U0173 – Lost Communication With Control Module “C”

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

Definition source: SAE J2012/J2012DA (industry standard)

DTC U0173 indicates that one control module on the vehicle network has detected a loss of communication with “Control Module C.” This is a network-level fault, meaning the issue is with message exchange over the communication bus rather than a single sensor signal value. The exact identity and role of “Control Module C,” the conditions that set the code, and which module stores U0173 can vary by vehicle, so you should confirm module naming, network topology, and test points using the correct service information. Do not assume a failed module from the code alone; most confirmed fixes come from locating power/ground, wiring/connector, or network integrity problems that interrupt data flow.

What Does U0173 Mean?

U0173 – Lost Communication With Control Module “C” means the vehicle has detected that expected network messages from Control Module C are not being received within the required timeframe. Under SAE J2012 DTC structure conventions, this is categorized as a communication/network fault rather than a component performance fault. In practice, the module that logs U0173 is typically one that relies on data or coordination from Control Module C; when that data stops arriving (or arrives corrupted), it flags a loss-of-communication condition. The code does not, by itself, prove whether the cause is a wiring issue, a power/ground problem at Control Module C, a bus fault, or an internal module failure.

Quick Reference

• Subsystem: Vehicle network communications (data bus messaging involving Control Module “C”).
• Common triggers: Module C powered down, open/short in network wiring, poor connector contact, excessive network interference, or bus termination issues (varies by vehicle).
• Likely root-cause buckets: Wiring/connector faults, power/ground supply issues to Module C, network bus integrity problems, module fault or configuration/software issue (as applicable).
• Severity: Varies; may be minor warning-only or may affect critical functions if Module C supports safety/driveability features.
• First checks: Scan for multiple U-codes, verify Module C presence on the scan tool, check module power/ground and connector security, inspect network harness routing/condition.
• Common mistakes: Replacing a module before verifying power/ground, misidentifying which unit is “Module C,” or ignoring additional network codes that point to the real fault location.

Theory of Operation

Modern vehicles use a shared communication network so control modules can exchange status and commands. Control Module C periodically broadcasts identifying and operating messages, while other modules listen for them and may request additional data as needed. Each receiving module expects certain messages to appear at predictable intervals and with valid formatting; if those messages stop arriving or are consistently unreadable, the receiving module interprets this as lost communication.

A U0173 fault is typically set when the receiving module’s internal communication monitor times out or detects repeated message errors related to Module C. Common reasons include Module C losing power or ground, a high-resistance connection that intermittently drops the module offline, damaged bus wiring that prevents messages from propagating, or a broader network integrity issue affecting multiple nodes. Specific monitor logic and reporting behavior vary by vehicle.

Symptoms

• Warning lights may illuminate, such as a general malfunction indicator or a system-specific warning related to functions supported by Module C.
• Feature loss may occur for functions that depend on data from Module C (exact features vary by vehicle).
• Intermittent operation may be noticed if the network drops in and out due to vibration-sensitive connections or harness movement.
• Multiple U-codes may be stored across several modules, indicating a broader network communication problem.
• No communication with Module C may be observed on a scan tool, or Module C may appear offline/not responding.
• Reduced performance may be present if other modules enter a fallback strategy when required data is missing.

Common Causes

• Open, shorted, or high-resistance wiring in the network communication circuits between modules
• Loose, backed-out, damaged, or corroded connector terminals at Control Module “C” or an inline junction
• Poor power feed or ground to Control Module “C” (including shared grounds or splices that affect multiple modules)
• Network bus fault affecting multiple modules (for example, damage near harness routing points or at connectors serving several controllers)
• Intermittent connection caused by harness movement, vibration, or connector strain/poor terminal tension
• Aftermarket/added electrical equipment or recent wiring work introducing a network disturbance or poor connection (varies by vehicle)
• Control Module “C” internal fault or software/firmware issue (consider after external circuit checks pass)
• Incorrect module configuration/coding or variant mismatch after a module replacement (varies by vehicle and service procedure)

Diagnosis Steps

Tools that help: a scan tool capable of full-module network scans and viewing communication status, a digital multimeter for power/ground and voltage-drop checks, and access to service information for connector views, network topology, and pinouts. If available, use a breakout/terminal test kit to avoid damaging terminals, plus basic hand tools for harness and connector inspection.

