U0402 on 2021 Jeep Wrangler: Implausible Data from TCM Causes & Fixes

What's Unique About the 2021-2021 Jeep WRANGLER

For the 2021 Jeep Wrangler (JL generation), this specific code is frequently tied to known software issues that Jeep has acknowledged and addressed. Multiple Technical Service Bulletins (TSBs) have been issued that call for reprogramming the vehicle's control modules as the solution. This makes a dealer software update the most critical first step in diagnosis, distinguishing it from other vehicles where a hardware failure might be the immediate suspect. The Jeep Gladiator (JT), a direct platform-mate, also experiences similar software-related transmission issues.

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 (Malfunction Indicator Lamp) is on.
• Harsh or erratic shifting, especially in lower gears.
• Vehicle enters 'limp mode', getting stuck in a single gear.
• Delayed engagement when shifting from Park to Drive.
• Inaccurate transmission temperature readings on the dash (e.g., dropping from 80F to -40F instantly).
• Flashing of other warning lights, like the traction control light.
• Vehicle may not start or may display a "Service Shifter" message.

Most Likely Causes

1. Outdated Control Module Software 🔴 High Probability Multiple TSBs (e.g., 18-022-24 REV. A, 18-012-23) have been issued by the manufacturer specifically to correct this code with a software update for the 2021 Wrangler. This is a widely acknowledged issue by the manufacturer.
   How to confirm: A Jeep dealership or a qualified shop with a wiTECH scan tool must check the current software version on the PCM and TCM against the latest available from the manufacturer. The tool will show if an update is available for the vehicle's VIN.
   Typical fix: Reprogramming (flashing) the Powertrain Control Module (PCM) and/or Transmission Control Module (TCM) with the latest software. This is a non-mechanical repair that takes approximately 30-60 minutes.
   Est. part cost: $0-$250 (Labor for flashing, may be covered under warranty)
2. Wiring or Connector Issues ⚪ Low Probability The CAN bus network wiring that connects the modules can become damaged, corroded, or have a loose connection, disrupting communication. Forum members have specifically identified the green 'star' connector behind the glove box and a large connector (XY230A) behind the right front fender liner as potential failure points.
   How to confirm: Visually inspect the wiring harness between the PCM and TCM. A multimeter can be used to check for continuity and proper voltage/resistance on the CAN High and CAN Low lines. Several owners on jlwranglerforums.com have fixed the issue by re-seating the pins in the green star connector 🎬 See how to fix CAN-C errors by replacing the star connector behind the glove box.
   Typical fix: Repair the damaged section of the wiring harness or clean/secure the corroded or loose connectors. In some cases, owners bypass the problematic connector (like XY230A) entirely. The star connector itself can be replaced with Mopar part #68321746AA. 🎬 Watch: Step-by-step removal and replacement of the green star connector
   Est. part cost: $20-$300
3. Failing Transmission Control Module (TCM) ⚪ Low Probability If software updates do not resolve the issue and wiring is confirmed good, the module itself can fail internally, sending corrupt data. One forum user reported a dealer replacing the TCM/valve body assembly twice before discovering the root cause was a wiring connector.
   How to confirm: This is typically diagnosed by exclusion. If the software is up-to-date and wiring is confirmed to be good, the TCM becomes the primary suspect. A dealer tool may also fail to communicate with the TCM at all.
   Typical fix: Replace the Transmission Control Module and program the new unit to the vehicle's VIN. The average cost for this repair at a shop is estimated between $948 and $1,023.
   Est. part cost: $500-$900
4. Low Battery Voltage or Poor Grounds ⚪ Low Probability Control modules require a stable voltage (typically 12.4V+ engine off, 13.5V-14.5V running) to operate correctly. A weak battery, failing alternator, or a bad ground can cause modules to send corrupt data, a common issue in modern electronically-dense vehicles.
   How to confirm: Test the battery voltage with the engine off and while running using a multimeter. Inspect the main battery terminals and chassis ground connections for corrosion or looseness. Some owners on forums have resolved various electronic issues by ensuring both main and auxiliary batteries are healthy.
   Typical fix: Charge or replace the battery, replace the alternator, or clean and tighten ground connections.
   Est. part cost: $0-$500

