U0100 on 2021 Ram 1500: Lost Communication with ECM/PCM Causes and Fixes

What's Unique About the 2021-2021 Ram 1500

For the 5th generation (DT) Ram 1500, including the 2021 model year, communication codes like U0100 are frequently linked to software issues affecting various modules. Ram has issued several recalls and service bulletins for software updates to fix communication-related problems. Specifically, TSB #08-037-21 for the 2021 model lists U0100 alongside other communication codes, pointing towards a network problem often involving the ABS control module. A major recall (24V-653 / FCA #75B) also addresses ABS module software that can disable the Electronic Stability Control (ESC) system, highlighting the interconnected nature of these modules and its role as a primary cause for this code.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Check Engine Light is on
• Vehicle will not start or has a no-crank condition
• Engine stalls intermittently while driving
• Multiple warning lights on the dashboard (ABS, Stability Control, Forward Collision Warning, etc.)
• Erratic gauge behavior or cluster going blank
• Reduced engine power or 'limp mode'
• Wipers turning on by themselves
• Center stack switch lights illuminated

Most Likely Causes

1. ABS Control Module Software Glitch 🔴 High Probability A known issue on 2019 and 2021-2024 Ram 1500s, subject to safety recall 24V-653 (FCA #75B). A software malfunction in the ABS module, supplied by Continental, can disrupt the network, disabling ESC and causing a cascade of communication codes, including U0100. An estimated 100% of vehicles in the recall period have the faulty software.
   How to confirm: A dealer can check the vehicle's VIN against recall 24V-653. An advanced scan tool may show faults originating from the ABS module. The ABS and ESC warning lights will typically be illuminated on the dash.
   Typical fix: Update the ABS control module software, which is performed by a dealer free of charge under the recall. The dealer uses a wiTECH scan tool to perform the flash.
   Est. part cost: $0
2. General Module Software Issue 🔴 High Probability The provided TSB #08-037-21 lists U0100 with a cluster of other communication codes (U0140, U0121), often resolved by a software update to the Hybrid Control Processor (HCP) or other modules. Stalling issues related to PCM software were also addressed by recall 37A (NHTSA #20V-191) for 5.7L eTorque models. [pass 1]
   How to confirm: A dealership can check for any applicable TSBs or recalls related to module software for the vehicle's specific VIN using the VIP database.
   Typical fix: Reprogramming the affected module (PCM, BCM, HCP) with the latest software version using a wiTECH scan tool.
   Est. part cost: $0 if under recall/warranty
3. Wiring Harness or Connector Issue 🟡 Medium Probability CAN bus communication relies on physical wiring. A single corroded pin, loose connector, or chafed wire can bring down the entire network. Owners have reported issues with loose PCM connectors after installing aftermarket tunes and chafing near engine components.
   How to confirm: Visually inspect major wiring harnesses for damage, especially around the engine, frame, and behind wheel wells. Check connectors at the PCM, BCM, and ABS module for corrosion or loose pins. A multimeter can be used to check for proper resistance (around 60 ohms) across the CAN lines (Pin 6 and Pin 14) at the OBD-II port. 🎬 See how to perform a CAN bus resistance test with a multimeter. A reading of 120 ohms suggests a terminating resistor or module is offline, while a reading near 0 ohms indicates a short between the two CAN lines.
   Typical fix: Repairing the damaged section of the harness or cleaning/replacing the faulty connector. In some cases, a faulty CAN bus "star connector" or junction block is the culprit. 🎬 Watch: Finding a communication failure at the star connector.
   Est. part cost: $10-$100
4. Weak or Failing Battery ⚪ Low Probability Modern vehicles are highly sensitive to voltage. A weak or dying battery can cause unpredictable electronic glitches and communication errors during startup or operation. Forum users frequently report that strange electrical issues, including communication codes, are often resolved by a new battery.
   How to confirm: Test the battery with a multimeter or a dedicated battery tester. A healthy, fully charged battery should read around 12.6 volts with the engine off. Ensure the charging system is also working correctly (approx. 13.5-14.5 volts with engine running).
   Typical fix: Recharge or replace the battery.
   Est. part cost: $150-$350

Rare But Worth Checking

• Failed Powertrain Control Module (PCM): While the code directly points to the PCM, the module itself is rarely the root cause. It should only be considered for replacement after all wiring, software, and other modules on the network have been thoroughly checked. A forum user with an aftermarket tune experienced U0100 and was advised to reseat the PCM connectors first.
• Failed ABS Control Module (Hardware): While the software is the most common failure point for the ABS module, hardware failure is still possible. A technician on a forum noted that they have seen the ABS module itself fail and crash the bus, requiring replacement. If the software flash under recall 24V-653 does not resolve the issue, the module hardware may be faulty.

