U0172 – Lost Communication With Control Module “B”

System: Network | Standard: ISO/SAE Controlled | Fault type: General | Location: Designator B

Definition source: SAE J2012/J2012DA (industry standard)

DTC U0172 indicates the vehicle detected a network communication loss with “Control Module B.” In practice, that means one or more other modules expected to hear messages from that module, but did not receive them within a required time window. The exact identity and role of “Control Module B” varies by vehicle architecture, so the affected features, symptoms, and the module’s physical location are not universal. Because the monitor strategy and network layout can differ, always confirm which unit is designated as “Module B” and what messages are considered missing by checking the vehicle’s service information and scan tool module list. This code describes a communication condition; it does not, by itself, prove a specific component has failed.

What Does U0172 Mean?

U0172 – Lost Communication With Control Module “B” means the vehicle’s network control strategy has determined that communication with a specific controller identified as “Control Module B” has been lost. Under SAE J2012 DTC structuring, “U” codes relate to network communication concerns, and this entry is used when expected data from that module is missing or invalid due to a communication timeout or lack of message reception. Depending on vehicle design, the setting module (the one that stores U0172) may be a gateway or any controller that depends on Module B’s messages to coordinate functions. The code indicates a network-level fault condition, not a confirmed internal failure of Module B.

Quick Reference

• Subsystem: Vehicle network communications (data bus) involving Control Module “B” and the modules that expect its messages.
• Common triggers: Module B not powering up, network wiring/connector issues, excessive resistance, intermittent open/short on bus lines, poor grounds, or a bus being pulled down by another node.
• Likely root-cause buckets: Wiring/connector faults, power/ground supply problems to Module B, network topology/gateway issues (varies by vehicle), or a module fault after basics are verified.
• Severity: Varies by vehicle; may range from warning lights and feature loss to reduced drivability if Module B is required for critical coordination.
• First checks: Scan for related U-codes, verify Module B is present on the scan tool, check fuses and grounds for Module B, and inspect network connectors for damage or looseness.
• Common mistakes: Replacing a module without confirming power/ground integrity, ignoring shared network faults, or focusing only on the module named in the code instead of the entire bus segment.

Theory of Operation

Modern vehicles use a communication network so multiple controllers can share sensor data and coordinate actions. Control modules broadcast messages at regular intervals, and other modules monitor those messages to confirm they are present and plausible. A controller may also act as a gateway, passing traffic between network segments depending on vehicle design.

For U0172, the setting module expects periodic communication from Control Module B. If those messages stop arriving for long enough—because Module B is offline, the network is disrupted, or the bus is being held in a faulted state—the setting module flags a lost-communication condition and stores U0172. The fault can be continuous or intermittent, and it can be caused by issues local to Module B (power/ground/connector) or by network-wide problems affecting multiple modules.

Symptoms

• Warning indicators: One or more warning lamps/messages may appear due to missing data on the network.
• Feature loss: Functions that rely on Control Module B’s messages may be disabled or operate in a limited mode.
• Communication failures: Scan tool may not connect to Control Module B or may show it as “not responding.”
• Multiple U-codes: Additional network-related DTCs may set in several modules when the bus is disrupted.
• Intermittent behavior: Symptoms may come and go with vibration, temperature changes, or movement of harnesses/connectors.
• Performance changes: Some vehicles may enter a default strategy (failsafe) if required network data is missing.

Common Causes

• Open, shorted, chafed, or high-resistance wiring in the network communication lines between modules (varies by vehicle network design)
• Poor terminal fit, corrosion, moisture intrusion, or partially seated connectors at Control Module “B” or at intermediate junctions/splices
• Power supply issue to Control Module “B” (blown fuse, faulty relay, voltage drop in the feed circuit, or intermittent supply interruption)
• Ground circuit problem for Control Module “B” (loose ground fastener, corrosion, damaged ground wire, or excessive ground-side voltage drop)
• Network backbone/junction fault (damaged splice pack, connector, or harness segment affecting multiple modules)
• Control Module “B” not powering up, resetting, or going offline intermittently due to internal fault or environmental/installation issues
• Improper prior repairs (aftermarket wiring taps, incorrect pinning, poor crimping, or insulation damage) impacting the network or module power/ground
• Software/configuration mismatch after module replacement or programming events (varies by vehicle; verify with service information)

Diagnosis Steps

Tools typically needed include a scan tool capable of full-network scans and accessing module communication status, a digital multimeter, and service information with wiring diagrams and connector views. Back-probing tools, terminal tension tools, and approved electrical contact cleaner are helpful. If available, a lab scope can help assess network signal integrity, but many faults can be isolated with scan data and voltage-drop testing.

