U0100 on 2009-2014 Honda Fit: Lost Communication With ECM/PCM Causes and Fixes

What's Unique About the 2009-2014 Honda Fit

The second-generation Honda Fit (2009-2014, chassis code GE) has a generally reliable electrical system, but like many modern cars, it is very sensitive to low voltage. The U0100 code is frequently triggered not by a failed computer, but by a simple issue like a dying battery or a corroded ground wire. The main engine/transmission to chassis ground strap, located under the battery and connected to the motor mount, is a common point of corrosion and failure that can cause this code. Owners often report a 'Christmas tree' effect on the dashboard, where multiple warning lights appear at once, which is a classic sign of a network communication problem stemming from a power supply issue.

Symptoms You May Notice

• Check Engine Light is on, sometimes flashing.
• Multiple other warning lights on simultaneously (ABS, VSA, Power Steering, etc.).
• Engine will not start (no-crank or crank-no-start).
• Engine stalls intermittently while driving.
• Transmission may not shift correctly or may be stuck in one gear (limp mode).
• Gauges on the instrument cluster, like the speedometer or tachometer, do not work.
• Cooling fans run constantly at high speed, even with a cold engine (fail-safe mode).
• Drastic drop in fuel economy.

Most Likely Causes

1. Weak or Failing Battery 🔴 High Probability → Shop Vehicle Battery Modern vehicle electronics require a stable voltage (typically above 12.4V) to communicate. A battery that is old, has a bad cell, or is low on charge is the most common trigger for communication codes like U0100. The small Group 151R battery used in many Fits has a lower capacity, making it more susceptible to voltage drops.
   How to confirm: Test the battery voltage with a multimeter. A healthy, fully charged battery should read at least 12.6 volts with the engine off. Anything below 12.4V warrants a recharge and re-test. Most auto parts stores can perform a load test to check the battery's health, which is a more definitive test.
   Typical fix: Replace the battery if it fails a load test.
   Est. part cost: $120-$250
2. Corroded or Loose Battery Terminals 🟡 Medium Probability → Shop Vehicle Battery Corrosion on the battery posts and terminals can restrict the flow of electricity, causing voltage drops that upset control modules, even if the battery itself is healthy.
   How to confirm: Visually inspect the battery terminals for white, blue, or green crusty buildup. Wiggle the terminals by hand to ensure they are tight and do not move.
   Typical fix: Disconnect the terminals (negative first), clean them and the battery posts with a wire brush and a baking soda/water solution, and reconnect them tightly (positive first). Apply dielectric grease to prevent future corrosion.
   Est. part cost: $0-$10
3. Poor PCM Power or Ground Connection 🟡 Medium Probability The PCM relies on clean ground connections to the chassis to function. The main engine-to-chassis ground strap on the GE Fit is located under the battery tray and is known to corrode or break, causing intermittent communication failures. The primary PCM ground point, G101, is located on the thermostat housing and is a known trouble spot.
   How to confirm: Locate the main PCM ground points (G101 on the engine, G201/G301 on the chassis) and inspect them for corrosion, looseness, or damage. A voltage drop test between the battery negative post and the PCM ground point should be less than 0.1V. The PCM itself has ground pins (PG1, PG2) that can be checked for continuity to ground, which should be less than 0.2 Ω.
   Typical fix: Remove, clean, and securely re-fasten the ground straps. In some cases, a corroded ground strap may need to be replaced.
   Est. part cost: $10-$30
4. Damaged Wiring Harness (CAN Bus) ⚪ Low Probability While not common, wires in the CAN bus harness can be damaged by rodents, chafing against other components, or previous improper repairs. Rodent damage is a frequently cited cause in forums due to the soy-based wire insulation used in some modern cars.
   How to confirm: Visually inspect the wiring harnesses leading to the PCM for any signs of damage, such as chewed wires, melted insulation, or corrosion in connectors. A professional may need to check CAN bus resistance (should be ~60 ohms between pins 6 and 14 of the OBD-II port with the battery disconnected) and check signals with an oscilloscope.
   Typical fix: Repair the damaged section of the wiring harness. This can be complex and may require professional service.
   Est. part cost: $50-$800

Rare But Worth Checking

• Failed Powertrain Control Module (PCM): → Shop Engine Control Module (ECM) This is the least likely cause. The PCM is a robust component and should only be suspected after all other possibilities (power, grounds, wiring) have been exhaustively ruled out.
• Interference from Aftermarket Electronics: Poorly installed remote starters, alarms, or audio equipment spliced into the vehicle's network can disrupt communication and trigger U-codes.
• Blown Fuse: Check fuses related to the ECU/PCM in both the under-hood and interior fuse panels. A blown fuse can cut power to the module, preventing it from communicating.

