U0100

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

Safe to drive

Do not rely on the vehicle. It can stall or refuse to restart without warning, and if the engine stops while moving you lose power steering assist and, after a few brake applications, power brake assist — the brakes still work but need far more pedal effort. Have it diagnosed before driving further, and tow it rather than drive it if it has already stalled once.

Urgency

Get it looked at now

Will it pass emissions

Not affected

If ignored

Modules that depend on engine data run on substitute values, so transmission shifting, stability control, charging, and immobilizer functions can behave unpredictably. The vehicle can stall or fail to start at any time, potentially somewhere unsafe, and a stall at speed takes power steering and vacuum brake assist with it. If the cause is a chafed bus wire, continued flexing turns an intermittent into a permanent no-start.

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. Which modules stored the code, and whether each is current or history

   This is the clue that gets destroyed first, so record it before touching anything. A code present only as history in several modules with a healthy battery usually points back to a past low-voltage event. A code current in every module points at the ECM's power, ground, or bus connection right now.

2. Battery state of charge, terminal condition, and cranking voltage

   This is the cheapest and most common cause of network dropouts. Modules reset when voltage sags, and a weak battery or a corroded terminal sets communication codes across the whole vehicle. Fix this before chasing wiring.

3. Whether the scan tool can communicate with the ECM at this moment

   If the tool talks to the ECM normally, the fault is intermittent and you need to catch it in the act. If the tool cannot reach the ECM either, the problem is present and testable.

4. Recent work and aftermarket installations

   Remote starters, alarms, stereo systems, trailer wiring, and dash cameras are frequently spliced into or near bus wiring. Disconnecting a recent add-on is a free test that resolves a meaningful share of these.

5. Fuses and ground connections feeding the engine controller

   A blown fuse or a loose, corroded engine-to-body ground strap takes the ECM off the network and costs almost nothing to check. Fuse locations and ground point positions vary by make.

1. Perform a full all-module scan and record status before clearing anything

   Tools Scan tool with all-system scan capability

   List every module reporting U0100 and whether each is current, pending, or history. Note any module the tool cannot reach at all. The pattern of who is complaining and who is silent points straight at where the network is broken. Do not clear anything yet — clearing at this point throws away the one piece of evidence that localizes the break.

   Expected

   A written list of every module storing U0100, its status, and every module the tool could not reach

2. Test and charge the battery, then confirm charging system output

   Tools Battery load tester, multimeter, terminal cleaner

   Load test the battery and clean the terminals. Confirm charging voltage is stable with the engine running. Only after the module list from step 1 is recorded, clear codes and retest. A large share of U0100 complaints do not return once the electrical supply is solid.

   Expected

   Battery passes load test, charging voltage steady and within the alternator's normal range

3. Measure network termination resistance at the diagnostic connector

   Tools Digital multimeter, factory network topology diagram

   With the key off and the battery disconnected, measure resistance between pin 6 and pin 14 of the OBD-II connector. A properly terminated high-speed CAN bus reads about 60 ohms because two 120 ohm terminating resistors sit in parallel. About 120 ohms means one terminator or one branch is open. Near zero means a short between the bus wires. A very high reading means the bus is open near the connector. This applies only to a bus that is directly wired to the diagnostic connector. Many vehicles put a gateway module between the DLC and the internal networks, in which case the reading at pins 6 and 14 reflects only the gateway's segment and may legitimately not be 60 ohms — and a healthy 60 ohms there proves nothing about the segment the ECM sits on. Check the network topology diagram for the vehicle and, where a gateway is present, measure at a connector on the segment the engine controller is actually on.

   Expected

   Approximately 60 ohms on a directly connected high-speed CAN segment

4. Verify ECM power feeds and grounds under load

   Tools Multimeter, factory wiring diagram, backprobe leads

   Confirm battery voltage at every ECM power supply pin and perform voltage drop tests on its grounds with the circuit loaded. A good ground drops well under about 0.2 volts under load. Connector pinouts vary entirely by make, so use factory wiring diagrams rather than probing blindly.

   Expected

   Full battery voltage at supply pins, minimal voltage drop on grounds

5. Scope the bus wires while recreating the fault

   Tools Two-channel lab scope

   Capture CAN high and CAN low waveforms. Healthy high-speed CAN shows the two lines mirroring each other around a shared idle level. A line stuck flat, a missing mirror, or corrupted frames identifies a wiring or module problem. Wiggle the harness during capture to catch intermittents.

   Expected

   Clean mirrored CAN high and CAN low signalling with no dropouts during wiggle testing

6. Isolate a module that is dragging the bus down

   Tools Multimeter, scan tool, factory network topology diagram

   If the bus is shorted or loaded, disconnect modules one at a time and re-measure. When the bus recovers after unplugging one module, that module or its branch wiring is the fault. Work methodically and reconnect each before moving on.

   Expected

   Bus resistance and communication return to normal when the faulty branch is isolated

7. Inspect the harness at flex, heat, and moisture points

   Tools Flashlight, pick set, inspection mirror

   Check the engine harness where it crosses to the body, door and hatch boots, areas near the exhaust, and any connector that can collect water. Corroded pins and rodent damage are common and are invisible until you open the connector.

   Expected

   Clean, dry, tight terminals with no chafing or green corrosion

8. Consider the engine controller only after everything upstream tests good

   Tools Factory-capable programming equipment

   Confirm correct power, ground, and bus wiring first. If the module has all three and still will not transmit, it has failed internally. Replacement almost always requires programming and often an immobilizer or key relearn, which is not a bolt-in job.

   Expected

   Documented good power, ground, and bus wiring, with a scope capture showing the module still not transmitting — an internal failure concluded from tests rather than assumed

Module replacement cost is dominated by programming and security relearn, which many independent shops subcontract to a dealer or a mobile programmer. Wiring repairs vary enormously depending on whether the damage is reachable or buried inside a loom. Ranges are US independent-shop, parts plus labor.

• Replacing the engine computer before verifying its power and ground supplies. A module that cannot be powered cannot talk, and the new one will not talk either.
• Ignoring a marginal battery. Low voltage during cranking resets modules and sets network codes throughout the vehicle.
• Clearing codes before recording which modules stored them, which throws away the most useful clue about where the break is.
• Reading 60 ohms at the diagnostic connector on a gateway-equipped vehicle and concluding the whole network is healthy, when that reading only covers the gateway's own segment.
• Probing bus wires by piercing the insulation, which creates a corrosion entry point and a future intermittent.
• Overlooking aftermarket electronics spliced into the harness, especially remote starters and alarms installed years earlier.
• Treating a history-only U0100 after a known dead battery or jump start as an active fault and chasing it into a teardown.

• ECM
• CAN Bus Wiring

• U0115Lost Communication with ECM/PCM "B"Severity level 4 of 5: Get it looked at now.high
• U0101Lost Communication with TCMSeverity 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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