1. Confirm the complaint and record all stored and pending DTCs from every module. Save freeze-frame or event data where available, and note whether U0172 is current or history.
2. Perform a full network/module scan and identify which modules are not reporting. Determine whether Control Module “B” is missing from the network list or present but reporting communication-related faults.
3. Check for symptoms of a broader network outage. If multiple “lost communication” codes are present, prioritize diagnosing shared power/ground, backbone wiring, or a common junction before focusing on a single module.
4. Clear DTCs and attempt to reproduce the fault under the conditions noted in the scan data (key cycle, engine running, road test if safe). Use live-data logging of network status/communication counters where the scan tool supports it.
5. Verify Control Module “B” power and ground. Using service information, identify all module power feeds and grounds. Perform voltage-drop testing on the power feed(s) and ground(s) while the circuit is loaded (key on and, if applicable, during crank or with commanded loads). Do not rely on unloaded voltage checks alone.
6. Inspect related fuses and relays for Control Module “B.” Confirm the fuse has power on the correct side(s) under the correct key state, and verify relay control and output where applicable. Replace only if testing shows a fault, then recheck for underlying causes.
7. Visually inspect connectors and harness routing at Control Module “B” and along the network path. Look for backed-out pins, bent terminals, water intrusion, abrasion points, and evidence of prior repairs. Correct any obvious issues and ensure connectors are fully seated and locked.
8. Perform a targeted wiggle test. With the scan tool monitoring Control Module “B” communication status and related live data, gently manipulate the harness, connectors, and junctions. If communication drops in/out, isolate the exact location and repair the wiring/terminal issue found.
9. Check the network circuits for opens/shorts/high resistance per service information. With the system powered down as required by the test procedure, verify continuity and isolation between the relevant network lines and to ground/power. If the procedure calls for it, measure circuit resistance end-to-end to identify excessive resistance due to corrosion or damaged conductors.
10. If wiring, connectors, power, and grounds test good, evaluate Control Module “B” itself. Confirm it is waking up and staying online, verify reference signals or wake lines if used on that platform, and follow service information for module reset, reconfiguration, or programming checks after replacement.

Professional tip: If U0172 is intermittent, focus on capturing the moment the module drops offline. Log network/module status while performing a controlled wiggle test and while cycling key states and electrical loads. Intermittent communication faults are commonly caused by poor terminal tension, marginal grounds, or harness stress near connectors, and these can appear “normal” during static checks unless the circuit is loaded and disturbed.

Possible Fixes & Repair Costs

Repair costs for U0172 vary widely because the fix depends on which network path failed, whether the issue is intermittent, and how accessible the wiring and module locations are. Diagnosis time, parts needs, and labor complexity can differ significantly by vehicle and configuration.

• Repair or replace damaged network wiring after confirming an open, short, or high resistance in the communication circuit
• Clean, secure, and re-pin connectors with poor terminal fit, corrosion, or backed-out pins at the affected module, splice points, or junctions
• Restore the control module’s power and ground feeds (including repairing blown fuses, loose grounds, or high-resistance power/ground connections) after voltage-drop testing
• Repair shared network power/ground or gateway-related connections if multiple modules show communication loss and testing supports a common point failure
• Update/reprogram software or perform a network relearn/setup only when service information calls for it and communication integrity has been verified
• Replace Control Module “B” only after confirming it is properly powered/grounded, the network lines test good, and the module still will not communicate

Can I Still Drive With U0172?

Sometimes the vehicle may still drive with U0172, but it depends on what functions Control Module “B” supports and how the vehicle responds to a network communication loss. If you have stalling, a no-start condition, reduced power, or warnings related to braking, steering, stability control, or critical safety systems, do not drive—have it diagnosed and repaired. If the only issue is an intermittent warning lamp with normal operation, drive cautiously and avoid long trips until the fault is confirmed, since intermittent network problems can worsen without warning.

What Happens If You Ignore U0172?

Ignoring U0172 can lead to recurring warning lights, intermittent feature loss, and unpredictable drivability changes if the communication failure spreads or becomes constant. A marginal connection can deteriorate into a hard fault, potentially causing additional network codes, failed inspections, reduced functionality, or an eventual no-start depending on vehicle design and the role of Control Module “B.”

Last updated: March 17, 2026

Vehicles Commonly Affected by U0172

• Vehicles with multiple high-speed and low-speed network segments linked by a gateway module
• Vehicles with advanced driver-assist, stability, or traction features that require frequent cross-module messaging
• Vehicles with high module count and multiple network splice packs or junction connectors
• Vehicles that have had recent electrical repairs, accessory installations, or collision/body work near harness routes
• Vehicles operated in high-humidity, high-corrosion, or heavy road-debris environments that can affect connectors
• Vehicles with modules mounted low in the body where moisture intrusion or connector contamination is more likely
• Vehicles with frequent battery issues, jump-start events, or charging system problems that can disrupt network operation
• Vehicles with known harness flex points (doors, liftgates, under-seat areas) where wiring can fatigue over time

For the most accurate repair path, identify which module the vehicle designates as Control Module “B” and confirm network, power, and ground integrity with service information before replacing any parts.