OBD-II Code U1400: Implausible Data from a Control Module

What Does U1400 Mean?

U1400 means one of your car's computers received a message from another computer that violates basic logic. Think of it like a garbled text message; the information is invalid. This prevents the receiving computer from trusting the data. For example, the transmission module receives a throttle position reading of 150%, flagging the data as faulty.

Technical definition: The official SAE/OBD-II definition for U1400 is "Invalid or Implausible Data Received From a Control Module". The specific data referenced (e.g., throttle position, wheel speed) and the module setting the code vary by the vehicle's manufacturer.

Can I Drive With U1400?

Yes, But With Caution. Yes, but only for a short distance to a safe location or a repair shop. This code often engages 'Limp Mode,' limiting speed to 35-40 mph and locking the transmission in a single gear. Driving for extended periods in this state overheats the transmission, causing thousands of dollars in internal damage. The underlying fault also disables critical safety systems like ABS.

Common Causes

• Weak or failing battery (Very Common) — Low or fluctuating system voltage is the primary cause of communication errors. 🎬 Watch: How a weak battery triggers these communication codes. Before a battery dies completely, it provides inconsistent power, which corrupts data signals on the CAN bus and triggers a U1400 code.
• Poor or corroded ground connections (Common) — A loose or corroded ground strap for a control module or the main chassis introduces electrical noise and voltage irregularities. This causes widespread communication faults and is a known issue on GM and VW models.
• Faulty sensor data (Wheel Speed or Steering Angle) (Common) — A failing wheel speed or steering angle sensor sends erratic data to the ABS module. The ABS module then broadcasts this 'implausible' data to other modules on the network, triggering the U1400 code. This is highly common on Stellantis (Chrysler/Dodge/Jeep) vehicles.
• Damaged CAN bus wiring or connectors (Common) — The wires connecting the vehicle's computers can be damaged by vibrations, heat, or moisture. A chafed wire rubbing against metal or a corroded connector pin distorts signals, leading to this code.
• Failing Totally Integrated Power Module (TIPM) (Less Common) — On 2008-2016 Chrysler, Dodge, and Jeep vehicles, the TIPM (fuse box) contains internal relays that fail, causing severe electrical issues and communication codes like U1400.
• Failing control module (Less Common) — A specific module, like the ABS module or Transmission Control Module (TCM), fails internally. It sends out corrupted data or stops communicating altogether, causing other modules to report a U1400 code.
• Interference from aftermarket electronics (Rare) — Poorly installed aftermarket accessories, such as remote starters or alarms, create electrical noise or interfere with the CAN bus network, corrupting data.

Symptoms

• Check Engine Light is on — The Malfunction Indicator Lamp (MIL) illuminates on your dashboard.
• ABS and/or Traction Control lights are on — Because this code often involves data from the braking system's computer, warning lights related to the ABS and traction control systems illuminate.
• Vehicle enters 'Limp Mode' — The engine reduces power and the transmission locks in one gear as a safety measure because critical sensor data is missing or invalid.
• Erratic transmission shifting — The Transmission Control Module receives bad data about engine speed or throttle position, causing harsh, incorrect, or delayed shifting.
• Instrument cluster behaving strangely — Gauges act erratically, warning lights flash randomly, or the entire cluster resets while driving.
• No-start or intermittent start condition — If the communication failure involves the Powertrain Control Module (PCM) or anti-theft system, the engine cranks but fails to start.

Diagnostic Flowchart

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

Common Fixes & Costs

• Replace Battery — Parts: $150-$350, Labor: $50-$100, ~0.5 hr book time (DIY)
• Repair/Clean Ground Connection — Parts: $5-$20, Labor: $100-$200, ~1.0 hr book time (DIY)
• Replace Wheel Speed Sensor — Parts: $50-$250, Labor: $100-$150, ~1.0 hr book time (DIY)
• Replace TIPM (Chrysler/Dodge/Jeep) — Parts: $700-$900, Labor: $150-$300, ~1.5 hr book time (Professional)
• Replace ABS Control Module — Parts: $600-$1,000, Labor: $150-$250, ~1.5 hr book time (Professional)
• Repair Wiring Harness — Parts: $20-$100, Labor: $250-$800, ~4.0 hr book time (Professional)

Used vs. New Parts: Buying Guide

When a used part is worth it: For expensive electronic modules like an ABS Module or TIPM, a used or remanufactured part from a reputable source offers significant savings. This is true for older vehicles where a new OEM part exceeds the car's value.

