U0100
Lost Communication With ECM/PCM AU0100 is a generic OBD-II network diagnostic trouble code: Lost Communication With ECM/PCM A. It is logged by the engine control unit when the network monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What U0100 means
U0100 is an SAE generic network communication fault stored by any module on the vehicle's Controller Area Network (CAN) bus when it cannot receive expected messages from the Engine Control Module or Powertrain Control Module (ECM/PCM). The CAN bus is the backbone network that allows all electronic control units — ABS, transmission control module, body control module, instrument cluster, and others — to share real-time data. When that link to the ECM/PCM is interrupted or absent, the detecting module logs U0100 to signal that critical powertrain data is missing.
The most common root causes are electrical rather than module failures: a blown fuse supplying the ECM, a corroded or broken ground connection, or damage to the two-wire CAN bus (CAN-High and CAN-Low) between modules. Physical connector damage, water intrusion, and chafed wiring in the engine harness are frequent contributors. Because the CAN bus uses a pair of twisted wires running throughout the vehicle, a single open or short can silence the ECM from the perspective of every other module simultaneously, making the fault appear widespread.
U0100 can be stored as a historical (memory) code even after the underlying fault has cleared, which is why the check engine light and associated symptoms may not be present at the time of diagnosis. The severity ranges from a momentary communication dropout — often caused by a loose connector — to a complete no-start condition where the ECM is genuinely offline. Always read and document all stored codes before clearing, as companion codes from other modules frequently point directly to the affected circuit.
Common causes
Most-frequently reported root causes when U0100 is logged.
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1
Loss of power supply to the ECM/PCM (blown fuse or relay)
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2
Poor or broken ECM/PCM ground connection
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3
Open circuit on the CAN-High or CAN-Low bus wire
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4
Corroded, damaged, or disconnected ECM/PCM wiring harness connector
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5
Short circuit on the CAN bus (wiring chafed against chassis ground)
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6
Water intrusion or corrosion inside the ECM connector or housing
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7
Failed or failing ECM/PCM (less common, after electrical causes are ruled out)
Symptoms drivers notice
How to diagnose U0100
A typical diagnostic flow when this code is present.
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1
Connect a scan tool and record all stored DTCs across all modules before clearing anything — companion codes often identify the exact circuit at fault
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2
Check whether the code is active or historical: attempt to communicate with the ECM/PCM directly; if the scan tool cannot connect, the ECM is likely unpowered or offline
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3
Inspect all ECM/PCM fuses and relays in both the under-hood and interior fuse boxes; replace any that are blown and retest
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4
Inspect ECM/PCM ground straps and chassis ground points for corrosion, looseness, or breakage; clean and torque to spec
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5
Perform a visual inspection of the ECM harness connector and the CAN bus wiring for damage, melted insulation, corrosion, or backed-out terminals; repair as needed
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6
With the ignition on (engine off), measure resistance between the CAN-High and CAN-Low pins at the OBD-II datalink connector (DLC) — a healthy network reads approximately 60 ohms; a reading far outside this range indicates a wiring or termination fault
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7
Only after all wiring, power, and ground checks pass and the fault persists, consult vehicle-specific service data and consider ECM/PCM testing or replacement by a qualified technician
Related network codes
- U0101 — Lost Communication with TCM
- U0102 — Lost Communication with Transfer Case Control Module
- U0103 — Lost Communication With Gear Shift Module
- U0104 — Lost Communication With Cruise Control Module
- U0105 — Lost Communication With Fuel Injector Control Module
- U0106 — Lost Communication With Glow Plug Control Module
Frequently asked questions
Can I drive with a U0100 code?
It depends on whether the code is historical or active. If the ECM/PCM is currently offline, the vehicle may stall, fail to start, or operate in a severely restricted state — driving is not safe. If the code is stored from a past event and the vehicle is running normally, driving to a shop for diagnosis is generally acceptable, but the fault should be addressed promptly because it can reoccur without warning.
Will clearing the code fix U0100?
No. Clearing the code only erases the stored record. If the underlying cause — a bad fuse, ground fault, or damaged wire — is still present, the code will return, often immediately. The root cause must be identified and repaired.
Does U0100 always mean the ECM/PCM needs to be replaced?
Rarely. The vast majority of U0100 cases are caused by electrical supply or wiring issues: a blown fuse, loose ground, or damaged CAN bus wire. An ECM/PCM failure is possible but uncommon and should only be considered after all wiring and power checks have been completed and passed.
Why are there so many other warning lights on at the same time as U0100?
The ECM/PCM continuously broadcasts engine and transmission data that other modules — ABS, stability control, transmission control, instrument cluster — rely on. When the ECM goes silent on the CAN bus, every dependent module detects a loss of communication and may log its own fault code and illuminate its own warning light. This is expected behavior, not multiple independent failures; resolving the ECM communication issue typically clears all the secondary warnings.
Disabling U0100 in software
RaceTune can permanently disable U0100 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
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