Diagnosis Steps

1. Check for active Technical Service Bulletins (TSBs) related to U0402 for your vehicle's VIN. This is the most important first step. Key TSBs include 18-022-24 REV. A and 18-012-23.
2. Using a professional scan tool, confirm the presence of U0402 and check for any other codes, especially U145D or U0001.
3. If TSBs exist, have a dealer or qualified shop with a wiTECH tool update the PCM and TCM software. Ensure the battery is on a charger maintaining 13.2-13.5 volts during the flash process.
4. If the code returns, check the vehicle's battery and charging system to ensure stable voltage.
5. Inspect the wiring harness and connectors. Pay close attention to the green star connector located behind the glove box and the XY230A connector behind the right front fender liner. Check for loose pins or corrosion. Many owners have success simply re-seating the connections.
6. If software, power, and wiring are all confirmed to be good, the Transmission Control Module (TCM) is the likely culprit and may need replacement and programming.

Parts You'll Likely Need

• Control Module Software Update — This is not a physical part, but a service. It is the most common fix documented in TSBs for the 2021 Wrangler to resolve communication errors.
  Trusted brands: Mopar (Dealer Service)
  OEM price range: $100-$250
  Aftermarket price range: N/A
• Transmission Control Module (TCM) (OEM #Part numbers are VIN-specific. Examples for other models/years include 68467808AA and 5150808AC, but must be verified.) — If a software update doesn't fix the problem, the module itself may have failed internally and is sending corrupt data. Replacement requires VIN-specific programming.
  Trusted brands: Mopar
  OEM price range: $700-$900
  Aftermarket price range: $400-$600
• CAN Bus Star Connector (OEM #68321746AA) — This connector, located behind the glove box, is a known failure point where solder joints can crack or pins can lose connection, causing CAN bus errors.
  Trusted brands: Mopar
  OEM price range: $20-$40
  Aftermarket price range: N/A

Related Codes That Often Appear With This One

• U145D — This code for 'Implausible Data Received From TCM On D-PT' is explicitly mentioned alongside U0402 in manufacturer TSBs for the 2021 Wrangler, indicating they stem from the same software issue.
• U0001 — This code for 'High Speed CAN Communication Bus' can appear alongside U0402 when the root cause is a physical wiring or connector issue, such as the star connector, indicating a broader network problem.

Technical Service Bulletins (TSBs) & Recalls

• 18-022-24 REV. A
• 18-012-23
• 18-051-22 REV. B

Platform-Specific Known Issues

• Multiple TSBs have been released by Jeep/Stellantis for the 2021 Wrangler to address code U0402 via a software flash. This includes TSB #18-022-24 REV. A, #18-012-23, and #18-051-22 REV. B.
• For the 4xe PHEV model, TSB #18-051-22 REV. B notes that this code can appear with symptoms like the vehicle not entering electric mode. The fix also involves a PCM software update but requires verifying that several other hybrid system modules are also updated.
• A common physical failure point is the green CAN bus 'star' connector behind the glove box, which can cause a host of communication codes, including U0402.
• Another documented wiring issue is the XY230A connector located behind the right front fender liner, which contains power, ground, and CAN bus wires for the TCM. Poor crimps on the pins can lead to TCM communication failure.

