U0101

A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.

Safe to drive

Drive only if you must, short distances at low speed. Limp mode means one fixed gear and possibly no reverse.

Urgency

Get it looked at now

Will it pass emissions

Not affected

If ignored

The vehicle stays in limp mode, which means poor acceleration, potentially no reverse, and high engine speeds at road speed if it is locked in a low gear. Merging or pulling into traffic without the ability to downshift is dangerous. If the cause is a corroded ground or a chafed harness, the problem tends to spread as more modules on the same circuit start dropping off, and the diagnosis gets harder and more expensive the longer it goes. If automatic transmission fluid is wicking into the harness, it keeps travelling toward the module.

Check engine light

Solid

• Multiple warning lights may illuminate
• Affected modules may go into limp mode or shut down
• Specific functions inoperative depending on module
• Diagnostic tools may show communication faults

Many vehicles show no symptom at all beyond the warning light. The absence of a symptom does not mean the fault is not real.

These are the causes this code can have. For the order to work through them in, see what to check first.

Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.

In order. Each of these is cheaper or faster than what follows it, and each one can make the rest unnecessary.

1. Battery condition, charging output, and main ground connections

   Low or unstable system voltage drops modules off the network and produces communication codes that vanish once voltage is correct. This causes more U-codes than broken wires do and it is the cheapest thing to check.

2. Whether the battery was recently disconnected, jump started, or replaced

   A transient voltage event sets a timeout code that may not represent a live fault. Clearing it and seeing whether it returns can save hours of chasing a fault that no longer exists.

3. A full scan of every module, noting who lost communication with whom

   The pattern localises the break. Only the engine module reporting a lost transmission module points at that branch, while many modules losing each other points at the network backbone or at power and ground.

4. The transmission control module's fuses, power feeds, and ground

   An unpowered module is silent, which looks identical to a broken network wire. A blown fuse or a corroded ground is common and quick to find.

5. Any aftermarket alarm, remote start, trailer brake controller, or stereo installation

   Accessory installs frequently splice into network wiring or into ignition feeds. This is a leading cause of communication faults on otherwise healthy vehicles.

1. Scan every module and map who has lost whom

   Tools Scan tool with all-module scan capability

   Do not stop at the engine module. Record every stored communication code across the whole vehicle and note which modules respond to the tool at all. The pattern of what is missing is the most useful single piece of information in this diagnosis.

   Expected

   A clear map showing whether the loss is confined to the transmission module's branch or affects the whole network.

2. Test the battery and charging system under load

   Tools Battery and charging tester, multimeter

   Load test the battery, verify charging voltage, and inspect and clean the main engine and body grounds. Everything measured afterwards is unreliable until this is confirmed good.

   Expected

   A battery that holds under load and charging voltage in the normal range for the vehicle.

3. Determine whether the transmission module talks to the scan tool directly

   Tools Scan tool with module-level access

   Attempt direct communication with the transmission module. A module that answers the tool but that the engine module cannot hear is a very different fault from a module that is silent to everything.

   Expected

   Either direct communication, which points at a routing or gateway issue, or complete silence, which points at power, ground, or that branch of the bus.

4. Verify power and ground at the transmission module connector

   Tools Multimeter, factory wiring diagram, back-probe leads

   Measure battery feed, ignition feed, and ground at the module connector with the circuits loaded, not just as an open-circuit voltage reading. Check the relevant fuses. Connector locations and pin assignments vary by vehicle and must come from factory wiring diagrams.

   Expected

   Full supply voltage under load and a ground with negligible voltage drop. High resistance on either explains a silent module.

5. Measure network resistance with the system asleep

   Tools Multimeter, factory network topology diagram

   With the key off and the modules fully asleep, measure resistance across the network high and low lines at the diagnostic connector. On a conventional two-terminated high-speed controller area network this reads roughly 60 ohms, since two 120 ohm terminating resistors sit in parallel. Around 120 ohms suggests a terminator or backbone is open, and near zero suggests a short between the lines. Verify the network architecture for the specific vehicle first, because not every bus is arranged this way.

   Expected

   A reading consistent with the documented architecture for that vehicle.

6. Inspect the transmission case connector and internal harness

   Tools Hand tools, inspection light, terminal test tools

   Open the connector at the transmission and look for automatic transmission fluid wicked up the internal harness, corroded terminals, and spread pins. This is a well-documented failure point on several designs and it is easy to walk past.

   Expected

   A clean, dry connector with tight terminals. Fluid inside the connector is a finding by itself.

7. Scope the network lines while the fault is present

   Tools Two-channel oscilloscope, back-probe leads

   Capture both network lines on an oscilloscope. A healthy high-speed pair shows the two signals mirroring each other around a resting level. A flat line, a line stuck at supply or ground, or one line missing entirely tells you which conductor has failed and roughly where.

   Expected

   Clean mirrored waveforms on both lines with correct amplitude, or a specific defect that localises the failure.

8. Isolate modules one at a time on the affected branch

   Tools Factory topology diagram, hand tools, scan tool

   A single failed module can pull the whole network down. Disconnect modules individually and watch whether communication returns. Work from the factory topology diagram so you know what you are removing.

   Expected

   Either communication restored when a specific module is unplugged, identifying it, or no change, which exonerates all of them.

9. Consider the transmission module last

   Tools Factory service information, programming equipment

   Only after power, ground, connectors, and network integrity all check out, and the module is confirmed silent at its own connector with verified good power and ground, does replacement make sense. Expect programming and adaptation afterwards, and confirm first whether the module is even a separate unit on this vehicle.

   Expected

   Every external cause documented as ruled out before the module is condemned.

The spread here is driven almost entirely by how far down the list the actual fault turns out to be. A corroded ground or a bad battery is cheap. A module replacement carries programming and adaptation costs on top of the part, and where the module sits inside the transmission the labour is substantial. Ruling out the cheap causes properly is worth the diagnostic time.

• Replacing the transmission control module first. The large majority of these codes are power, ground, connector, or wiring faults.
• Skipping the battery and charging test. A weak battery sets a spray of communication codes across the vehicle.
• Measuring network resistance with the key on or the modules awake, which gives a reading that means nothing.
• Overlooking aftermarket electrical accessories spliced into the network or into ignition feeds.
• Missing automatic transmission fluid wicked into the case connector, which is a known and easily overlooked failure point.
• Treating a code set once during a jump start or a dead battery as a live fault. Clear it and see whether it comes back before spending money.
• Assuming the vehicle has a standalone transmission module at all. On many vehicles the function is inside the engine module or a mechatronic unit inside the transmission.

• TCM
• CAN Bus Wiring

• U0100Lost Communication with ECM/PCM "A"Severity level 4 of 5: Get it looked at now.high
• U0102Lost Communication with Transfer Case Control ModuleSeverity level 4 of 5: Get it looked at now.high

Definition

Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.

Diagnostic guidance

Written and reviewed in-house. See our editorial policy.

Review status

Reviewed

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