U0101 on 2009-2013 Infiniti FX: Lost Communication With TCM Causes and Fixes

What's Unique About the 2009-2013 Infiniti FX

For the second-generation Infiniti FX (S51), the U0101 code follows the standard manufacturer definition. However, owner experiences and mechanic reports strongly suggest these vehicles are sensitive to voltage drops. A weak battery or a failing alternator can be a primary trigger for this communication code before the TCM itself is suspected. A notable issue for this platform is a leaking power steering pump, which can drip fluid directly onto the alternator, causing it to fail prematurely and lead to the voltage issues that trigger a U0101 code. The TCM is located inside the transmission as part of the valve body assembly, making it more difficult to access for testing.

Symptoms You May Notice

• Check Engine Light is on
• Transmission warning light is illuminated
• Vehicle enters 'limp mode' with severely limited speed
• Harsh, erratic, or delayed shifting
• Transmission may get stuck in one gear (e.g., 2nd or 3rd gear)
• The gear selector indicator (PRNDL) on the dashboard is blank or incorrect
• Vehicle may not start or may crank but fail to turn over
• Sudden loss of power or stalling while driving

Most Likely Causes

1. Low Battery Voltage or Failing Alternator 🔴 High Probability Control modules require a stable voltage to communicate. Intermittent drops in voltage from a weak battery or faulty alternator are a very common cause for network codes like U0101. A specific issue is the power steering pump leaking onto the alternator, causing it to fail.
   How to confirm: Test the battery with a multimeter; it should read ~12.6V with the engine off and ~13.7-14.7V with the engine running. If the voltage is low or fluctuates wildly, the battery or alternator is suspect. Check for signs of fluid contamination on the alternator.
   Typical fix: Charge or replace the battery. If the charging voltage is low, inspect the alternator for fluid damage and replace it if necessary. Address the source of any fluid leaks.
   Est. part cost: $150-$400
2. Poor Ground Connection 🟡 Medium Probability The TCM and other modules rely on a clean ground connection to the chassis. Over time, these ground points can rust or become loose, creating high resistance and communication faults. Owners have resolved U0101 issues by finding and cleaning/tightening a single loose ground bolt.
   How to confirm: Locate the main chassis and engine ground straps, particularly the one near the battery tray area. Check that they are clean, tight, and free of corrosion. Test for voltage drop between the TCM ground pin and the negative battery terminal; it should be less than 0.1 volts.
   Typical fix: Remove the ground strap, clean the contact points on the chassis and the terminal itself with a wire brush, and re-secure it tightly.
   Est. part cost: $5-$25
3. Corroded or Damaged Wiring/Connectors 🟡 Medium Probability The main wiring harness connector to the transmission can be exposed to moisture, road salt, and heat, leading to corrosion on connector pins or breaks in the wires. The large vertical connector on the transmission can loosen over time.
   How to confirm: Visually inspect the main electrical connector on the transmission housing. Check for green/white corrosion, frayed wires, or loose pins. Test for power, ground, and CAN signal continuity at the TCM connector with a multimeter.
   Typical fix: Clean the connectors with an electrical contact cleaner and apply dielectric grease. Repair or replace the damaged section of the wiring harness. Ensure the connector is securely seated.
   Est. part cost: $20-$500
4. Blown Fuse ⚪ Low Probability A short circuit in a related component can blow a fuse that supplies power to the TCM. In one documented case on a sister platform, a short during a navigation system installation blew a 'meter' fuse, causing U0101.
   How to confirm: Check the vehicle's fuse boxes (in the cabin kick panel and engine bay IPDM) for any fuses related to the TCM, AT, or Meter. A blown fuse indicates a power supply interruption.
   Typical fix: Replace the blown fuse. If it blows again immediately, it indicates a short circuit in the wiring that must be traced.
   Est. part cost: $1-$5

Rare But Worth Checking

• Failed Transmission Control Module (TCM) / Valve Body: While this is the part the code points to, the TCM itself is often the last thing to fail. All power, ground, and wiring issues must be ruled out completely before condemning the module. On the FX, the TCM is integrated with the transmission valve body, requiring replacement as a single unit. This is often the confirmed fix when accompanied by other codes like P0720 (Output Speed Sensor).

Diagnosis Steps

1. Scan for all codes in all modules. Note any other 'U' codes, as they can help pinpoint a network-wide issue versus a single module problem.
2. Check battery health and charging system voltage. Ensure the battery is fully charged and the alternator is providing stable voltage (13.7V-14.7V). Inspect the alternator for any signs of fluid leaks from above (e.g., power steering pump).
3. Inspect all fuses related to the transmission ('A/T') and instrument cluster ('Meter') in both the driver's side kick panel fuse box and the IPDM in the engine bay.
4. Locate and inspect main chassis ground points. A critical one is located near the battery tray. Ensure they are clean, tight, and free of corrosion.
5. Locate the Transmission Control Module (TCM). On the 2009-2013 FX, it is located inside the transmission oil pan, integrated with the valve body.
6. Inspect the main round electrical connector on the outside of the transmission case. Check for corrosion, moisture, or bent/damaged pins. Ensure it is securely fastened.
7. Using a multimeter and a wiring diagram, verify that the TCM is receiving proper battery voltage and has a solid ground connection at the main transmission connector.
8. Test the two CAN bus wires at the TCM connector for proper resistance (should be ~60 ohms with the battery disconnected) and voltage (CAN High ~2.5-3.5V, CAN Low ~1.5-2.5V with key on).
9. If power, ground, and network wiring are all confirmed to be good, the TCM/Valve Body assembly is the most likely cause of the failure.

