U0100 on 2014-2020 Acura MDX: Lost Communication With ECM/PCM Causes and Fixes

What's Unique About the 2014-2020 Acura MDX

For the third-generation Acura MDX, this code often points towards foundational electrical issues rather than a single, common part failure. While no specific recall targets U0100 directly, this platform has known service bulletins (TSBs) that involve updating the PCM software. TSB 23-001 addresses an issue where the vehicle may not restart after an auto idle-stop event, a problem caused by software mis-set criteria, especially with low battery health. TSB 20-039 for the MDX Sport Hybrid corrects a software issue preventing certain transmission DTCs from being cleared. These bulletins suggest that ensuring the PCM has the latest software from an Acura dealer is a crucial diagnostic step if simple electrical checks don't resolve the issue.

Symptoms You May Notice

• Check Engine Light is on
• Multiple other warning lights may illuminate (ABS, VSA, Power Steering, Electric Park Brake, etc.).
• Engine will not start, or cranks but does not fire.
• Engine may stall intermittently while driving.
• Transmission may shift erratically or get stuck in one gear
• Gear selector indicator may flash.
🎬 Watch: How to resolve multiple dashboard warning lights on your MDX
• Loss of features like cruise control or remote start
• Vehicle enters 'limp mode' with drastically reduced power.

Most Likely Causes

1. Weak or Failing Battery 🔴 High Probability → Shop Vehicle Battery Modern vehicles like the MDX are highly sensitive to voltage. A battery that is even slightly weak can cause communication errors between modules before it's unable to start the engine. TSB 23-001 explicitly notes that low battery health can trigger related starting issues.
   How to confirm: Test the battery with a multimeter or a digital battery tester. A healthy, fully charged battery should read approximately 12.6 volts with the engine off. Any reading below 12.2V suggests a weak battery. A load test is more definitive, as a battery can show good voltage but fail under load.
   Typical fix: Recharge or replace the 12-volt battery. An AGM (Absorbent Glass Mat) battery is typically required.
   Est. part cost: $150-$350
2. Corroded or Loose Battery Terminals / Ground Straps 🟡 Medium Probability → Shop Engine Ground Strap Engine bay grounds can corrode over time, creating resistance that disrupts module communication. Key ground points to check are the main battery-to-chassis ground, and additional grounds near the transmission mount and under the air intake.
   How to confirm: Visually inspect the battery terminals for white or blue-green corrosion. Wiggle the terminals to check for tightness. Follow the main negative cable to the chassis and engine block, ensuring the connections are clean and tight. A video on jump-starting the 2017 MDX shows the official engine ground hanger location under the engine cover.
   Typical fix: Disconnect the battery (negative first), clean the terminals and posts with a wire brush and baking soda solution, and re-secure them. Clean chassis and engine ground points with a wire brush until shiny metal is visible. 🎬 Watch: The right way to clean corroded battery terminals
   Est. part cost: $5-$20
3. Blown Fuse or Faulty PGM-FI Main Relay 🟡 Medium Probability
   How to confirm: Consult the owner's manual for the location of the fuse boxes (under-hood and interior). Inspect all fuses related to the ECM/PCM, PGM-FI, and other control modules. A mechanic on a 2017 MDX with U0100 specifically checked 'PGM-FI main relay 1' and 'PGM-FI main relay 2' as a key diagnostic step.
   Typical fix: Replace any blown fuses. Swap the PGM-FI main relay with a known good relay of the same type (e.g., from the horn or another non-critical system) to see if the problem resolves.
   Est. part cost: $5-$70
4. Damaged Wiring Harness or Connectors ⚪ Low Probability Wiring can be damaged by rodents, chafing against engine components, or previous repair work. The two CAN bus wires are a twisted pair; any damage to this pair can disrupt the entire network.
   How to confirm: Visually inspect the wiring harness leading to the PCM (located in the RH engine compartment) for any signs of damage, melting, or rodent activity. Check the PCM connectors for corrosion or bent pins. A technician on Reddit checked continuity on CAN wires between the PCM and modules like the TCM, VSA, and gateway.
   Typical fix: Repair the damaged section of wiring. This often requires professional service to ensure a durable and safe repair.
   Est. part cost: $100-$1000+