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

Donor quality checklist:

• Verify the part number matches exactly; modules look identical but have different internal programming.
• Purchase from a seller offering a warranty (30-90 days) and a clear return policy.
• For Chrysler/Dodge/Jeep, a used TIPM requires programming to enable specific features (e.g., fog lights).
• Avoid parts from flood or fire-damaged cars.

Decision logic:

• If The vehicle is less than 8 years old and the part is under $400 new → Buy new for the longer warranty and guaranteed compatibility.
• If The vehicle is over 10 years old and the new OEM part costs over $800 → A remanufactured module with a 1-year or lifetime warranty is the best value.
• If The part requires VIN programming to function (common for ABS modules and TIPMs) → Favor a 'rebuild and return' service for your original module, avoiding programming issues entirely.

Warranty tradeoff: Used parts from a junkyard offer a 30-90 day warranty. Remanufactured modules come with a 1-year to lifetime warranty. New OEM parts carry a 1-2 year warranty but at a significantly higher price.

Worst-case if a used part fails: $300-$800 if a used or improperly remanufactured module fails, including repeat labor costs.

What Happens If You Wait — Timeline

1. 0-1 month: MIL is on, ABS/Traction Control lights illuminate. Vehicle occasionally enters limp mode or exhibits strange electrical behavior. Safety systems are unreliable. (MPG impact: 0-5%% · Added cost: $0)
2. 1-3 months: Limp mode becomes frequent, making driving unsafe. If the cause is a failing alternator, continued driving leads to a dead battery and a no-start condition. (MPG impact: 10-25% (if in limp mode)% · Added cost: $150-$250 (for tow and diagnostic time))
3. 3-6 months: Driving in limp mode causes the transmission to suffer heat damage from operating in a single gear, wearing out clutches and seals. (MPG impact: 10-25%% · Added cost: $2000-$4500 (for potential transmission rebuild))
4. 6+ months: A persistent electrical issue causes cascading failures, damaging multiple expensive control modules (PCM, BCM, ABS). (MPG impact: N/A (vehicle likely undrivable)% · Added cost: $3000-$5000+ (for multiple module replacements))

Cost of Not Fixing It

• 0-1 month: Continued unpredictable behavior of safety systems like ABS and Traction Control. The vehicle randomly enters 'Limp Mode,' creating a safety hazard in traffic. (Added cost: Negligible, but high safety risk.)
• 1-6 months: If the vehicle is frequently driven in 'Limp Mode,' the transmission suffers from overheating. This leads to premature wear of internal clutches and seals, requiring a transmission rebuild. (Added cost: $2000-$4500)
• 6+ months: An underlying electrical issue (like a bad ground or failing alternator) cascades, damaging other sensitive control modules on the network. (Added cost: $1000-$3000+)