Diagnosis Steps

1. Check Battery Health: Using a multimeter, verify battery voltage is above 12.4V. Perform a load test to confirm the battery is healthy.
2. Inspect Battery Terminals: Check for and clean any corrosion from the battery posts and terminals. Ensure they are tight.
3. Scan for All Codes: Use an OBD-II scanner to pull all codes from all modules, not just the engine. Note which modules are reporting a loss of communication.
4. Inspect Main Grounds: Locate and inspect the main battery-to-chassis and engine-to-chassis ground straps. The key ground strap is under the battery tray, connecting to the transmission mount. The critical PCM ground G101 is on the thermostat housing. Check all PCM ground connections for tightness and corrosion.
5. Check Fuses: Inspect all fuses related to the PCM, ECM, and other control modules in both the interior and under-hood fuse boxes.
6. Visual Wiring Inspection: Carefully inspect the wiring harness leading to the PCM for any visible signs of damage, such as chafing, melting, or rodent damage.
7. Advanced Diagnostics (Professional): If the above steps don't reveal the issue, a technician will need to check the CAN bus resistance (should be ~60 Ohms between pins 6 and 14 of the DLC) and check for proper signals on the CAN High and CAN Low lines with an oscilloscope.
8. Suspect PCM: Only after all wiring, power, and ground issues have been ruled out should the PCM itself be considered the point of failure.

Parts You'll Likely Need

• 12V Battery (OEM #31500-SCV-A01 (Group 151R)) — A weak or failing battery is the most frequent cause of network communication codes on the Honda Fit.
  Trusted brands: Interstate, DieHard, Duralast
  OEM price range: $180-$250
  Aftermarket price range: $120-$200
• Powertrain Control Module (PCM) (OEM #e.g., 37820-RP3-A75, 37820-RP3-A41, 37820-RP3-A43 (Varies by year/trans)) — This is the last-resort replacement if the module itself has failed internally, after all power, ground, and wiring issues are ruled out. A replacement PCM will require programming by a dealer or specialized shop.
  Trusted brands: Honda (OEM)
  OEM price range: $800-$1200
  Aftermarket price range: $400-$600 (Remanufactured)
• Ground Strap (OEM #32111-RB0-000 (Example, verify by VIN)) — A corroded or broken engine or chassis ground strap can interrupt the PCM's essential ground path. The strap under the battery is a common failure point. The G101 ground on the thermostat housing is also critical.
  Trusted brands: Honda (OEM), Standard Motor Products
  OEM price range: $20-$40
  Aftermarket price range: $10-$25

Related Codes That Often Appear With This One

• U0101 — Lost Communication with TCM (Transmission Control Module). This often appears with U0100 because the network is down, affecting multiple modules.
• U0122 — Lost Communication with Vehicle Dynamics Control (VSA) Module. This is another module on the same CAN bus that loses communication when the network fails.
• U0155 — Lost Communication With Instrument Panel Cluster (IPC) Control Module. This indicates the gauges have also lost communication with the PCM.

Platform-Specific Known Issues

• source — A user with a 2009 Fit experienced a no-start and multiple U-codes. The initial diagnosis pointed towards a bad PCM, but the community strongly advised checking the battery and all ground connections first. The user eventually found a loose ground connection, which resolved the issue completely, avoiding a costly PCM replacement. This highlights the importance of checking basics first.
• source — Multiple owners report that the ground strap located under the battery tray, which connects the transmission to the chassis, is a primary culprit. It is exposed to battery fumes and moisture, causing it to corrode and eventually break. Symptoms are often intermittent at first. A visual inspection requires removing the battery and its tray. Replacing this strap is a common, effective, and low-cost fix for U0100.

