U2603 – Invalid signal received from another control module, Bus signal/message faults, Message missing (Volvo)

Volvo-specific code — factory diagnostic data

U2603 means the Volvo XC40 has a communication problem, not a confirmed bad part. In plain terms, the ESM may stop seeing a message it expects from another control module, which can affect related functions and trigger warning messages or network faults during a scan. According to Volvo factory diagnostic data, this manufacturer-specific code means invalid signal received from another control module, bus signal/message faults, message missing. The wording matters. It tells you a message did not arrive correctly or did not arrive at all. It does not tell you which component failed. You must verify the network path, module power, grounds, and message source before replacing anything.

U2603 Quick Answer

On a Volvo XC40, U2603 means the ESM detected a missing or invalid network message from another module. Diagnose the message source, network wiring, and module power and ground before condemning the ESM or any other controller.

What Does U2603 Mean?

The official Volvo definition says the module received an invalid signal from another control module, with a bus signal or message fault and a missing message condition. In practice, the ESM expected data over the vehicle network and did not get a valid update within the allowed logic window. That can cause the module to limit its function, set a fault history entry, or stop acting on that message.

Technically, this is a manufacturer-specific network DTC in the U-family. The scan description is the working definition for diagnosis. The ESM does not measure a failed speaker, sensor, or actuator directly with this code. Instead, it monitors message validity, timing, and presence from another controller. The FTB subtype -87 points to a highly diagnostic message-fault condition and should be treated as subtype information only. It supports the missing-message direction, but it still does not identify the root cause by itself.

Theory of Operation

Under normal conditions, Volvo modules share operating data across the network in timed message cycles. The ESM depends on those messages to know vehicle state, command status, and when its function should run or stay inactive. Each module expects specific identifiers and valid data fields. When the correct message arrives on time, the ESM accepts it and continues normal operation.

This code sets when that communication chain breaks down. A source module may drop offline, a connector may add resistance, a power or ground fault may interrupt message transmission, or the network may corrupt the data enough that the ESM rejects it. Volvo uses broad U-code wording by design. That means the code points to a communication trouble area, not a final answer. Good diagnosis must prove whether the problem comes from the sending module, the network path, or the ESM’s ability to receive and process the message.

Symptoms

Communication faults often show up first on the scan tool, then in related vehicle functions.

• Scan tool behavior: The ESM or another related module may drop in and out of the module list, show intermittent communication, or store companion network codes.
• Warning messages: The cluster may display temporary system messages tied to network communication or reduced function.
• Stored U-codes: Multiple modules may log communication or missing-message faults at the same time.
• Intermittent operation: Exterior sound system behavior may become inconsistent if the ESM loses required network inputs.
• No obvious drivability issue: The XC40 may drive normally while the fault remains active in module memory.
• Cold or wet weather pattern: The fault may appear after moisture intrusion or temperature-related connector movement.
• Ignition-cycle reset: The issue may clear temporarily after a key cycle, then return when the missing message condition repeats.

Common Causes

• Intermittent CAN communication loss: A brief open, short, or unstable bias on the network can make the ESM miss a required Volvo bus message and set U2603.
• Poor ESM power feed: Low supply voltage at the Exterior Sound Module can corrupt message processing and make valid network traffic appear invalid or missing.
• High-resistance ESM ground: A weak ground causes module resets, logic instability, and dropped communication even when static voltage checks look normal.
• Connector terminal spread or corrosion: Loose, oxidized, or water-damaged terminals add resistance and interrupt CAN circuits or module power and ground paths.
• Harness damage near body pass-through points: Chafed wiring, pinch damage, or vibration wear can create intermittent opens or shorts in the network path.
• Fault in the transmitting module: Another Volvo control module may stop broadcasting the message that the ESM expects, which leaves the ESM reporting the message as missing.
• Network wake-up or sleep-state issue: If a module does not wake on time or drops offline early, the expected message may disappear during key transitions.
• Incorrect software configuration or incomplete programming: A Volvo module with the wrong calibration or configuration can send implausible data or fail to send the required message set.
• CAN bus short to power or ground: A partial short can distort bus voltage and block message traffic without completely taking the network offline.

Diagnosis Steps

You need a full-function scan tool with Volvo network access, a quality DVOM, wiring diagrams, and service information. A lab scope helps with intermittent bus faults. Use freeze frame to capture vehicle speed, ignition state, and related DTCs. Freeze frame shows when the code set. A scan-tool snapshot captures live data during your road test.