Rare But Worth Checking

• Faulty Powertrain Control Module (PCM): → Shop Engine Control Module (ECM) The PCM itself can fail, but this is rare and should be the last resort. A real-world diagnostic on a 2017 MDX involved swapping the PCM, but it did not immediately solve the issue, highlighting the need to exhaust other options first. A replacement PCM must be programmed to the vehicle's immobilizer system by a dealer or qualified locksmith.
• Outdated PCM Software: Acura has issued software updates for the PCM on this generation of MDX. TSB 23-001 updates the PGM-FI software to fix an idle-stop restart issue. TSB 20-039 for the Sport Hybrid model updates software for an odd gear monitor fault. It is possible that a software glitch could cause communication problems. A dealer can check if any updates are pending for your vehicle's VIN.
• Faulty Aftermarket Electronics: Improperly installed remote starters, alarms, or GPS trackers spliced into the vehicle's wiring can interfere with the CAN bus network and cause communication codes. These devices can cause voltage drops or introduce noise into the network.

Diagnosis Steps

1. Check the Battery: Start by testing the 12V battery's state of charge and health. Ensure it reads at least 12.6V engine-off and passes a load test. A weak battery is the most common cause.
2. Inspect Battery Terminals and Grounds: Clean any corrosion from the battery terminals and ensure they are tight. Inspect the main engine and chassis ground straps for a clean, secure connection. Key locations are battery-to-chassis and near the transmission mount.
3. Scan All Modules: Use a high-quality OBD-II scanner that can read codes from all vehicle modules (ABS, TCM, BCM, etc.). Note all modules that are logging U0100 or other communication codes.
4. Check Fuses and Relays: Inspect all fuses related to the PCM/ECM in both under-hood and interior fuse panels. On this MDX, pay special attention to the PGM-FI main relays in the engine bay fuse box.
5. Visual Wiring Inspection: Carefully inspect the wiring harness connected to the PCM (RH engine compartment). Look for any signs of physical damage, chafing, or corrosion, paying close attention to the twisted pair of CAN bus wires.
6. Check for Software Updates: Contact an Acura dealership with your VIN to determine if there are any open recalls or PCM software updates, such as TSB 23-001 or TSB 20-039.
7. Advanced Diagnostics (Professional): If the above steps do not resolve the issue, a technician will need to perform advanced checks. This includes: a) Verifying power and ground directly at the PCM connector pins (e.g., a mechanic noted Pin B11 is a main ground on a '17 MDX). b) Testing the CAN bus resistance at the OBD-II port (pins 6 and 14). With the battery disconnected, it should be ~60 Ohms. A reading of 120 Ohms indicates an open circuit or a problem with one of the two terminating resistors.

Parts You'll Likely Need

• 12V AGM Battery (OEM #31500-TZ7-AGM100M) — A weak or failing battery is the most common trigger for network communication codes on modern, electronically-dense vehicles.
  Trusted brands: Acura (OEM), Interstate

Related Codes That Often Appear With This One

• U0101 — Lost Communication with TCM (Transmission Control Module). The TCM and PCM are on the same high-speed CAN network and often lose communication together, pointing to a network-wide issue rather than a single module failure. A Reddit thread on a 2017 MDX showed U0100 and U0101 appearing together across multiple modules.
• U0122 — Lost Communication with Vehicle Dynamics Control Module (VSA). The VSA module is another critical component on the CAN bus that relies on data from the PCM.
• B1196 — Gauge Control Module lost communication with battery (LIN bus connected). This was seen alongside U0100 in a detailed forum post, indicating a widespread electrical or communication failure, potentially originating from the gauge cluster or a shared power/ground issue.

Technical Service Bulletins (TSBs) & Recalls

• 23-001: Acura issued this update because under certain circumstances, particularly with low battery health, the vehicle may not restart after an auto idle-stop event. The software update adjusts the parameters for when idle-stop engages. This is highly relevant as low voltage is a primary trigger for U0100.
• 20-039: This TSB addresses a software calibration issue in the PCM of Sport Hybrid models that prevents certain permanent transmission-related DTCs from being cleared, which could cause a failed emissions test. While not directly causing U0100, it shows a history of PCM software fixes for this platform.
• 23-002: This is a warranty extension related to TSB 23-001. If the software update does not fix the no-restart issue, this covers the replacement of the starter and associated relays, and a valve adjustment. It lists the PGM-FI relay part number 39794-T5A-J01.

Platform-Specific Known Issues

• Acura has issued service bulletins for this MDX generation to update the PCM software for issues related to idle-stop functionality (TSB 23-001) and clearing of diagnostic codes on hybrid models (TSB 20-039). While not a direct fix for U0100, it highlights that PCM software is a known area for updates.