Diagnosis Steps

1. Check for Recalls: Enter the vehicle's VIN on the NHTSA website (nhtsa.gov/recalls) or call a Ram dealer to check for open recalls, especially 24V-653 (ABS software) and 37A (PCM software). This is the most critical first step.
2. Scan for All Codes: Use an OBD-II scanner capable of reading codes from all modules (ABS, BCM, TCM, etc.), not just the engine. Note all U-codes present to understand the scope of the communication failure.
3. Check Battery Health: Test the battery voltage and inspect terminals for corrosion. Ensure the battery is fully charged (approx. 12.6V engine off) and the alternator is charging correctly (13.2-13.5V during software flashes).
4. Inspect Wiring and Grounds: Perform a thorough visual inspection of the main wiring harnesses connected to the PCM, ABS module, and Body Control Module (BCM). Look for any signs of chafing, melting, or corrosion. Check that main chassis and engine ground straps are clean and tight.
5. Check CAN Bus Resistance: With the battery disconnected, use a multimeter to measure the resistance between Pin 6 (CAN High) and Pin 14 (CAN Low) on the OBD-II port. A reading of approximately 60 ohms indicates the network's two 120-ohm terminating resistors are intact. A reading of 120 ohms suggests one resistor (or the module containing it) is offline.
6. Isolate Modules (Advanced): If a specific module is suspected (e.g., the ABS module), a technician may disconnect it to see if communication with the PCM is restored. On Ram trucks, this can also involve disconnecting legs of the network at the star connectors (junction points) to isolate the fault.
7. Professional Diagnosis: If the cause is not an obvious recall or battery issue, professional diagnosis with an oscilloscope or advanced scan tool (wiTECH) is needed to analyze network traffic and pinpoint the source of the failure.

Parts You'll Likely Need

• ABS Control Module (OEM #Multiple, VIN-specific. Supplier is Continental Automotive Systems.) — A known failure point due to software issues that can disrupt the entire CAN network, as identified in TSB 08-037-21 and recall 24V-653. The fix is usually a free software update, but hardware failure is possible and would require replacement.
  Trusted brands: Mopar
  OEM price range: $500-$900
  Aftermarket price range: $300-$600

Related Codes That Often Appear With This One

• U0140 — Lost Communication With Body Control Module. Its presence alongside U0100 suggests a wider network problem, not just a single module failure. This code is specifically mentioned in TSB 08-037-21.
• U0121 — Lost Communication With Anti-Lock Brake System (ABS) Control Module. This often indicates the ABS module itself may be the source of the network disruption, which is a very common scenario for this truck.
• U0101 — Lost Communication with TCM (Transmission Control Module). The TCM and ECM are on the same high-speed network, so a bus failure often affects both.

Technical Service Bulletins (TSBs) & Recalls

• 08-037-21: Flash: Hybrid Control Processor (HCP) Updates
• 18-037-16 REV. A: Software anomaly causing 'No crank/No start' with U0100 (Referenced for similar platform issues)
• Recall 24V-653 / 75B: ABS Module Configuration Mismatch
• Recall 37A / 20V-191: PCM Update for Engine Stall Condition [pass 1]

Platform-Specific Known Issues

• TSB #08-037-21: Addresses a condition where U0100 appears with other communication codes (U0140, U1818, U0102, U1003), often pointing to a network issue related to the ABS module or requiring a Hybrid Control Processor (HCP) software update.
• Safety Recall 24V-653 (FCA #75B): A software malfunction in the ABS control module can disable the Electronic Stability Control (ESC) system and cause network communication faults. The fix is a free software update at a dealership. This recall affects 2019 and 2021-2024 models.
• Safety Recall 37A (NHTSA #20V-191): For 2021 models with the 5.7L eTorque engine, an over-rich fuel condition caused by PCM software could lead to engine stall. The fix is a PCM software update. [pass 1]
• Owner Forum Experience: A user on 5thGenRams.com reported a brand new 2021 Ram 1500 Limited with eTorque had over 30 error codes, including communication issues, within 24 hours of purchase. The dealer identified a problem with the 'charging pack' and replaced the entire truck.