Mechanic-Grade Diagnostic Values

• CAN Bus Network Resistance — expected: Approximately 60 Ω. Failure: A reading of 120 Ω indicates a missing terminating resistor or an open circuit. A reading near 40 Ω suggests a third, unintended resistor is on the network. A reading of 0 Ω indicates a short between CAN High and Low lines.
• CAN Bus Voltage (Key On, Engine Off) — expected: CAN High: ~2.5-3.5V; CAN Low: ~2.5-1.5V. Both lines should idle near 2.5V and fluctuate during communication.. Failure: Voltages that are stuck high, stuck low, or do not mirror each other indicate a wiring short or a faulty module pulling the bus voltage down.
• TCM Power Supply Voltage (Ignition On) — expected: 11.5V - 13.5V. Failure: Voltage below 11.5V can cause the module to send corrupt data or reset unexpectedly.
• Alternator Output Voltage (Engine Running) — expected: 13.5V - 14.5V. Failure: Voltage outside this range indicates a charging system problem that can lead to unstable module operation.

Hidden / Shadow Codes Worth Checking

• U0402-00: The '-00' suffix stands for 'No Sub Type Information'. It is part of the standard DTC display on FCA vehicles and confirms the general 'Implausible Data' fault without adding more specific detail. Its presence is expected when reading the code with a capable scanner. (see via Professional scan tool like wiTECH or other high-end OBD-II scanners with manufacturer-specific capabilities.)

Scan Tool Commands That Help

• wiTECH or equivalent bidirectional scanner: Bidirectional Gear Selection Test — With the vehicle stationary, a technician can command the TCM to cycle through gears. If the transmission does not respond or reports an error, it helps confirm a functional problem with the TCM or its immediate connections, separating it from a more intermittent data corruption issue.

Wiring & Ground Locations

• Green Star Connector (C4 C20) — Located behind the glove box. It serves as a central junction for multiple CAN bus circuits.. This is a widely documented failure point. Poor pin connections or cracked solder joints inside this connector can interrupt communication between the TCM and other modules, directly causing code U0402.
• Connector XY230A — Large electrical connector located behind the right front fender liner.. This connector contains critical power, ground, and CAN bus circuits for the Transmission Control Module. Poorly crimped pins or corrosion here can lead to a total loss of communication or implausible data from the TCM.
• G904A / G906A — These are primary dash ground points located in the driver and passenger footwell areas, often behind the kick panels.. While not the direct ground for the TCM (which is on the transmission harness), a poor connection at these major chassis ground points can introduce electrical noise and voltage instability across the entire CAN network, leading to communication codes like U0402.

Real Owner Repair Stories

• jlwranglerforums.com user 'Shelly Williams' and 'Jeep Wick' (2021 Jeep Wrangler Rubicon) — U0402 code, hard shifting in lower gears, transmission temperature reading on dash would suddenly drop from 80F to -40F, causing the vehicle to jerk.
  ❌ Tried (didn't work) Initial owner troubleshooting.
  ✅ What actually fixed it The dealership replaced the entire transmission under warranty. This highlights how this electronic/communication code can lead to a major component replacement, which may be a misdiagnosis of the root cause.
• jlwranglerforums.com user referencing 'alphawolff' (Jeep Wrangler JL (year not specified, but JL platform)) — U0402 code present.
  ❌ Tried (didn't work) Not specified, but the user went straight to wiring diagnosis.
  ✅ What actually fixed it The owner moved the TCM CAN bus wires (violet/white and yellow) to unused empty slots within the green star connector behind the glove box. They also disconnected and re-seated the large connector (XY230A) behind the right fender liner. This resolved the code.

When the Usual Fixes Don't Work

• While the most common manufacturer-recommended fix for U0402 is a software update via a TSB, this is not always the root cause. Multiple owner-documented repairs show that physical wiring faults, particularly at the green star connector behind the glove box, can be the true source of the problem. In these cases, a software flash would not have fixed the issue, and a dealership might proceed to unnecessarily replace modules. A physical inspection and electrical testing of the CAN bus connectors should be performed if a software update does not resolve the code or if symptoms point heavily toward a connection issue (like temperature readings dropping out instantly).

OEM Part Supersession History

• N/A → 68321746AA — This is the current part number for the 20-way green CAN bus star connector.
  Heads up: This part is widely used across many 2017-2026 Stellantis (Mopar) vehicles, including Chrysler, Jeep, and Ram models, for various CAN bus junction applications.