Diagnosis Steps

1. Check and Charge the Battery
   A low battery is the most common cause. Check the battery's voltage with a multimeter. It must be above 12.4 volts with the engine off. Have the battery load-tested at an auto parts store to ensure it is healthy under load.
   Tools: Multimeter, Battery Charger (Beginner)
2. Read All Fault Codes from All Modules
   Use an advanced OBD-II scanner to read codes from all modules (ABS, TCM, BCM, TIPM). U1400 is a secondary code; look for other codes pointing to a specific module or sensor (e.g., a wheel speed sensor code) to find the primary fault.
   Tools: Advanced OBD-II Scanner (Beginner)
3. Perform a Voltage Drop Test on Grounds
   Find hidden resistance in battery cables and ground connections. Connect the multimeter's positive lead to the negative battery post and the negative lead to a clean metal point on the engine block. Crank the engine. A reading above 0.2 volts indicates excessive resistance in the main ground strap.
   Tools: Multimeter (Advanced)
4. Perform a Thorough Visual Inspection
   Inspect the wiring harnesses going to the main control modules, especially the ABS module, TCM, and wheel speed sensors. Look for chafed wires, corrosion on connectors, loose grounds, or a buildup of metallic debris on sensor tips.
   Tools: Flashlight, Basic Hand Tools (Intermediate)
5. [PRO TIP] Check Network Resistance
   Disconnect the battery. Measure the resistance between Pin 6 (CAN High) and Pin 14 (CAN Low) on the OBD-II port. A healthy network reads 60 ohms. A reading of 120 ohms indicates an open circuit, while 0 ohms indicates a short circuit between the CAN lines. If the reading is not 60 ohms, take the vehicle to an electrical diagnostic specialist.
   Tools: Multimeter (Advanced)
6. [PRO TIP] Check CAN Bus Voltages
   With the battery connected and the key 'ON' (engine off), check voltages on the OBD-II port. CAN High (Pin 6 to ground) is approximately 2.6-3.5 volts and CAN Low (Pin 14 to ground) is approximately 1.5-2.4 volts. If either voltage is stuck at 0V, 5V, or 12V, there is a short in the network wiring.
   Tools: Multimeter or Oscilloscope (Advanced)
7. [PRO TIP] Check for AC Ripple Voltage
   A failing alternator produces excessive electrical noise (AC ripple) that corrupts data. With the engine running at 1500 RPM and headlights on, set your multimeter to AC Volts across the battery terminals. A reading above 0.5 Volts (500 millivolts) indicates a failing alternator diode.
   Tools: Multimeter (Advanced)
8. Isolate the Faulty Module
   If the network resistance is incorrect (0 or 120 ohms), unplug modules one by one to see when the resistance returns to 60 ohms. For example, if unplugging the ABS module returns resistance from 0 to 60 ohms, the ABS module is shorted internally.
   Tools: Multimeter, Wiring Diagram, Basic Hand Tools (Advanced)

When This Code Triggers (Freeze-Frame Conditions)

• Engine Coolant Temp: 180-210°F (The engine is fully warmed up, as communication faults are evaluated once all systems are at normal operating temperature.)
• Vehicle Speed: 35-55 mph (The code triggers during steady-state cruising when modules expect consistent data from wheel speed and other sensors.)
• RPM: 1500-2500 (Occurs at a constant engine speed, where implausible data (like a sudden drop in a sensor reading) is easier for the ECU to identify as an error.)
• Battery Voltage: 11.8-12.4V (Key On, Engine Off) or 13.2-14.5V (Engine Running) (Freeze frame shows low or fluctuating voltage, directly pointing to a battery or alternator issue as the root cause.)

Related Codes

• U0001 / U0002 — These codes indicate a general network failure. If present, diagnose them first. They represent a total network breakdown, whereas U1400 points to corrupt data on a network that is still partially functioning.
• U0121 — 'Lost Communication With ABS Module'. If present with U1400, the problem originates in the ABS system. If you cannot communicate with the ABS module at all, U0121 is the primary fault. If you can communicate, U1400 is primary, meaning the ABS module is online but sending bad data.
• U0101 — 'Lost Communication with TCM'. Appears on Chrysler/Dodge/Jeep vehicles when the transmission module sets U1400 because it receives unbelievable data from the engine computer. Check for engine-related codes first.
• P0500 — 'Vehicle Speed Sensor 'A' Malfunction'. If P0500 is present alongside U1400, the vehicle speed sensor is the root cause. The faulty sensor sends erratic data which other modules report as implausible.

Climate & Environmental Factors

• High Heat / Hot Climates: Heat accelerates the chemical reactions inside a battery, causing fluid to evaporate and internal plates to degrade. This leads to a higher incidence of low-voltage related U-codes.
• Cold Weather / Freezing Climates: Cold thickens engine oil, increasing the power required to start the engine, while simultaneously reducing the battery's available power. This sudden demand on a weak battery drops voltage low enough to trigger communication faults.
• Humidity and Road Salt (Rust Belt): Salt and humidity create a highly corrosive electrolyte that attacks wiring harnesses, connector pins, and ground points. This corrosion creates high resistance, causing intermittent signals and U1400 codes.

How to Talk to a Mechanic About This Code

Say this: "I have a U1400 network communication code, and my ABS and traction control lights are also on. I'd like to authorize one hour of diagnostic time to identify the source of the communication error. Please scan all modules—especially the ABS, TCM, and BCM—for primary fault codes before suggesting any repairs."

This signals that you understand U1400 is a secondary code. It directs the technician to perform a comprehensive diagnosis across the entire vehicle network, rather than incorrectly focusing on the first module that stored the U1400 code.