Parts You'll Likely Need

• Battery — A weak or failing battery is a primary cause of module communication errors.
  Trusted brands: Interstate, DieHard, Bosch
  OEM price range: $200-$300
  Aftermarket price range: $150-$250
• Transmission Control Module / Valve Body Assembly (OEM #31705-X079B (supercedes 31705-14X8A, 31705-X072C, etc.)) — If all wiring, power, and ground checks pass, the module itself has likely failed. On these Infiniti vehicles, the TCM is integrated with the valve body inside the transmission and must be replaced as a unit.
  Trusted brands: Hitachi (OEM), Bosch, Remanufactured by companies like Flagship One
  OEM price range: $800-$1500
  Aftermarket price range: $600-$900
• Alternator — Often fails due to fluid contamination from a leaking power steering pump, leading to voltage issues that trigger communication codes.
  Trusted brands: Bosch, Denso, Hitachi
  OEM price range: $400-$700
  Aftermarket price range: $250-$450

Related Codes That Often Appear With This One

• U0100 — Lost Communication with ECM/PCM. If other modules are also setting communication codes, it can point to a broader network issue on the CAN bus, a bad ground, or a failing IPDM (Intelligent Power Distribution Module).
• U1000 — A generic CAN Communication Line code often seen on Nissan/Infiniti vehicles. It frequently appears alongside other U-codes and points to a network-wide problem.
• U0122 — Lost Communication with Vehicle Dynamic Control Module. This often appears with other U-codes, indicating a widespread network problem rather than a single module failure.
• P0720 — Output Speed Sensor Circuit Malfunction. When seen with U0101, this strongly suggests an internal failure of the TCM/Valve Body assembly, as the speed sensor is part of that unit.

Platform-Specific Known Issues

• Power Steering Fluid Leak Causing Alternator Failure: A well-documented issue on this platform is the power steering high-pressure hose or pump leaking fluid directly onto the alternator located below it. This fluid ingress causes premature alternator failure, leading to voltage instability and a host of communication codes, including U0101. If diagnosing U0101, it is critical to inspect the alternator for wetness or grime and check the power steering system for leaks.
• TCM Integrated with Valve Body: Unlike some vehicles with an externally accessible TCM, the 2009-2013 FX has its TCM integrated into the transmission valve body, located inside the oil pan. This makes diagnosis more labor-intensive as the pan must be dropped to access the unit. It also means that a failed TCM requires replacing the entire expensive valve body assembly.

Mechanic-Grade Diagnostic Values

• CAN Bus Network Resistance — expected: ~60 Ohms across CAN_H and CAN_L pins at the transmission connector (with battery disconnected).. Failure: A reading of ~120 Ohms indicates a break in one of the CAN wires or a missing terminating resistor. A reading significantly lower than 60 Ohms suggests a short between the CAN lines.
• CAN Bus Voltage (Key On, Engine Off) — expected: CAN_H: ~2.5V to 3.5V. CAN_L: ~1.5V to 2.5V.. Failure: Voltages outside these ranges, or a lack of voltage, points to a wiring short to power/ground or a module failure on the bus.
• TCM Power Supply at Transmission Connector — expected: 12V+ (Battery Voltage) with ignition on.. Failure: Low or no voltage indicates a blown fuse, a faulty IPDM, or a break in the power wire.
• TCM Ground Circuit Voltage Drop — expected: Less than 0.1 Volts (100mV) between the ground pin at the transmission connector and the negative battery terminal.. Failure: Higher voltage indicates high resistance in the ground circuit, often due to corrosion or a loose connection.

Scan Tool Commands That Help

• Infiniti CONSULT-III / CONSULT-III plus: Programming > Before Replace ECU — Used to save the existing TCM's configuration and VIN data before removing it. This is the first step when replacing the valve body/TCM with a new 'blank' unit, assuming the old module is still communicative.
• Infiniti CONSULT-III / CONSULT-III plus: Programming > After Replace ECU — Used after installing a new 'blank' TCM to write the saved configuration data to it. If the old module was dead, this function is used to select the correct software part number (obtained from the parts catalog) and program the new module from scratch.
• Infiniti CONSULT-III / CONSULT-III plus: Work Support > G-Sensor Learning / Clutch Point Learning — These functions are required after replacing and programming a new TCM/Valve Body to calibrate the new hardware to the vehicle, ensuring proper shift quality.