Technical Service Bulletins (TSBs) & Recalls

• Bulletin #A18-006 — While issued for other Honda models, this manufacturer bulletin notes that an internal software issue with the multipurpose camera unit can trigger a "Lost Communication with the PCM" message and a possible DTC U0100-F1.

Mechanic-Grade Diagnostic Values

• CAN Bus Network Resistance — expected: ~60 Ω (Ohms). Failure: A reading of ~120 Ω indicates one of the two terminating resistors is offline or there's an open circuit. A reading near 0 Ω indicates a short between the CAN High and CAN Low wires.
• CAN Bus Voltage (Key On, Engine Off) — expected: CAN-H (Pin 6 to Ground): ~2.5V to 3.5V. CAN-L (Pin 14 to Ground): ~1.5V to 2.5V.. Failure: Voltages outside these ranges, or a line stuck at 0V or 5V, indicate a short or open circuit in the harness.
• PCM Ground Circuit Voltage Drop — expected: < 0.1 Volts (100 millivolts). Failure: A reading higher than 0.1V indicates excessive resistance in the ground circuit, often due to corrosion or a loose connection at points like G101.
• PCM Ground Pin Continuity — expected: < 0.2 Ω (Ohms). Failure: Resistance higher than this indicates a poor ground connection that needs to be cleaned or repaired.

Scan Tool Commands That Help

• Honda Diagnostic System (HDS) or equivalent J2534 tool: Replace ECM/PCM — This function is mandatory after installing a new or used PCM. It is used to write the vehicle's VIN to the new module and perform the immobilizer relearn procedure so the engine will start.
• Honda Diagnostic System (HDS): Immobilizer Setup Tool > Add/Delete Keys — As part of the PCM replacement, all existing keys must be re-registered to the new PCM and immobilizer unit. This function manages the key programming.
• Autel or similar professional scanner: Immobilizer > Replace ECM/PCM — Used to sync a replacement (often used) PCM with the car's immobilizer system to resolve a no-start condition after swapping the module.

Wiring & Ground Locations

• G101 — On the engine block, specifically on the thermostat housing.. This is a primary power and logic ground for the PCM. Corrosion or looseness at this single point is a well-documented cause of intermittent stalling and communication codes.
• G201 — Behind the right side of the front bumper.. Grounds several components in the engine compartment harness, including the ABS/VSA modulator-control unit. A fault here can cause multiple U-codes.
• G301 — Located in the engine compartment, grounding components like the wiper motor and fan motors.. While not a direct PCM ground, high current draw or faults in these circuits can introduce electrical noise that may disrupt network communication if the ground is poor.
• Main Engine/Transmission Ground — A braided strap running from the transmission housing/mount area (under the battery tray) to the chassis frame rail.. This is the main ground path for the entire powertrain. Corrosion or breakage of this strap is a very common cause of U0100 and a variety of other electrical issues.
• DLC (OBD-II Port) — Under the driver's side dashboard, to the left of the steering column.. Pins 6 (CAN-H) and 14 (CAN-L) are the direct access points for testing the CAN bus network's resistance and voltage.

Real Owner Repair Stories

• Reddit user in r/MechanicAdvice (2012 Honda Fit Sport with ~400,000 miles) — Intermittent voltage sag when multiple accessories (lights, heat, radio) were on, leading to power steering failure and engine stalling at idle. The car would always restart immediately. The radio cutting out was the first symptom.
  ❌ Tried (didn't work) The owner had previously replaced the immobilizer, neutral safety switch, and the entire master wiring harness due to rodent damage, which fixed a prior issue but not this new one.
  ✅ What actually fixed it The final diagnosis pointed to a failing alternator that could not keep up with the electrical load at idle, causing system voltage to drop and modules to lose communication. Replacing the alternator resolved the stalling and electrical failures.
• YouTube video by 'Pine Hollow Auto Diagnostics' (2013 Honda Fit) — Start-and-stall condition, U0100 code present.
  ❌ Tried (didn't work) Initial diagnosis was a bad PCM.
  ✅ What actually fixed it The root cause was a shorted fuel injector which damaged the injector driver circuit inside the original PCM. The fix required replacing both the faulty fuel injector and the PCM. A used PCM was installed and programmed using an Autel scanner to write the VIN and sync the immobilizer, which resolved the code and the stalling.
• NHTSA ODI #11442354 — An owner reported a "Lost Communication with ECM/PCM 'A'" error and requested the manufacturer address the issue immediately.
• NHTSA ODI #11670114 — An owner of a similar Honda model reported that after receiving codes P0172, U0038, and U0100, they were told replacing fuel injectors would fix the car, but the issues persisted.
• NHTSA ODI #11589674 — A report describes a vehicle that, upon starting, rebooted and cleared errors, but a dealership found a long list of logged communication codes including U0100-00, U0100-F1, U0101-00, and U0122-00.
• NHTSA ODI #11496856 — A diagnostic scan on a Honda vehicle revealed multiple permanent communication codes, including U0100, U012D, and U0111, affecting the electric power train and steering modules.

