By iCarsoft Official Store Technical Team, Automotive Diagnostics Editors · Updated October 15, 2026 · Technical sources checked
Quick Answer
The U0138 code means lost communication with all terrain control module. It does not prove a failed All Terrain Control Module, a failed actuator, or safe system operation. Save the scan, identify the exact architecture, and test the physical and electrical paths before selecting a repair.
What Does the U0138 Code Mean?
U0138 is listed as lost communication with all terrain control module in the Autel code-reference table. The full fault record determines the test branch.
A terrain selector is an input to a control system, while U0138 records a communication relationship. A failed mode selection and an unavailable module can occur together, but one observation does not prove the cause of the other. Determine whether only terrain-control messages are missing or several modules on a shared segment disappear together.
The latter pattern calls for a network investigation before treating the selector as a standalone switch problem. Changes in module visibility during an approved isolation test are evidence about a branch, not permission to disconnect components at random. Keep transfer-case position, actual driveline behavior and scan-tool availability as separate observations.
Keep the sender, reporting receiver and physical function separate. A component name on a scan is not a replacement instruction.
Ford TSB 21-2412 covers certain 2021 Ranger 4WD vehicles built from April 6 through July 26, 2021. It includes U0138 among a group of CAN communication symptoms and describes an internal ATCM/mode-selector condition that can affect network behavior. The prescribed isolation result matters; the code alone is not an instruction to replace the terrain controller. The model, drive configuration and build-date limits must all match.
Use the documented case to choose a question, then prove the branch on the actual vehicle.
Save the full module wording and suffix before clearing; they can change the test path.
Separate U0138 from adjacent fault families
| Code | General description | Key distinction |
|---|---|---|
| U0138 | Lost Communication With All Terrain Control Module | Confirm the complete OEM wording, receiver and installed sender. |
| U0073 | Control Module Communication Bus A Off | Bus state differs from one sender missing at a receiver. |
| U0100 | Lost Communication With ECM/PCM | A different message source; do not combine replacement decisions. |
| P0562 | System Voltage Low | A voltage record requires its own measured confirmation. |
Record the complete U0138 description. The suffix, reporting module and companion faults can change the diagnostic path. Use the manufacturer's full wording and test procedure for the exact vehicle.
U0138 Symptoms and Likely Causes
Record U0138 symptoms and fault status; symptoms alone cannot identify a failed part.
Check whether companion faults share a proven cause.
Record what changed in the all terrain control module system, the exact warning and when it appeared. Absence of a warning does not prove all messages arrived.
A history event needs its original conditions preserved. A current missing sender requires diagnosis in the prescribed wake and power state.
Review the actual record of terrain-selector, console, transfer-case or wiring service. Timing narrows inspection but does not prove the new component is defective.
Save the vehicle-wide report. Several missing messages may share a prerequisite; one receiving module may also have a separate software or configuration issue.
Practical cause-checking order
- Incorrect identity or equipment assumption: Record whether the problem affects mode selection only or multiple CAN modules at the same time, then check the Ranger build and 4WD restrictions before applying the cited example.
- Unstable feed, ground or wake state: Verify the sender’s permitted supplies, grounds and wake conditions. Record the actual operating state and OEM test limits; a sleeping sender cannot be judged from the wrong state.
- Connector or network-path interruption: Inspect the missing-message branch, including disturbed connectors and routing. Prove terminal contact and the electrical path with the prescribed checks; no universal CAN value or connector pin is assumed.
- Applicable configuration or software issue: A qualified technician should follow the manufacturer's branch-isolation sequence and compare the network before and after the permitted isolation. Do not use random unplugging to select a part.
- Internal sender or receiver fault: Consider controller failure after the required external paths pass. Request the failed-test evidence and distinguish a genuine offline ECU from scan-tool coverage or access limitations.
Did U0138 appear after recent terrain-selector, console, transfer-case or wiring service? Recheck the documented connections, parts and setup steps affected by that work. Record battery and software history; a post-repair code does not prove the new part is defective.
How Serious Is U0138, and Can You Drive?
