result in a new part number. The fix is identified by the unit's serial number. A CMU with an affected part number (e.g., BDGF-66-9C0) but a serial number *outside* the specified faulty range is considered a good part.
• BDGF-66-9C0* → Same P/N, but with improved internal DRAM. — Defective internal circuit board element (DRAM) causing boot failures.
  Heads up: For 2019-2020 Japan-built Mazda3s, units with part number BDGF-66-9C0* and serial numbers between 622110 and 624748 are known to be defective per TSB 16-004/22.
• BFWM-66-9C0* → Same P/N, but with improved internal DRAM. — Defective internal circuit board element (DRAM) causing boot failures.
  Heads up: For 2020 Japan-built Mazda3s, units with part number BFWM-66-9C0* and serial numbers between 512261 and 512520 are known to be defective per TSB 16-004/22.

Model Year Variations Within This Range

• 2019 only: When updating the CMU software from a version older than 7000C0A-NA01_10040, it is mandatory to also reprogram the TCU and BCM. This dependency is specific to the 2019 model year to prevent setting a communication error after the update.