Mechanic-Grade Diagnostic Values

• CAN Bus Network Resistance — expected: ~60 Ohms. Failure: A reading of ~120 Ohms indicates a missing terminating resistor or an open in the circuit. A reading near 0 Ohms indicates a short between CAN High and CAN Low wires.
• PCM Main Ground Resistance — expected: < 0.5 Ohms. Failure: Higher resistance indicates a poor ground connection, which can cause intermittent module resets and communication loss.
• PCM Power-Up Voltage — expected: Battery Voltage (approx. 12.6V). Failure: Low or no voltage indicates a problem with the PGM-FI main relay, a fuse, or the wiring to the PCM.
• CAN High (CAN-H) Voltage — expected: ~2.5V to 3.5V. Failure: Voltage outside this range can indicate a short to power, ground, or a faulty module transceiver.
• CAN Low (CAN-L) Voltage — expected: ~1.5V to 2.5V. Failure: Voltage outside this range can indicate a short to power, ground, or a faulty module transceiver.

Hidden / Shadow Codes Worth Checking

• B1196: Gauge Control Module lost communication with battery (LIN bus connected). (see via This code is logged in the Gauge Control Module and requires a professional scan tool capable of reading codes from all vehicle modules, not just the PCM. Its presence alongside U0100 suggests a systemic electrical issue or that the Gauge Control Module itself may be the source of the network disruption.)

Scan Tool Commands That Help

• Acura HDS, Autel IM508/608, or equivalent professional scanner: Replace ECM/PCM — This function is mandatory after installing a new or used PCM. It writes the vehicle's VIN to the replacement module and synchronizes it with the immobilizer control unit and the keys. Failure to perform this step will result in a no-start condition with a flashing green key light on the dashboard.

Wiring & Ground Locations

• PCM — In the engine compartment on the right-hand (passenger) side.. This is the primary module that has lost communication. All power, ground, and network connections to it must be verified.
• PCM Connector B, Pin 11 — On the larger of the two main PCM connectors, this is a black wire.. This is a primary ground for the PCM. A technician measured its resistance to ground as a key diagnostic step. A poor connection here will cause the PCM to fail.
• PCM Connector B, Pin 10 — On the larger of the two main PCM connectors, this is a yellow wire.. This is a key power supply pin for the PCM, controlled by the PGM-FI main relay. Verifying voltage here confirms the relay and fuse are working.
• CAN Bus Terminating Resistors — The high-speed CAN bus has two 120-Ohm resistors. One is typically located within the PCM itself. The other is located in another module at the opposite end of the network, often the VSA (ABS) control unit or the Gauge Control Module.. The network requires exactly two terminating resistors to function. If a module containing a resistor fails or is disconnected, the total bus resistance will change from 60 Ohms to 120 Ohms, causing communication to become unstable or fail completely.

Real Owner Repair Stories

• Reddit user /u/SpicyChips69, r/MechanicAdvice (2017 Acura MDX, 3.5L SH-AWD, 160,000km) — No start, multiple warning lights on the dash, gear selector flashing with key on.
  ❌ Tried (didn't work) Replaced weak battery, Checked all fuses, Swapped PGM-FI main relays 1 & 2, Checked CAN wire continuity between PCM and multiple other modules (TCM, VSA, Gateway, etc.), Cleaned and checked all engine compartment grounds, Verified power and ground at PCM connector pins B10 and B11, Replaced PCM with a known-good unit
  ✅ What actually fixed it The user, a mechanic, posted seeking advice after all standard and advanced diagnostic steps failed to resolve the issue. The story did not have a final resolution posted, but it serves as a powerful example of how a U0100 can persist even after the most common culprits, including the PCM itself, are eliminated. The presence of a B1196 code for the Gauge Control Module was a strong clue that the fault may have been in another module or a very difficult-to-find wiring issue.
• NHTSA ODI #11499832 — An owner of a related Acura model reported that the vehicle was inspected and scanned for multiple communication codes including U0100, U0101, and U0122, noting that there were no prior warning lamps or symptoms indicating a problem with the system before the codes appeared.