Mechanic-Grade Diagnostic Values

• CAN Bus Network Resistance — expected: 53.5 to 67 Ohms. Failure: A reading of ~120 Ohms indicates an open circuit or missing terminating resistor. A reading near 0 Ohms indicates a short between the CAN high and low wires.
• Battery Voltage During Module Flashing — expected: Between 13.2 and 13.5 Volts. Failure: Voltage dropping below 13.2V or climbing above 13.5V can cause the flash process to fail or corrupt the module.
• System Operating Voltage (Engine Running) — expected: 13.0V to 14.5V. Failure: Voltage below 12.6V while running indicates a charging system issue that can cause communication codes. On eTorque models, voltage can fluctuate, but should not stay at 12.6-12.7V for extended periods under load.

Scan Tool Commands That Help

• wiTECH 2.0: Network Scan / View Topology — This is the first step after connecting to the vehicle. It provides a visual map of all control modules on the network and shows which ones are not communicating (typically shown in red), immediately identifying the scope of the U0100 fault.
• wiTECH 2.0: Reprogram Module (Flash) — Used to apply software updates to specific modules like the PCM, ABS, or BCM to fix known bugs that cause communication codes. This is the required fix for many TSBs and recalls.
• wiTECH 2.0: Clear All DTCs — After a module has been reprogrammed (flashed), the tool will automatically prompt the technician to clear all fault codes that may have been set in other modules during the update process.

Wiring & Ground Locations

• CAN Bus Star Connectors (Hubs) — Located under the dashboard near the steering column shaft. There are typically two hubs, one for the CAN-C (high speed) and one for the CAN-IHS (interior high speed) bus.. These hubs are the central junction point for the entire network. A failure in the star connector itself, or a fault in one branch, can be isolated by disconnecting individual connectors from the hub to see if communication is restored to the rest of the network.
• Security Gateway Module (SGW) Bypass Connector — Located under the driver's side dash, to the left of the steering wheel.. While not a direct cause, technicians and advanced DIYers use a bypass cable here to gain full diagnostic access to all CAN buses, which is necessary for in-depth troubleshooting of communication faults beyond what standard OBD-II readers can see.
• Primary Engine/Transmission Ground — A braided ground strap often connects the transmission case to the chassis frame mount, typically accessible from the driver's side wheel well.. A poor primary ground for the powertrain can cause intermittent and widespread electrical issues, including communication codes, especially during high-current events like starting.
• Cabin Ground Studs — Multiple ground points are located behind the instrument panel trim on the driver's side, and also under the center seat or console area.. Modules located inside the cabin, such as the Body Control Module (BCM) which acts as a gateway, rely on these grounds. A loose or corroded cabin ground can disrupt the entire network.

Real Owner Repair Stories

• RamForum.com user (2015 Ram 1500) — Intermittent stalling, no-crank, 'Service EBT' message, and blinking immobilizer light.
  ❌ Tried (didn't work) Re-seating the PCM connector after inspecting for corrosion (pins were clean).
  ✅ What actually fixed it The owner found that the issue was a loose PCM wiring harness connector. Wiggling the connector would kill the engine. A temporary fix was achieved by using a zip tie to pull the harness connector tight against the PCM, restoring consistent pin contact. The underlying issue was likely spread female pins within the connector.
• RamForum.com user (Ram 1500 with aftermarket Hemifever tune) — Code U0100 appeared after installing a new tune for long tube headers.
  ❌ Tried (didn't work) Clearing the code.
  ✅ What actually fixed it The user was advised to disconnect the battery, then unclip and reseat the PCM connectors to ensure a solid connection. This is a common first step after flashing a new tune, as the process can sometimes lead to a temporary communication glitch that reseating the physical connection resolves.
• CarKiller.com user question (2019 Ram 1500 5.7L) — No-start condition, dash lights up, security control module light on, code U0100.
  ❌ Tried (didn't work) Dealer replaced the security control module., Dealer replaced the ECM., Dealer replaced the PCM., New key fob was programmed., Battery was upgraded.
  ✅ What actually fixed it The vehicle was still not fixed after extensive parts replacement. The expert response indicated this is a classic sign of a CAN bus wiring fault (open or short) that was missed during diagnosis, highlighting a common misdiagnosis pattern where modules are replaced instead of tracing the underlying wiring or network fault.

OEM Part Supersession History

• 68084591AA, 68140666AA → 68230877AA — Standard part evolution and supplier changes.
  Heads up: This is a Mopar part number for a 'Connector Can Buss' or 'Star Connector', which fits a wide range of Chrysler/Ram vehicles.