Avoid saying:

• 'My check engine light is on, can you fix it?'
• 'The code is U1400, please replace the computer.'
• 'Just do whatever it takes to fix it.'

Questions to ask before authorizing the repair:

• What other fault codes did you find, and in which specific modules?
• What diagnostic tests did you perform to confirm that the recommended part is the root cause?
• Did you check the battery, main grounds, and CAN bus resistance first?
• Can you show me the damaged part or the test results (e.g., voltage drop reading, wiring damage)?
• Does the recommended repair require module programming, and is that included in the quote?

Where to Take It: Dealer vs Independent vs Chain

• Dealer: A strong option if you suspect a software issue or a known TSB exists, but the most expensive path for common wiring or sensor faults.
  Best for: Vehicles under warranty., Complex, manufacturer-specific electrical issues (e.g., TIPM failures, known software bugs)., When the required repair involves programming or flashing a new module.
  Downsides: Highest labor rates, often 1.5-2x more than independent shops., Defaults to replacing an entire expensive module when a simpler wiring or ground repair suffices. (Typical cost: +50% vs. baseline)
• Independent Shop: The best choice, provided you find a shop that specializes in automotive electrical systems or diagnostics. Look for ASE certifications.
  Best for: Out-of-warranty vehicles where cost is a major factor., Diagnosing common U1400 causes like bad batteries, grounds, or wheel speed sensors.
  Downsides: Diagnostic skill for complex network issues varies greatly., May not have access to the latest manufacturer-specific software updates. (Typical cost: +0% vs. baseline)
• Chain Shop: AVOID for diagnosis. They can replace a battery if that is the confirmed cause, but do not authorize them to diagnose a U1400 code.
  Best for: Simple, unambiguous repairs like battery replacement.
  Downsides: Technicians lack the advanced training and tools for network diagnostics., High pressure to meet sales targets leads to misdiagnosis., Unlikely to perform detailed wiring or ground repairs. (Typical cost: -10% vs. baseline)

When to Walk Away From the Repair

If the estimated repair cost exceeds 50% of the car's current private-party value, seriously consider selling or trading in the vehicle instead of repairing it.

• Car worth $4000, fix is $2200: Walk away. The repair cost is 55% of the car's value, making it a poor financial investment.
• Car worth $12000, fix is $1800: Fix it. The repair cost is only 15% of the car's value and is a worthwhile investment.
• Car worth $2500, fix is $1500: Walk away. At 60% of the car's value, this repair is too expensive.

What Scan Tool You Need for This Code

Minimum: A scanner that reads codes from ALL vehicle modules (Engine, ABS, Transmission, Body Control). A basic engine-only code reader is insufficient.

A $20 code reader only shows the U1400 code from the engine computer. It cannot see the primary fault code (like a bad wheel speed sensor) stored in the ABS or transmission module.

Budget: BlueDriver Pro (~$99) — Connects to your smartphone and reads/clears codes from all major modules. It graphs live data, helpful for spotting erratic sensor readings.

Mid-range: Foxwell NT630 Plus / Innova 5610 (~$150-350) — Offers full system diagnostics plus bidirectional controls for functions like ABS brake bleeding. Allows you to command specific modules to perform tests.

Professional: Autel MaxiCOM MK808 / TopDon Phoenix Lite (~$450-800) — Provides full bidirectional control to activate and test individual modules and sensors. Offers OEM-level diagnostics, including module programming and configuration.

How to Clear the Code After You Fix It

1. Ensure the underlying fault (e.g., bad sensor, corroded wire) is fixed.
2. Reconnect the battery if it was disconnected for the repair.
3. Use an OBD-II scan tool to formally clear all Diagnostic Trouble Codes (DTCs) from all modules.
4. Perform a complete drive cycle to allow the vehicle's readiness monitors to reset.

Drive cycle (~20 minutes): A generic drive cycle includes a cold start (vehicle sat for 8+ hours), 2-3 minutes of idling, followed by mixed city driving and 10-15 minutes of steady highway driving between 45-65 mph. Allow the vehicle to cool down completely and repeat if necessary.

Readiness monitors affected: Comprehensive Component Monitor, Misfire Monitor

Before emissions retest: drive at least 50 miles to fully set monitors.