1. Verify the complaint and capture data. Record all stored and pending DTCs from every module, plus freeze-frame or event data if available. Note which modules report U0173 and whether any other “lost communication” codes are present, as this helps determine if the issue is isolated to Control Module “C” or broader network-wide.
2. Confirm whether Control Module “C” is reachable. Attempt to communicate directly with Control Module “C” using the scan tool. If it will not respond, treat the issue as either loss of power/ground to that module or a network connection problem at/near that module.
3. Check for pattern clues in the network scan. Look for multiple modules offline, a group of modules on the same network segment missing, or a single missing module. A cluster of offline modules suggests a shared power/ground, splice, or network trunk issue, while only one missing module often points to its connector, wiring branch, or module power/ground.
4. Clear codes and perform a controlled retest. Clear DTCs, then cycle the ignition and retest. If possible, reproduce the conditions under which the code sets. If U0173 does not return, treat it as intermittent and prioritize connector fit, harness strain points, and evidence of recent work.
5. Perform a visual inspection at the module and harness routing points. With the ignition off, inspect Control Module “C” connectors and the harness leading to it. Look for bent pins, pushed-out terminals, moisture/corrosion, damaged seals, abrasion, pinch points, or evidence of previous repairs. Correct obvious physical issues before deeper testing.
6. Verify Control Module “C” power and ground integrity. Using service information, identify all module B+ feeds, ignition feeds, and grounds. Check for missing power/ground and then perform voltage-drop testing on the grounds and feeds under load (key on, and with as much electrical load as practical). Excessive drop indicates resistance in a fuse connection, splice, ground eyelet, or connector terminal.
7. Check fuses and related distribution points the right way. Don’t rely only on visual fuse checks. Confirm power is present at the relevant fuse(s) and that it carries load, then inspect any relay, junction block, or splice that supplies Control Module “C.” A marginal connection can pass a quick check but fail during vibration or temperature change.
8. Evaluate the network circuits to Control Module “C.” With the ignition off and modules asleep per service guidance, check the network wiring for opens/shorts between Control Module “C” and the next connector/splice using the correct pinout. Also inspect for short-to-power or short-to-ground conditions suggested by melted insulation, crushed harness sections, or contamination bridging terminals.
9. Wiggle test for intermittents while monitoring live data. Reconnect everything, key on, and monitor communication status/heartbeat or module presence on the scan tool. Gently manipulate the harness at the module connector, nearby bends, and known rub points. If communication drops in/out, isolate the exact section by narrowing the movement area, then re-check terminal tension and conductor integrity.
10. Log a short road test if safe and applicable. If the vehicle can be driven safely, record a data log showing module online/offline status and any related network DTCs. A repeatable dropout during acceleration, bumps, or turning often indicates harness strain, poor terminal fit, or a ground shifting under load.
11. Make a module conclusion only after external checks pass. If power, ground, and network wiring to Control Module “C” test good, connectors are confirmed sound (including terminal tension), and the fault persists with consistent loss of communication, then consider Control Module “C” failure or software/configuration issues. Follow service procedures for configuration verification and replacement only after confirming inputs/outputs and network integrity.

Professional tip: Treat U0173 as a network integrity problem first, not an automatic module replacement. The highest-yield approach is to confirm module reachability, then prove power/ground quality with voltage-drop testing and identify intermittent connector/terminal faults with a live-data wiggle test. If multiple communication codes appear, diagnose the shared power/ground or network trunk before focusing on any single module.

Possible Fixes & Repair Costs

Repair cost for U0173 varies widely because the correct fix depends on which network path is failing, whether a power/ground issue is present, and how accessible the affected wiring and module are. Confirm the fault with testing before replacing parts.

• Repair or replace damaged network wiring (opens, shorts, chafing) in the communication circuit to Control Module “C”
• Clean, reseat, and secure network connectors; correct backed-out terminals, poor pin fit, or corrosion after verifying with inspection and testing
• Repair verified power feed or ground faults for Control Module “C” (including high-resistance connections found via voltage-drop testing)
• Correct shared network issues affecting multiple modules (for example, a splice or junction point with high resistance or intermittent contact)
• Restore proper connector retention/strain relief and harness routing to prevent repeat intermittent communication loss
• Reprogram/reconfigure a module only if service information requires it and diagnostics confirm software/configuration as the cause
• Replace Control Module “C” only after network integrity and module power/grounds have been proven good and communication remains lost

Can I Still Drive With U0173?

You may be able to drive with U0173 if the vehicle operates normally and no critical warning indicators are present, but the risk depends on what Control Module “C” supports (varies by vehicle). If you have stalling, a no-start condition, reduced-power behavior, or warnings related to braking or steering, do not drive; have the vehicle diagnosed and repaired to prevent a sudden loss of functions.

What Happens If You Ignore U0173?

Ignoring U0173 can lead to intermittent or progressive loss of functions that rely on Control Module “C,” potentially escalating from occasional warning lights to recurring drivability issues or a no-start. Continued network faults can also complicate diagnosis later, create multiple communication-related codes, and mask the root cause until wiring damage or poor connections worsen.

Last updated: March 17, 2026

Vehicles Commonly Affected by U0173

• Vehicles with multiple networked control modules communicating over a shared data bus
• Vehicles with modules located in moisture-prone areas (under carpet, low in the cabin, near bulkheads), increasing connector risk
• Vehicles with tight harness routing near moving components where chafing can occur
• Vehicles with recent collision repair or body/trim work where connectors may be left partially seated
• Vehicles with aftermarket electrical additions that tap power/ground or disturb network wiring (varies by installation quality)
• High-mileage vehicles with vibration-related connector fretting or terminal tension loss over time
• Vehicles with previous battery/charging issues that can trigger network resets or expose weak power/ground connections
• Vehicles that have had module replacement without proper configuration steps where required by service information

For the most reliable repair, treat U0173 as a communication-loss problem: verify network wiring and connector integrity, confirm Control Module “C” has stable power and ground under load, and only replace parts after tests prove the root cause.