U0171 – Lost Communication With Control Module “A”

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

Definition source: SAE J2012/J2012DA (industry standard)

DTC U0171 indicates the vehicle network has detected a lost communication condition involving Control Module “A”. In practical terms, one or more modules stopped receiving expected messages from that module for long enough to set a fault, or the network determined those messages were invalid or absent. Exactly which physical module is “A”, what messages are monitored, and how long the timeout is before the code sets varies by vehicle and by network design. Because the meaning is standardized but the implementation is not, always confirm module identity, network topology, and test procedures in the appropriate service information before replacing parts or reprogramming modules.

What Does U0171 Mean?

U0171 – Lost Communication With Control Module “A” means the onboard network detected that communication with Control Module “A” was lost. This is a network-type diagnostic trouble code, where modules exchange data over a communication bus and each module expects certain periodic messages from others. When those expected messages from Control Module “A” are not received (or are judged unusable) within the criteria defined by the vehicle’s diagnostics, U0171 is stored. The SAE J2012 DTC structure standardizes how the code is labeled, but the specific module designated as “A” and the monitored message set are determined by the vehicle’s configuration.

Quick Reference

• Subsystem: Vehicle communication network (data bus messaging to/from Control Module “A”).
• Common triggers: Module “A” offline, network bus open/short/high resistance, poor terminal contact, or module “A” power/ground interruption.
• Likely root-cause buckets: Wiring/connector faults, power/ground supply issues, network bus integrity problems, module fault, configuration/software issues (varies by vehicle).
• Severity: Varies widely; may be a warning-only condition or may disable features that rely on Module “A”.
• First checks: Scan for related U-codes, confirm which module is “A”, check battery/charging health, inspect module power/ground and network connectors.
• Common mistakes: Replacing a module before verifying power/ground, ignoring network-wide codes, or overlooking intermittent connector/terminal issues.

Theory of Operation

Modern vehicles use a serial data network so control modules can share information, coordinate functions, and report diagnostic status. Control Module “A” periodically transmits specific messages and also responds to requests from other modules. Receiving modules monitor message presence, sequence, and basic validity; if messages from Module “A” are missing, corrupted, or not acknowledged according to the platform’s diagnostic logic, the network monitor can flag a loss of communication.

U0171 typically sets when Module “A” cannot transmit or other modules cannot reliably receive its messages due to a bus fault, connector/terminal issue, or Module “A” losing power or ground. Some platforms also set communication codes after module resets, low system voltage events, or configuration mismatches, so confirming the failure is current and repeatable is essential.

Symptoms

• Warning light: Malfunction indicator or a general warning message illuminated (varies by vehicle).
• Feature loss: One or more vehicle functions dependent on Control Module “A” may be unavailable or limited.
• Multiple codes: Additional network DTCs stored in several modules, often alongside U0171.
• Intermittent behavior: Symptoms that come and go with vibration, temperature change, or harness movement.
• No communication: Scan tool may fail to communicate with Control Module “A” while other modules respond.
• Reduced operation: Some systems may enter a backup strategy, reduced performance mode, or disable certain assists (varies by vehicle).

Common Causes

• Open, shorted, or high-resistance wiring in the network communication circuits between modules
• Poor terminal fit, corrosion, water intrusion, or connector damage at the affected module or at network junction/connectors
• Power supply issue to Control Module “A” (blown fuse, faulty relay, open feed, or excessive voltage drop on the feed circuit)
• Ground circuit problem for Control Module “A” (loose ground point, corrosion, damaged ground wire, or excessive voltage drop)
• Intermittent harness fault (chafing, pinch points, vibration-related opens) causing the module to drop off the network
• Network backbone fault affecting multiple nodes (damaged splice/junction, twisted-pair damage, or short between network conductors)
• Control Module “A” internal fault that prevents it from communicating (hardware or internal power regulation issue)
• Software/configuration issue after module replacement or programming that leaves the module offline (varies by vehicle)

Diagnosis Steps

Useful tools include a scan tool with full network functions (module list/topology, ability to read codes from all modules, and live data), a digital multimeter for voltage-drop testing, and backprobing leads. Service information is required for module location, connector views, pinouts, fuse/ground identification, and the vehicle-specific network layout. If available, a breakout lead or harness pin kit helps reduce terminal damage during testing.