Watch out for:

• Disconnecting the battery clears the code but resets all readiness monitors to 'Not Ready', causing an automatic emissions test failure.
• The code returns immediately if the root cause of the implausible data is not repaired.
• Some vehicles require a specific, manufacturer-defined drive cycle to reset certain monitors.

Will This Fail Emissions / State Inspection?

Yes — this code typically fails an OBD-II emissions inspection.

• California: An illuminated Check Engine Light results in an automatic failure. After repairs, a full drive cycle must be completed to set the readiness monitors before a retest.
• New York: The NYS DMV inspection includes an OBD-II scan. Any U-code illuminating the Check Engine Light causes an automatic failure.
• Texas: In counties requiring emissions testing, an illuminated Check Engine Light is an automatic failure. You must complete a drive cycle after clearing codes.

Most Commonly Affected Vehicles

• Jeep Grand Cherokee (2006-2015) — Often related to faulty wheel speed sensors or issues with the ABS module. The 2011-2013 models are subject to a TIPM fuel pump relay recall.
• Dodge Ram 1500/2500/3500 (2007-2012) — Frequently sets an 'Implausible TPS Signal' from the ECM, which is then flagged by the Transmission Control Module (TCM).
• Chrysler Town & Country / Dodge Grand Caravan (2008-2016) — Highly susceptible to TIPM (fuse box) failure, causing random stalling, lights flickering, and no-start conditions.
• Chrysler / Dodge 300 / Charger / Magnum (2005-2010) — Typically set by the TCM when it receives corrupted throttle position data from the engine computer over the network.
• Volkswagen Tiguan / Jetta / Crafter (2015-2020) — U1400 often means 'Function Restricted due to Insufficient Voltage', pointing directly to a weak battery, failing alternator, or poor ground connection.
• Chevrolet / GMC Silverado / Sierra (2007-2014) — Network codes are caused by a poor ground connection at location G218 (under the driver's side dash) or chafed wiring harnesses near the engine (TSB 08-07-30-021H).
• Ford F-150, Focus, Explorer (2010-2018) — Related to communication issues between the PCM and other modules like the ABS or instrument cluster, frequently caused by wiring problems or corrosion at module connectors.
• Hyundai / Kia Sonata, Optima, Forte (2011-2019) — Triggered by intermittent communication failures. A software update for one of the control modules is often required to resolve communication conflicts.

Manufacturer-Specific Notes

• Chrysler / Dodge / Jeep / Ram: U1400 is commonly defined as 'Implausible TPS Signal Received' or relates to wheel speed sensor data. Be highly suspicious of the TIPM (fuse box) on 2008-2016 models. NHTSA recall 14V-530 addresses the fuel pump relay inside the TIPM on 2011-2013 Durango and Grand Cherokee models.
• Volkswagen / Audi: VW and Audi use U1400 to mean 'Function Restricted due to Insufficient Voltage'. The first diagnostic step is to thoroughly test the battery and charging system, followed by an inspection of main ground points.
• General Motors (GMC / Chevrolet): U1400 is a symptom of a broader network communication problem addressed in TSB 08-07-30-021H. The most common culprits are a poor BCM ground (G218) under the driver's dash or a chafed wiring harness in the engine bay.
• Ford: U-codes point to wiring issues. Inspect harnesses for chafing, especially where they pass through the firewall or near suspension components. Corrosion in the large connectors at the PCM or BCM is a common failure point.

Real Owner Stories

2006 Jeep Grand Cherokee with multiple warning lights

ABS, ESP, BAS, and 'Service 4WD System' lights illuminated. An advanced scanner pulled code C100A pointing to the left front wheel speed sensor.

Outcome: Replacing the sensor and clearing the codes resolved all warning lights.

Lesson: A basic OBD-II scanner cannot see ABS codes. An advanced scanner is essential for diagnosing U-codes. A faulty wheel speed sensor triggers a cascade of seemingly unrelated warning lights.

2015 Chevy Silverado with intermittent 'Service Stabilitrak' message

The 'Service Stabilitrak' message flashed on the dash, accompanied by the radio and instrument cluster flashing. The issue happened more often in the rain.

Outcome: Cleaning the poor ground connection resolved the intermittent electrical issues.

Lesson: Intermittent electrical gremlins that seem weather-related are often caused by a poor ground. Always check and clean main chassis and module grounds before replacing expensive parts.

