U0100Lost Communication With ECM/PCM “A”
Caution
What it means
Modern cars run on a network — the CAN bus — where control modules constantly talk to each other. U0100 is stored by another module, such as the transmission, ABS, or body computer, saying it stopped hearing from the engine computer. The problem may be wiring, power to the ECM, or the ECM itself: it's a network fault, not an engine fault as such.
Is it safe to drive?
Caution
Unpredictable is the honest word. Some cars store U0100 from a momentary glitch and drive normally; others stall, refuse to start, or drop major systems without warning while driving. If the code is current and the car shows symptoms — stalling, dead gauges, clusters of warning lights — don't trust it on a highway until it's diagnosed. If it's an old stored code and everything works, have it looked at without panic.
Common causes
Most likely first.
- Battery or charging problems — low system voltage garbles the network; check this first
- A blown fuse or a bad ground feeding the ECM/PCM
- Damaged CAN bus wiring — rodent damage and connector corrosion are the classics
- A failing ECM/PCM
- Aftermarket electronics tapped into the bus and interfering
What the live data shows
The pattern of communication is the diagnostic: polling every module shows which ones answer and which don't, and U0100 appearing in several modules at once points at the bus or the ECM's power supply rather than any one sensor. The control-module voltage PID is worth reading too — the whole network misbehaves when system voltage sags. And if the engine module itself won't answer the scanner at all, that's the finding.
A scanner that reads all 139 standard sensors shows you these readings live. ScanMyCar charts them as you drive, keeps the freeze-frame stored with the code, and pulls codes from every module — free, with any cheap Bluetooth adapter.
Typical fixes
Cheapest first, where honesty allows.
- Load-test the battery and clean and tighten its terminals and the main grounds — the cheap cause
- Check the ECM's fuses
- Inspect accessible CAN wiring for chafing and rodent damage; unplug recently added aftermarket electronics
- From there, professional network diagnosis — condemning an ECM is a last step, not a first one
A guide, not a diagnosis. This page describes what U0100 generally means across OBD-II cars. What's actually wrong with your car depends on its live data and its history — and any repair decision deserves a proper diagnosis, whether that's you with a scanner or a professional with a lift.