1. Confirm the complaint and scan everything: Perform a complete vehicle scan, not just a single module. Record U0171 and any accompanying U-codes, battery/ignition-related codes, or low-voltage history. Save freeze-frame or event data if the platform provides it.
2. Determine whether Control Module “A” is reachable: Attempt direct communication with the suspected module. Note whether it is missing from the module list/topology or present but intermittently dropping offline. This helps separate a hard offline condition from an intermittent network issue.
3. Check for a broader network problem: If multiple modules report lost-communication codes at the same time, treat this as a network-wide fault first (backbone, power supply, or grounds). If only U0171 is present, focus on the specific module branch, connector, and its power/ground.
4. Verify battery and charging health (quick triage): Inspect battery terminals and main grounds for looseness or corrosion and check for signs of low system voltage in scan data where available. Communication faults commonly set during undervoltage events; confirm whether the code is current or only stored/history.
5. Inspect fuses, relays, and wake-up feeds for Module “A”: Using service information, identify all power feeds (constant and switched) for the module. Check related fuses/relays under load where possible. A fuse that tests good with no load can still fail under load due to poor contact or a compromised fuse link.
6. Perform voltage-drop testing on power and ground: With the module connected and the circuit loaded (key on, and ideally during the time the module should be awake), perform voltage-drop tests on the module’s power feed(s) and ground return(s). Excessive drop indicates high resistance in wiring, connectors, or ground points that can prevent stable communication without fully killing the circuit.
7. Visual and physical connector/harness inspection: Inspect Module “A” connectors and nearby harness routing for pushed-out pins, bent terminals, moisture/corrosion, damaged locks, or chafing at brackets and pass-throughs. Repair obvious mechanical issues before deeper electrical checks.
8. Wiggle test while monitoring module presence: With a scan tool displaying module list/topology and/or network status PIDs, gently wiggle the harness at the module connector, at known junction/splice areas, and along suspected rub points. If the module drops offline or communication errors spike during movement, isolate the section by repeating the test on smaller harness segments.
9. Check the network circuits for opens/shorts (key off as required): Using the correct pinouts, verify continuity of the communication lines between Module “A” and the network junction, and check for shorts between communication conductors and to power/ground as appropriate. Follow service information for the correct conditions (key off, modules asleep) to avoid misleading readings.
10. Evaluate live data logging for intermittent conditions: If the fault is intermittent, road test or run the vehicle . Correlate the dropouts with vibration, steering movement, temperature, or electrical loads to narrow the harness area or power/ground weakness.
11. Rule out module-specific failure only after basics pass: If power, ground, and network circuits test good and the module still will not communicate (or repeatedly drops offline), follow service information for any required resets, relearn/provisioning, or programming checks. Only then consider the possibility of an internal module fault.

Professional tip: If U0171 appears alongside multiple “lost communication” codes, avoid chasing each module individually. Start by stabilizing system voltage, then verify the network backbone and shared power/ground distribution points. Once the network is healthy and other modules remain online, U0171 will be easier to isolate to a single branch, connector, or the module itself.

Possible Fixes & Repair Costs

Repair cost and effort for U0171 can vary widely because the root cause may be as simple as a connector issue or as involved as module power/ground restoration or network repairs. Accurate diagnosis determines whether parts are needed and how much labor is involved.

• Clean, reseat, and secure connectors at Control Module “A” and any related network junctions; correct terminal spread, corrosion, or poor pin fit
• Repair or replace damaged network wiring (shorts, opens, chafing) between Control Module “A” and the rest of the network
• Restore proper power and ground to Control Module “A” by repairing feeds, grounds, splices, or fuse/relay/connector issues (as applicable)
• Repair high resistance issues found via voltage-drop testing at grounds, power feeds, or inline connections supporting the module
• Correct poor routing or inadequate harness retention that allows intermittent contact during vibration (after confirming with a wiggle test)
• Perform module configuration, setup, or relearn procedures if required by service information after network/power repairs (varies by vehicle)
• Replace Control Module “A” only after confirming power/ground integrity and network line health; verify programming requirements before replacement

Can I Still Drive With U0171?

Sometimes the vehicle may still drive with U0171, but it depends on what Control Module “A” does and how the vehicle reacts to the lost communication (this varies by vehicle). If you have warning lights related to braking, steering, stability control, shifting, reduced power, stalling, a no-start condition, or multiple modules showing communication faults, do not continue driving; have the vehicle towed or serviced promptly. If the issue appears intermittent, avoid long trips and prioritize diagnosis because the fault can worsen without warning.

What Happens If You Ignore U0171?

Ignoring U0171 can lead to intermittent or persistent loss of functions managed or supported by Control Module “A,” including fail-safe operation, reduced performance, and loss of certain driver-assist or safety-related features (varies by vehicle). Communication faults can also cascade into additional network codes, complicating diagnosis, and intermittent faults may progress to a no-start or frequent drivability interruptions if the underlying wiring or power/ground issue degrades.

Last updated: March 17, 2026

Vehicles Commonly Affected by U0171

• Vehicles using a multi-module CAN (or similar) network where critical functions are distributed across several controllers
• Vehicles with multiple network buses connected through gateways or network junction connectors
• Vehicles with modules mounted in vibration-prone areas where connector pin fit can loosen over time
• Vehicles exposed to moisture, temperature swings, or contamination that can affect terminals and splices
• Vehicles with previous wiring repairs, accessory installations, or harness modifications near network wiring
• Vehicles with underbody harness routing that is susceptible to chafing, abrasion, or impact damage
• Vehicles with high electrical load usage where poor grounds or marginal power feeds can surface as communication loss
• Vehicles that have recently had control modules replaced or programmed, where setup/configuration steps may be required (varies by vehicle)

If U0171 returns after repairs, recheck for intermittent connection issues with live-data logging and a careful wiggle test, and confirm voltage-drop results on the module’s power and ground paths under load.