Model Year Variations Within This Range

• 2012-2014: The 2012 model year introduced a mid-cycle refresh with cosmetic changes, including different front bumper covers, grilles, and headlight housing colors (black became standard). While the core electrical architecture and common causes for U0100 remained the same, some body panels and fender liners have different part numbers between pre-facelift (2009-2011) and post-facelift (2012-2014) models, which is relevant if diagnosing wiring damage from a collision.

Diagnostic Flowchart

Other Known Issues on This Vehicle

Issues unrelated to this code that are worth knowing about as an owner of this generation:

• Lost Motion Springs Failure 🔴 High — Common on 2009-2010 models, leading to a recall. Can cause engine noise, damage, or stalling. (Ref: NHTSA Campaign ID: 11V094000)
• VTC Actuator Rattle on Cold Start 🟠 Medium — A rattling noise for a few seconds on cold starts. While more prominent on the 3rd gen Fit, the L-series engine in the 2nd gen can also experience this. If left unaddressed, it can lead to premature timing chain wear. (Ref: Honda TSB #16-088 (applies to later models but describes the same issue))
• Loose Spark Plugs 🔴 High → Shop Spark Plug — Some vehicles were reported to have spark plugs that were not torqued correctly from the factory, which could loosen over time, causing a misfire and potentially damaging the ignition coil or cylinder head threads.
• Water Leaks into Cabin/Hatch Area 🟡 Low — Commonly reported issue where water can leak into the spare tire well or front footwells due to failed body seams/seals, particularly around the roof ditches and hatch hinges.
• Weak Motor Mounts 🟠 Medium — The engine and transmission mounts, particularly on manual transmission models, can wear out prematurely (as early as 80k miles), leading to excessive engine vibration and noise.

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: A used Powertrain Control Module (PCM) from a junkyard or online recycler can be a cost-effective option, but ONLY if you have a clear plan for programming. Some services can clone your original PCM's data to the used unit, creating a plug-and-play replacement. Otherwise, you will need a dealer or a well-equipped independent shop with a tool like the Honda HDS or a capable Autel scanner to program the used module to your car.

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

What to inspect on the donor part:

• Match the OEM part number on the used PCM EXACTLY to your original module. Different numbers may fit but not function correctly.
• Source the part from a vehicle with a known history, preferably one that was not involved in a flood or front-end collision.
• Inspect the connectors on the used PCM for any signs of corrosion, bent pins, or physical damage.
• Ask the seller if they offer a warranty or return policy in case the unit is dead on arrival.

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

• Powertrain Control Module (PCM): While used OEM is an option, avoid new aftermarket PCMs. They often have software or hardware incompatibilities. Stick with a new Honda part or a properly programmed/cloned used Honda part.

Brands owners have reported issues with on this vehicle:

• Generic, unbranded PCMs from online marketplaces. These are a significant gamble and often lead to more diagnostic headaches.

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 — Often appears alongside U0100 when there is a total loss of communication or a shared power/ground issue affecting multiple modules like the TCM.
• U0155 — Communication code for the instrument panel cluster; frequently triggers with U0100 when the CAN bus is interrupted or battery voltage is critically low.
• P0128 — Related to the thermostat area where the critical G101 PCM ground is located; poor grounding at this location can cause both sensor errors and U0100.
• B1234 — Body-related codes (ABS, VSA, Power Steering) often surface simultaneously with U0100 when the vehicle's network communication is compromised.

Frequently Asked Questions