2012 Dodge Ram with a no-start condition after a fuel pump replacement

A shop replaced a failed fuel pump. The next day, the new fuel pump ran continuously with the truck off, draining the battery. The shop diagnosed a failed TIPM and quoted $1200.

Outcome: The external relay kit bypassed the faulty internal relay in the TIPM, fixing the problem.

Lesson: On Chrysler/Dodge/Jeep vehicles, specific internal TIPM faults (like the fuel pump relay) are fixed with inexpensive bypass kits, avoiding a costly full replacement.

2019 VW Crafter with 'Function Restricted due to Insufficient Voltage' message

After the battery went completely flat and was jump-started, the U1400 code appeared repeatedly.

Outcome: The issue resolved itself as the battery management system recalibrated.

Lesson: On VW/Audi vehicles, U1400 points directly to a low voltage event. If you had a dead battery, fully charge it, clear the codes, and drive for a few days before assuming a serious fault exists.

How to Prevent This Code From Triggering

• Test Battery Annually (Once per year, especially before winter) — A load test identifies a weakening battery before its voltage becomes unstable enough to corrupt module communication.
• Clean Battery Terminals and Main Grounds (Every 2-3 years or during battery replacement) — Corrosion creates resistance, leading to voltage drops that cause communication faults. Cleaning to bare metal and protecting with dielectric grease ensures a solid electrical foundation.
• Apply Dielectric Grease to Critical Connectors (When servicing related components) — Applying dielectric grease to exposed connectors blocks moisture and prevents pin corrosion, a primary cause of implausible signals in wet climates.
• Secure and Inspect Wiring Harnesses (During any under-hood or under-vehicle maintenance) — Securing a loose harness with a zip tie prevents chafing against sharp metal edges or hot exhaust components, avoiding intermittent shorts.

Frequently Asked Questions

Can I fix a U1400 code myself?

Yes, if the cause is a weak battery or a visibly corroded ground wire. However, CAN bus wiring or control module faults require advanced diagnostic tools like an oscilloscope. Take the vehicle to an electrical diagnostic specialist if basic voltage and ground checks pass.

What is the most common misdiagnosis for U1400?

The most common mistake is replacing the control module that stored the U1400 code. This module is usually just the messenger reporting bad data it received from another source. Always find the primary code (like a wheel speed sensor fault) before replacing expensive computers.

Can a bad ground cause a U1400 code?

Absolutely. A loose or corroded ground creates an unstable voltage reference for the modules, leading to corrupted data. This is a well-documented problem on many GM trucks and VWs.

My car has a U1400 and won't start, what's wrong?

A no-start condition indicates a severe communication breakdown on a critical part of the network. This points to a failed Powertrain Control Module (PCM), a shorted CAN bus taking the network down, or a faulty anti-theft system. Tow the vehicle to a shop for network diagnostics.

What is a TIPM and can it cause U1400?

The Totally Integrated Power Module (TIPM) is the advanced fuse box used in many 2008-2016 Chrysler, Dodge, and Jeep vehicles. It is notorious for internal relay failures that cause network communication codes like U1400. Replacing or bypassing the faulty internal relays often resolves the issue.

Can a bad alternator cause a U1400 code?

Yes. A failing alternator produces excessive AC ripple or unstable voltage, which corrupts data signals on the CAN bus. A ripple voltage above 0.5V AC on a multimeter confirms a failing alternator diode.

My scanner only shows U1400. What should I do?

A basic code reader only shows generic codes from the engine computer. U1400 often originates in another module like the ABS or TCM. You must use an advanced scanner capable of reading all vehicle modules to find the root cause.

Will clearing the code make it go away?

Clearing the code turns the check engine light off temporarily, but it returns immediately if the underlying hardware or wiring problem remains. It is a useful diagnostic step to see if the fault is active, but it is not a repair.

Key Takeaways

• Code U1400 triggers when a control module receives invalid data, such as an impossible 0 mph reading while driving 60 mph.
• Test your battery voltage first; a drop below 12.4V is the leading cause of U1400 communication errors.
• Scan all vehicle modules (ABS, TCM, BCM) because U1400 is a secondary code pointing to a primary failure elsewhere.
• Never replace the module that stored the U1400 code without diagnosing the network, as it is almost always just the messenger.
• Limit driving to under 10 miles if the vehicle enters Limp Mode, as operating locked in 2nd or 3rd gear causes severe transmission overheating.