"I Checked Everything" — The Actual Cause

• In a documented case on a 2017 MDX, a technician performed all the standard electrical checks—battery, fuses, relays, grounds, and even PCM replacement—but the U0100 code persisted across multiple modules. This indicates that the root cause was not a simple component failure that standard diagnostics would find. The actual cause in such scenarios is often an intermittent short/open in a section of the wiring harness that is difficult to access, or a failure of another module on the network (like the ABS or Gauge Cluster) that corrupts the data bus, bringing down communication for the PCM.

When the Usual Fixes Don't Work

• While a faulty PCM is a possible cause, it should be treated as the last resort. A detailed diagnostic on a 2017 MDX showed that even after a known-good PCM was installed, the no-start condition and U0100 codes remained. This strongly refutes the idea of simply swapping the PCM as a quick fix and reinforces that the underlying cause is almost always related to the foundational electrical system: wiring, grounds, or another module on the network.

OEM Part Supersession History

• 37820-5J6-A71, A72, A73, A74, A75, A76 → 37820-5J6-A77 — Incremental hardware or software revisions.
  Heads up: These part numbers are for AWD models without the Advance Package. They are not compatible with FWD or Advance models.
• 37820-5J6-C91, C92, C93 → 37820-5J6-C94 — Incremental hardware or software revisions.
  Heads up: These part numbers are specifically for models with AWD and the Advance trim package.

Model Year Variations Within This Range

• 2017-2020: The 2017 model year refresh introduced a 9-speed automatic transmission sourced from ZF, replacing the previous 6-speed. This involves a different Transmission Control Module (TCM) and associated software logic, which can affect network diagnostics if U0101 (Lost Communication with TCM) is also present.
• 2014-2020: PCM part numbers are highly specific to the model year, drivetrain (AWD vs FWD), and trim level (e.g., Base/Tech vs Advance/AcuraWatch). Using a PCM from a vehicle with different options will result in incompatibility and a no-start condition, even after programming.

Diagnostic Flowchart

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: A used OEM PCM from a salvage yard is a smart, cost-effective choice for this repair, as the modules are not high-wear items and can be significantly cheaper than a new unit from the dealer. Used relays and fuse boxes are also perfectly acceptable.

Donor-vehicle mileage cap: roughly under 150000 miles for the part to have meaningful remaining life.

What to inspect on the donor part:

• **Part Number Match:** The OEM part number on the sticker of the donor PCM must EXACTLY match the original or be a direct supersession. Check compatibility for year, drivetrain (AWD/FWD), and trim.
• **No Physical Damage:** Inspect the donor module for any signs of water intrusion, corrosion on the pins, or cracks in the casing.
• **Reputable Source:** Purchase from a reputable auto recycler that offers a short-term warranty to protect against receiving a dead-on-arrival part.

OEM-only on this vehicle (don't cheap out):

• Powertrain Control Module (PCM): While a used OEM unit is fine, avoid unbranded 'remanufactured' units from unknown online sellers. The complexity of programming makes sticking with genuine Acura hardware essential.

Aftermarket brands forum-validated for this vehicle:

• For this specific fault, no aftermarket parts are generally recommended for the PCM itself. Stick to genuine new or used Acura parts.

Brands owners have reported issues with on this vehicle:

• Be cautious of generic, no-name PCM/ECM repair services or modules that promise 'plug-and-play' without requiring VIN programming or immobilizer syncing, as this is not possible on this platform.

Real Owner Stories

Aggregated from forums and TSBs cited above. Mileages and costs reflect what owners reported in those sources.

Related OBD-II Codes

• U0101 — Lost Communication with TCM (Transmission Control Module). This code frequently appears with U0100 because a network-wide issue, such as a weak battery, bad ground, or CAN bus wiring fault, will disrupt communication with multiple critical modules at the same time.
• U0122 — Lost Communication with Vehicle Dynamics Control (VSA) Module. Often triggered alongside U0100 by a low voltage condition from a weak battery, which causes multiple modules to report communication loss simultaneously.
• U0131 — Lost Communication with Power Steering Control Module. A common companion code when a systemic electrical issue like a failing battery or corroded ground strap disrupts the entire CAN bus network, causing many warning lights to appear.
• P0685 — ECM/PCM Power Relay Control Circuit/Open. A fault with the PGM-FI main relay can cause this code and is a direct cause for U0100, as the PCM loses power and cannot communicate with any other modules on the network.
• P0700 — Transmission Control System Malfunction. This is a general code from the TCM. It can be set when the TCM loses communication with the PCM (U0100) and enters a fail-safe mode, often resulting in erratic shifting or being stuck in one gear.

Frequently Asked Questions