Stop when the selected driveline state cannot be confirmed, unexpected engagement occurs, or steering, braking or propulsion changes. Do not attempt an off-road obstacle or wheel-slip event to reproduce a communication code. The DTC alone cannot establish a safe driving distance.
Record warning messages, operating conditions and service history. Do not induce wheel slip, emergency braking or a handling event on public roads. A red stop warning or abnormal control overrides any verification plan.
Stop and arrange a tow if required lighting, braking, steering, propulsion or another safety function is abnormal.
Stop when the selected driveline state cannot be confirmed, unexpected engagement occurs, or steering, braking or propulsion changes. Do not attempt an off-road obstacle or wheel-slip event to reproduce a communication code.
How to Diagnose U0138 in the Right Order
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Preserve the original event
Save U0138, its complete suffix, reporting ECU, current/history status, available event data and companion faults before clearing anything. Record the symptom timeline and the current vehicle warnings.
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Identify equipment, sender and receiver
Record whether the problem affects mode selection only or multiple CAN modules at the same time, then check the Ranger build and 4WD restrictions before applying the cited example. Obtain the VIN diagram and current OEM definition. Compare the installed equipment with the scan report, and note any mismatch explicitly.
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Check electrical prerequisites
Use the permitted feed, ground and wake checks for the identified message source. Secure the vehicle and respect its electrical safety state before access. Record loaded or static test conditions as required by the OEM procedure; do not borrow another model's pin assignments, thresholds or isolation sequence.
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Test the relevant communication branch
A qualified technician should follow the manufacturer's branch-isolation sequence and compare the network before and after the permitted isolation. Do not use random unplugging to select a part. Keep physical measurements separate from ECU-reported data. If the diagnostic tool cannot access the ECU, establish its coverage and authorization before concluding that the ECU is offline. Avoid arbitrary module disconnection that can create new faults and obscure the baseline.
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Choose a proven correction
Separate selector or controller work from transfer-case mechanical work. Ask which measurement or controlled isolation demonstrates that the proposed component is responsible. Use the failed path to choose wiring, terminal, supply, software, setup or controller work.
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Verify communication and function
Confirm the requested and actual driveline modes through the prescribed safe procedure, then recheck the complete network. Restored communication alone does not establish correct transfer-case engagement. Save a final multi-module report and compare it with the original record. Confirm required setup and the actual function under the manufacturer's safe conditions.
Specifications are vehicle-specific. Use the exact VIN specifications, approved test points and required operating state. No generic value or drive cycle can replace the applicable procedure. Manufacturer examples here do not establish specifications for other applications.
U0138 Repairs and Cost Planning
A U0138 repair must address the demonstrated failure. Do not replace an assembly from the code alone.
Recheck installation and required setup after repair. Separate selector or controller work from transfer-case mechanical work. Ask which measurement or controlled isolation demonstrates that the proposed component is responsible.
Ask the shop to name the failed branch and the measurement supporting each proposed repair.
Authorize diagnosis before parts while the cause remains uncertain.
| Possible path | Quote basis |
|---|---|
| Restore a demonstrated feed, ground, connector or network path | VIN-specific quote |
| Apply matching OEM configuration/software instructions when required | VIN-specific quote |
| Replace an isolated controller only after the approved diagnostic decision | VIN-specific quote |
These are alternative repair paths, not prices to add together. Request a VIN-specific quote separating diagnosis, parts, labor, fluids, taxes, required setup and verification. Ask which failed test supports the proposed repair and whether warranty coverage applies.
Using iCarsoft CR Max P for U0138
On a supported vehicle, iCarsoft CR Max P can save accessible faults, available freeze frame, live data and diagnostic reports. Preserve the U0138 evidence before clearing.
Compare the saved and final module records. Scan data supports diagnosis but does not replace the OEM reference, physical measurement or safety-system functional check.
Preserve the terrain-controller availability and shared network behavior evidence
iCarsoft CR Max P
Review supported codes and relevant live data together. Confirm the exact VIN, controller access, PIDs and any requested active test with iCarsoft before purchase; generic product capabilities do not guarantee a U0138-specific function.
