U0100 on 2014-2015 Ram 3500: Causes for Lost Communication with ECM/PCM

What's Unique About the 2014-2015 Ram 3500

For this specific generation of Ram 3500 (and its 2500 sibling), a known software glitch is a significant cause of the U0100 code. Ram issued Technical Service Bulletin #18-037-16 REV. A acknowledging that this code could appear and cause a 'No crank/No start' issue, particularly on models with the 6.7L Cummins engine and 68RFE automatic transmission (Sales Code DG7). This makes checking for the software update a unique and critical first step for these trucks, unlike other vehicles where U0100 is almost always a physical wiring or component fault.

Diagnostic Flowchart

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

Symptoms You May Notice

• No crank/No start condition
• Engine stalls intermittently
• Check Engine Light is on
• Multiple warning lights on the dashboard (ABS, Stability Control, etc.)
• Erratic gauge behavior
• Vehicle enters 'limp mode' with reduced power
• Transmission shift indicator on dash may disappear
• Aftermarket tuner may report 'no code' even when one is present

Most Likely Causes

1. Outdated Module Software 🔴 High Probability Ram released TSB #18-037-16 REV. A specifically to address a no-start condition associated with U0100 on 2015 models with the 6.7L Cummins (ETK) and 68RFE transmission (DG7), indicating a known software bug.
   How to confirm: A Ram dealership or a properly equipped independent shop can check the current software calibration version against the latest available version using a wiTECH scan tool.
   Typical fix: Reprogramming the Powertrain Control Module (PCM) with the updated software provided in the TSB.
   Est. part cost: $0 (if under warranty) - $200
2. CAN Bus Wiring Fault 🟡 Medium Probability Wiring harnesses in the engine bay are subject to heat, vibration, and potential chafing. Owners report issues with harnesses rubbing against engine components, AC lines, or the firewall, leading to shorts or open circuits in the CAN bus wiring.
   How to confirm: Visually inspect the wiring harness to the PCM for damage. A technician will use a multimeter to check for proper resistance (between 53.5 and 67 ohms) across pins 6 and 14 of the OBD-II port and check for continuity on the CAN bus circuits.
   Typical fix: Repairing the damaged section of the wiring harness or cleaning and securing corroded connectors.
   Est. part cost: $5-$50
3. Weak or Failing Battery / Poor Connections 🟡 Medium Probability Control modules require stable voltage (above 12.4V) to communicate properly. Low battery voltage, often triggered by extreme cold, can cause unpredictable communication errors across the network. Owners on RamForum.com have reported strange electrical issues and codes appearing during cold snaps that were resolved with a new battery.
   How to confirm: Test the battery voltage with a multimeter; it should be above 12.4 volts with the engine off. Load test the battery. Check battery terminals and all major ground connections for corrosion and tightness.
   Typical fix: Recharge or replace the battery. Clean or tighten battery terminals and ground straps.
   Est. part cost: $150-$400
4. Faulty Powertrain Control Module (PCM) ⚪ Low Probability
   How to confirm: This is a process of elimination. If the software is up to date, the battery is good, and all wiring has been tested and confirmed to be intact, the module itself is the likely culprit.
   Typical fix: Replacing the PCM. The new module will require programming to the vehicle's specific VIN and configuration.
   Est. part cost: $800-$1200

Rare But Worth Checking

• Faulty ABS Module:
• Faulty TIPM (Totally Integrated Power Module):
• Aftermarket Tuner/Programming Issue:

Diagnosis Steps

1. Check for Technical Service Bulletins (TSBs): The first step for a 2015 model is to check if TSB #18-037-16 REV. A has been performed. If not, this software update is the most likely fix for a no-start condition.
2. Scan for All Codes: Use a high-level scan tool (like AlfaOBD or wiTECH) to read codes from all modules. Note which modules are not communicating.
3. Check Battery and Grounds: Verify the battery has a full charge (over 12.4V) and that all battery terminals, chassis grounds, and engine ground straps are clean, tight, and free of corrosion.
4. Test the CAN Bus Resistance: Turn the ignition off and disconnect the battery. Use a multimeter to measure the resistance between Pin 6 (CAN High) and Pin 14 (CAN Low) of the OBD-II diagnostic port. A healthy network should read between 53.5 and 67 ohms. A reading of ~120 ohms indicates a missing terminating resistor or an open circuit. A reading near 0 ohms indicates a short between the CAN wires.
5. Inspect Wiring: Perform a thorough visual inspection of the main wiring harness connected to the PCM. Pay close attention to areas where it might rub against the engine block, firewall, or A/C lines. Look for any signs of chafing, melting, or corrosion.
6. Verify PCM Power and Ground: Using a wiring diagram, check that the PCM itself is receiving proper battery voltage and has solid ground connections at its connector pins.
7. Isolate Modules: If the bus resistance is incorrect, a technician may disconnect modules (e.g., ABS module, TIPM) one by one while monitoring the resistance. When the resistance returns to ~60 ohms, the last module disconnected is likely the one causing the fault.
8. Replace PCM: If all other steps fail—software is updated, wiring is intact, the battery is good, and other modules are not at fault—the PCM itself is the most likely cause and should be replaced and programmed.