Wiring & Ground Locations

• IPDM E/R (Intelligent Power Distribution Module Engine Room) — In the engine compartment, typically on the passenger side, near the battery and firewall.. This module contains the fuses and relays that supply power to the TCM. An internal failure of the IPDM can cut power to the transmission module, causing a U0101 code even if the TCM and its direct wiring are fine.
• Main Engine/Chassis Ground (Under Battery Tray) — Underneath the battery and battery tray. Requires removing the battery to access.. This is a primary ground point for multiple systems. It is highly susceptible to corrosion from battery fumes or water ingress. A poor connection here can cause intermittent communication faults across many modules, including the TCM.
• Front Engine Ground Point — On the front of the engine block, where several ground wires converge onto a single bolt.. A documented case showed that a loose bolt at this location, particularly after installing aftermarket grounding kits or other engine work, can cause a U0101 no-start condition.
• Transmission Main Electrical Connector — A large, round, multi-pin connector on the exterior of the transmission housing.. This is the single point of connection for all power, ground, and CAN bus signals to the internal TCM. All diagnostic voltage and resistance tests for the TCM circuit are performed at this connector's pins.

Real Owner Repair Stories

• YouTube user 'its.sonnyy' (Infiniti FX (S51 generation)) — No-start condition, all power accessories worked but engine would not crank. U0101 code was present.
  ❌ Tried (didn't work) Suspected a bad battery, but swapping it did not help., Suspected a bad alternator, which was contaminated with power steering fluid., Initially suspected a failed ECU or TCM based on the code definition.
  ✅ What actually fixed it The owner had recently installed aftermarket engine bay dress-up bolts and had forgotten to properly tighten a critical ground wire bolt on the chassis. Tightening the loose ground bolt resolved the no-start condition and the U0101 code.

OEM Part Supersession History

• 31705-14X8A, 31705-X072C, 31705-14X4B, 31705-74X8E → 31705-X079B — Revisions for improved reliability and performance. The TCM/Valve Body assembly is updated periodically by the manufacturer.
  Heads up: While valve bodies for the RE7R01A transmission may be physically interchangeable across different Nissan/Infiniti models, the TCM programming is often VIN-specific. Installing a used unit with a mismatched part number or from a different model can result in shifting problems or a persistent U0101 code, even if the part is functional.

Model Year Variations Within This Range

• 2009-2013: The primary variation is not in the physical hardware but in the TCM software. Infiniti released multiple software updates to address shift quality and reliability. When replacing the TCM/Valve Body, the new unit must be programmed with the latest software version compatible with the vehicle's VIN using the CONSULT-III plus tool.

Diagnostic Flowchart

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: A used TCM/Valve Body assembly is a viable, cost-saving option only if it is an exact part number match and sourced from a reputable dismantler who can verify the donor vehicle did not have transmission issues. It is a gamble, as hidden electronic faults are common.

Donor-vehicle mileage cap: roughly under 100000 miles for the part to have meaningful remaining life.

What to inspect on the donor part:

• Verify the part number on the used unit matches the required part for your vehicle.
• Inspect the main electrical connector for any signs of corrosion, bent pins, or fluid contamination.
• Ask the seller if the donor vehicle had a known history of coolant mixing with transmission fluid, a known issue that destroys these units.

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

• While the valve body can be used or remanufactured, the programming software must be from the OEM (Infiniti). There is no reliable aftermarket programming solution.

Aftermarket brands forum-validated for this vehicle:

• Hitachi (Original Equipment Manufacturer)
• Street Smart Transmission (Remanufactured, often pre-programmed)
• Flagship One (Remanufactured, often pre-programmed)

Brands owners have reported issues with on this vehicle:

• Unbranded, generic units from online marketplaces that do not offer VIN-specific programming. These are very high-risk and likely to be incompatible.

Real Owner Stories

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

Related OBD-II Codes

• U1000 — A common companion code on Nissan/Infiniti vehicles indicating a general CAN communication circuit failure. It is often triggered by the same root causes as U0101, such as low system voltage or a poor ground connection.
• P0562 — Indicates System Voltage Low. This code directly supports the most common cause for U0101 on this vehicle: a failing alternator (often due to a power steering fluid leak) or a weak battery causing voltage instability on the communication network.
• P0700 — This is a generic Transmission Control System Malfunction code set by the Engine Control Module (ECM). The ECM sets this code to illuminate the Check Engine Light when it detects a fault reported by the TCM, or in this case, when it can no longer communicate with the TCM at all.
• U0155 — Lost Communication With Instrument Panel Cluster (IPC) Control Module. This code is directly related to a potential cause cited in the context where a blown 'meter' fuse, which powers the instrument cluster, led to a U0101 code.
• U0073 — Control Module Communication Bus 'A' Off. This is a more general code indicating the entire CAN bus network is malfunctioning. A fault causing U0101, like a major wiring short or a bad ground, could be severe enough to bring down the whole network and set this code as well.

Frequently Asked Questions