Check current CR Max P price and availabilityCheck CR Max P compatibility by VINThe official product page describes supported full-system diagnostics, available live data, freeze frame and diagnostic reports; exact controller coverage and active tests depend on the VIN and software. Confirm the requested function by VIN before purchase. Follow OEM prerequisites and stop for abnormal operation.
Know the measurement boundary. Scan data describes ECU-provided information; it does not directly prove the physical condition of the terrain-controller availability and shared network behavior. Where required, use approved system scan access, the VIN wiring diagram and the specified electrical test equipment and compare reported values with the relevant measured evidence.
Verify the U0138 Repair
After repair, verify installation, clearance and required setup. Save the final findings and clear only erasable records as instructed. Battery disconnection is not a repair.
Run the manufacturer's applicable verification conditions in a safe workshop or route. Compare the original operating state with current data for the terrain-controller availability and shared network behavior, and check current and pending faults in related modules. A clean start or idle period does not prove the repair; use the OEM functional check for the affected system.
Compare the original and final fault status. Record the functional-check result and whether the original event returned.
If U0138 returns, compare its suffix, status, failure order and operating conditions with the saved baseline before replacing another part. Revisit the failed branch and inspect anything disturbed. Confirm the prescribed affected-system functional verification separately from code clearing.
Verification follows the system involved. After saving the original evidence, use the OEM procedure to clear erasable records, complete any required calibration or functional check, and compare the final module status with the original warning and fault conditions.
Related Terrain-Controller Availability And Shared Network Behavior Code Guides
Compare the related monitor families below. Shared symptoms or components do not make their repair procedures interchangeable.
U0138 Frequently Asked Questions
Does U0138 prove the terrain selector or transfer case is defective?
No. It names a communication loss. Save the module response pattern and identify the installed terrain-control architecture before deciding whether the next branch concerns supply, wiring, network loading, software or the controller.
What should I check first for U0138?
Save every module's codes and freeze frame before clearing. Confirm the full definition, suffix and VIN architecture. Then inspect recent terrain-selector, console, transfer-case or wiring service, connectors and shared supplies. Those facts determine whether the next test needs approved system scan access, the VIN wiring diagram and the specified electrical test equipment or a manufacturer-specific functional procedure.
Can I drive with the U0138 code?
Stop when the selected driveline state cannot be confirmed, unexpected engagement occurs, or steering, braking or propulsion changes. Do not attempt an off-road obstacle or wheel-slip event to reproduce a communication code. The DTC alone cannot establish a safe driving distance.
Why did U0138 return after a part was replaced?
A returned U0138 needs its suffix, reporting module and operating state compared with the baseline. Confirm the requested and actual driveline modes through the prescribed safe procedure, then recheck the complete network. Restored communication alone does not establish correct transfer-case engagement.
What should a U0138 repair estimate include?
Ask for the failed-test evidence and a VIN-specific breakdown of diagnosis, parts, labor, taxes, required setup and verification. A code alone cannot establish the repair price.
Can CR Max P diagnose and clear U0138?
On a supported vehicle, CR Max P can organize available faults, freeze frame, live data and reports, and can clear erasable records after repair. Confirm exact ECU coverage. It cannot replace the OEM functional procedure or physical measurements for the terrain-controller availability and shared network behavior.
The iCarsoft Official Store Technical Team maintains the store's fault-code library. This guide organizes published technical references and verified product information; it does not document a hands-on test of a particular vehicle or claim an individual engineer's review.
Technical and cost references
- Autel AL519 DTC definitions — communication-code terminology. Definition table only; no universal test limits or failed-part conclusion.
- Ford TSB 21-2412 — bounded 2021 Ranger CAN communication condition. Bounded OEM application example; scope and exceptions explained in the meaning section.
Product information: Official iCarsoft CR Max P page.
Editorial note: General diagnostic education, not a substitute for the owner's manual, current VIN-specific service information or professional inspection. Test limits, procedures, coverage and costs vary. Related articles provide context; apply only the specifications and repair procedures that match the vehicle. No universal failure rate, repair price or outcome is implied.