1. Confirm U2603 in the ESM and record all stored, pending, and history codes in every responding module. Save freeze frame data, especially vehicle speed, ignition state, and any companion network faults. If the code only shows as pending, treat it as an intermittent event. If it returns immediately at key-on, suspect a hard fault.
2. Check the relevant fuses, power distribution feeds, and network scan status before touching the ESM connector. On a Volvo XC40, verify whether the ESM appears on the scan tool network map and whether any expected partner module drops offline. A missing module on the network scan changes your direction fast.
3. Verify ESM power and ground under load. Do not rely on unloaded voltage or continuity alone. Perform voltage-drop testing with the circuit operating. Ground drop should stay below 0.1 volt. A weak ground can show battery voltage with no load and fail once the module wakes up.
4. Inspect the ESM connector, nearby harness routing, and shared network branches. Look for water entry, backed-out terminals, terminal spread, fretting, corrosion, or impact damage. Pay close attention to body pass-through areas and points where the harness flexes or rubs.
5. Check whether the fault points to a missing message from another module or a local ESM communication problem. Compare network-related codes across modules. If several modules complain about one controller, focus on that transmitter and its power, ground, and network path before blaming the ESM.
6. Test CAN bus resistance with the ignition off and the battery disconnected. Measure between CAN+ and CAN- at an accessible module connector. A healthy bus reads about 60 ohms. A reading near 120 ohms or OL suggests an open or missing termination. A very low reading points to a shorted bus segment or module.
7. Turn the ignition on and check CAN bias voltage to ground. Communication bias exists only with the circuit powered, so ignition-off voltage readings do not count. On a healthy network, CAN+ and CAN- each sit near 2.5 volts to ground. A line stuck high, stuck low, or badly split from the other line indicates a wiring or module fault.
8. Use the scan tool to monitor ESM communication status and related live data while you wiggle the harness. Watch for module dropouts, message timeout counters, and ignition-state changes. Trigger a manual snapshot during the wiggle test or road test if the fault acts intermittent.
9. Isolate the faulted area if bus values look wrong. Disconnect suspect modules one at a time only when service information supports that step. Recheck resistance and bias after each change. If the network normalizes when one branch or module disconnects, verify that branch before replacing anything.
10. After repairs, clear codes and perform a complete network scan. Confirm that the ESM communicates normally, no related modules drop offline, and U2603 does not return during key cycles and a road test. Recheck pending and confirmed status at the end.

Professional tip: U2603 does not prove the ESM failed. On Volvo vehicles, this code often reports what the ESM did not receive, not what part caused it. Find the missing message source, verify bus integrity, and prove clean power and grounds before you consider module replacement or programming.

Possible Fixes

• Repair CAN wiring faults: Fix any verified open, short, or high-resistance section in the network branch serving the ESM or the transmitting module.
• Clean and restore connector integrity: Correct corrosion, terminal spread, poor pin fit, or water intrusion at the ESM or related network connectors.
• Restore power or ground to the affected module: Repair the confirmed feed, splice, fuse connection, or ground point that fails voltage-drop testing under load.
• Repair the transmitting module circuit path: If another Volvo control module stops sending the expected message, fix its power, ground, connector, or network path first.
• Correct software or configuration faults: Update or program the affected Volvo module only after you verify that the wiring and network operate correctly.
• Replace a module only after proof testing: Replace the ESM or the message-source module only when bus integrity, powers, grounds, and configuration all test good and the module still fails communication checks.

Can I Still Drive With U2603?

You usually can drive a Volvo XC40 with U2603, but you should not ignore it. This code means the ESM has missed or rejected a message from another control module. That may only disable one network-dependent function, or it may point to a wider communication fault. If the vehicle drives normally and no warning lamps for braking, steering, propulsion, or driver assistance are active, the risk often stays limited to reduced convenience features. Stop using the vehicle if multiple modules lose communication, warning messages stack up, or the network fault appears with charging, propulsion, braking, or steering symptoms. A communication code never proves a bad module by itself. Confirm power, ground, connector condition, and network integrity first.

How Serious Is This Code?

U2603 ranges from minor to significant, depending on which message the Volvo ESM cannot receive and whether other modules also report bus faults. In many cases, it acts more like an inconvenience. The exterior sound function may set a warning and store a network code without changing basic drivability. The risk rises fast when this code appears with several U-codes, modules that drop offline, or faults tied to propulsion, charging, brake control, or steering support. On Volvo platforms, a missing message often reflects a shared network problem rather than a failed sound module. Treat it as a system fault, not a parts code. If driver assistance warnings also appear, verify proper operation after repair because any affected networked feature must communicate correctly before you can trust it.

Common Misdiagnoses

Technicians often replace the ESM first because the code sets there. That wastes money when the real fault sits in the message source, the network path, or the ESM power and ground feeds. Another common mistake is reading only one module. Volvo network faults require a full-vehicle scan, because the module that complains rarely causes the problem. Shops also miss intermittent connector tension issues, water entry, and high resistance at grounds because they check continuity with the circuit unloaded. That method misses voltage drop faults. Others condemn a control module when the scan tool cannot communicate, even though the module simply lost power or network bias. Avoid these errors by checking module communication status, topology-related codes, loaded circuit tests, and connector condition before any replacement decision.

Most Likely Fix

The most common confirmed repair is not ESM replacement. It is correction of the reason the expected message went missing. On a Volvo XC40, that often means repairing a poor power or ground supply to the ESM or the source module, fixing corrosion or terminal spread at a network connector, or repairing damaged communication wiring. If testing proves the network and supplies are stable, then software updates, configuration correction, or module replacement may become valid next steps. After the repair, clear codes and verify the fix under the conditions that let the affected message run. Enable criteria vary by platform and system, so check Volvo service information to know when the monitor should pass.

Repair Costs

Network and communication fault repairs vary by root cause — wiring/connectors are often the source, but module-level repairs or replacements can be significantly more expensive.

Last updated: April 11, 2026