MOST ring completed, Signal missing, General fault information, No sub-type information

What U1A25 means

Volvo vehicles equipped with the High-Speed MOST ring (used for audio, navigation, and in-car entertainment communication on XC90, XC60 II, and S90/V90 platforms among others) use a fibre-optic bus to connect modules such as the audio control module (ACM), telephone module, DVD player, and rear seat entertainment units. U1A25 indicates the ring is physically complete (the light is going around the loop) but the PCM or CEM is not receiving a valid data signal from at least one expected node — classified as 'Signal missing, General fault information' with no further sub-type detail. This commonly occurs when a module on the MOST ring has failed internally, has lost its power/ground supply, or when a fibre-optic connector is dirty or partially seated. Because MOST is a ring topology, one dead node disrupts all data traffic behind it in the ring.

Symptoms

• Audio system, navigation, or in-car entertainment features non-functional or intermittently unresponsive
• Volvo infotainment screen blank or stuck on boot screen
• Telephone or Bluetooth connectivity unavailable through the vehicle system
• No fault experienced in driving dynamics, engine, or transmission operation
• U1A25 stored alongside other U-series or entertainment-module codes in the CEM

Common causes

• Failed audio control module (ACM) or navigation module that has dropped off the MOST ring — the most common cause
• Broken, kinked, or dirty fibre-optic connector or cable in the MOST ring — often in the boot area where cables flex with the tailgate
• Loss of power or ground supply to one of the MOST ring nodes — causes the node to drop off the ring
• Software corruption in a MOST ring module requiring a re-flash or module replacement
• Corroded or loose electrical connector at a MOST ring module causing intermittent signal loss

Diagnostic approach

1. Use VIDA/DiCE or a Volvo-capable tool to identify which MOST ring node is missing — Read the full fault code list across all modules. U1A25 will typically be accompanied by a more specific fault indicating which node (audio, navigation, telephone, RSE, etc.) is not responding. Identify the missing node as a starting point for physical diagnosis.
2. Check the power and ground supply to the suspect module — Locate the identified missing module and verify its supply voltage and ground circuit with a multimeter. A module that has lost power will drop off the MOST ring and trigger U1A25. Check the relevant fuse in the junction box and the module's main connector integrity.
3. Inspect the fibre-optic MOST ring connectors — MOST ring cables are thin orange fibre-optic leads, typically found in the roof lining, behind the dashboard, and along the boot/tailgate area. Inspect each connector for a fully seated connection — they push-lock and can partially disengage over time with vibration. Clean the fibre-optic ends with a lint-free cloth if exposed; even a fingerprint can attenuate the optical signal.
4. Attempt a module reset or update via VIDA — Some U1A25 faults on Volvo platforms are caused by a module entering a corrupt state after a voltage event. VIDA can attempt a remote reset of infotainment modules. If the module does not respond to a reset, it may require a software re-flash or replacement.

Make & model notes

Volvo: XC90 (2016+) and S90/V90 (2017+) with the Sensus Connect system: the MOST ring connects the media player, amplifier, and internet module. Failed amplifier modules (the Bowers & Wilkins or standard unit behind the dashboard) are a frequent cause of U1A25 on these platforms. The amplifier can be accessed from the passenger footwell on most trim levels.

Volvo: Older XC90 (2003–2014) and V70/S80 with MOST: the rear entertainment module and optional CD changer in the boot are common failure points. Fibre-optic cables running to the tailgate are particularly prone to kinking or cracking as the boot lid flexes repeatedly over many years.

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