Parts You'll Likely Need

• Powertrain Control Module (PCM) Software Update — A software glitch is a documented cause for U0100 on these trucks, leading to a no-start condition. This is often the first and only fix needed.
  Trusted brands: Mopar
  OEM price range: $100-$200
  Aftermarket price range: N/A
• Powertrain Control Module (PCM) (OEM #68170201AB (For 2013-2015 Ram 2500/3500 w/ 6.7L Cummins))

Related Codes That Often Appear With This One

• U0101 — Lost Communication with TCM. The Transmission Control Module (TCM) is on the same high-speed CAN C bus, and a network-wide problem will affect both it and the PCM.
• U0140 — Lost Communication with Body Control Module. This indicates a broader network communication failure that could be caused by a bad module or wiring short.
• U0121 — Lost Communication With Anti-Lock Brake System (ABS) Control Module. This can appear alongside U0100 if the ABS module is the source of the network failure or if the network is down entirely.

Technical Service Bulletins (TSBs) & Recalls

• 18-037-16 REV. A: Provides an enhanced software update to fix a 'No crank/No start' issue related to U0100 - Lost Communication With ECM/PCM for 2015 Ram 2500/3500 with the 6.7L Cummins engine.

Platform-Specific Known Issues

• A software anomaly can cause a 'No crank/No start' condition accompanied by code U0100, which is correctable via a software update as detailed in Ram TSB #18-037-16 REV. A, particularly for 2015 models with the 6.7L engine and 68RFE transmission.
• Owners on forums like CumminsForum and RamForum have reported the main engine wiring harness chafing near the A/C components or engine block, causing a short in the CAN bus wires and triggering the U0100 code.

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 circuit.
• CAN C High (CAN C+) Voltage to Ground — expected: ~2.5 to 3.0 VDC (Key On). Failure: Significant deviation from this range indicates a bus problem.
• CAN C Low (CAN C-) Voltage to Ground — expected: ~2.0 to 2.5 VDC (Key On). Failure: Significant deviation from this range indicates a bus problem.
• Battery Voltage — expected: 12.6 Volts or higher (engine off). Failure: Voltage below 12.4V can cause communication issues. Voltage should be between 10.0 and 16.0 volts for diagnostics.

Wiring & Ground Locations

• CAN C Bus Wires — The CAN C Bus (+) is a white wire with a light green stripe, and the CAN C Bus (-) is a white wire with an orange stripe. They are a twisted pair.. These are the primary communication wires for the PCM. Damage or shorts in this twisted pair will directly cause a U0100 code.
• PCM Fuses — In the under-hood fuse box (TIPM), fuses F21, F22, F57, F64, F72, F77, and F78 are related to the PCM/ECM.. A blown fuse will cut power to the PCM, causing a total loss of communication and a U0100 code.
• PCM Ground Wires — A key ground connection point is located on the passenger side, with a wire coming from the harness below the PCM, running under the intake box along the fender, and connecting to the negative battery terminal.. A poor ground connection for the PCM is a common cause of communication failures and intermittent electrical issues.
• Splice S169 — Located on the right rear of the engine.. This is a known splice point in the harness. Corrosion or failure at splice points can interrupt signals on the CAN bus.

Real Owner Repair Stories

• RamForum.com user 'Joe Merchak' (2018 Ram (similar architecture)) — Truck would not start, then started but threw a Check Engine Light with code U0100 and had no power. Multiple other seemingly unrelated codes were stored.
  ❌ Tried (didn't work) Clearing codes (they would return), Initial visual inspection
  ✅ What actually fixed it The dealership technician diagnosed and replaced a faulty ABS module. The tech noted this was a known issue where a bad ABS module 'crashes the bus', causing widespread communication codes.
• RamForum.com user 'Rufi' (2006 Dodge Ram (older but demonstrates a relevant principle)) — No crank, no start, and a 'no bus' message on the instrument cluster.
  ❌ Tried (didn't work) Initial CAN bus diagnostics
  ✅ What actually fixed it The problem was solved by replacing the instrument cluster. The previous owner had installed an incorrect cluster from a 2007 model. Although physically identical, it was electronically incompatible and brought down the entire communication bus.