U0100 Code: Lost Communication With ECM/PCM

What happens if you don't fix it

Symptoms you can actually notice

• No start
• Multiple warning lights at once
• Random stalling
• Scan tool cannot reach the ECU

What the computer is actually complaining about

The engine computer talks to the rest of the car over a two wire CAN bus. Every module puts messages on those wires with an identifier, and the others listen. When a module stops hearing the engine computer for a set time, it logs U0100. Power, ground, or the bus wiring can cause it, and the computer itself is the rarest answer.

Most likely causes, cheapest check first

What you need in your hands

• OBD-II scanner with module list

  A $25 Bluetooth reader plus a free app shows which modules answer. If the engine module is missing from the list, you have your confirmation.

• Digital multimeter

  A basic meter runs $15 to $25. It checks battery resting voltage, the ground straps, and the two CAN wires at the port.

• Battery and ground strap Optional

  A battery runs $80 to $200 and a ground strap $10 to $30. Replace the strap first, because a corroded one costs almost nothing.

Test it yourself, step by step

Do these in order. Each step tells you what to do, what to look at, what counts as fine, what counts as broken, and what you will actually see.

1. 1 Turn the key off and leave the car sitting for one hour with the doors closed. Then touch the multimeter probes to the battery posts under the hood, red to positive and black to negative.

   Look at

   The resting voltage number on the display, and whether it holds steady for ten seconds.

   Fine if

   The meter reads 12.6 volts or higher and stays there.

   Broken if

   Below 12.2 volts, or the number keeps falling while you watch. A weak battery drops modules offline and sets U0100 on start.

   What you'll see A steady number like 12.6 on the display. Anything in the low 12s or below means the battery is the first thing to fix.

2. 2 Look at the negative battery cable where it bolts to the inner fender on the driver side, next to the battery tray. Undo the 10mm bolt and lift the strap end off the metal.

   Look at

   The color of the metal under the strap end, and whether the braided cable is green inside the insulation.

   Fine if

   Clean bare metal under the strap, and the cable strands still copper colored.

   Broken if

   White powder, green crust, or black corrosion between the strap and the fender. That raises resistance and starves the modules.

   What you'll see A flat metal tab with a bolt hole. Clean metal shines, corrosion looks like chalk or green fur on the strands.

3. 3 Plug the scanner into the trapezoid port under the dash on the driver side, above your left knee. Key to ON and open the module list, the screen that names every computer the tool can reach.

   Look at

   Which modules answer, and which ones show as no communication.

   Fine if

   Engine, transmission, ABS, and body modules all answer within a few seconds.

   Broken if

   The engine computer is missing from the list while other modules answer. That means the ECU lost power or ground, not that the bus is dead.

   What you'll see A list of five or six module names, each with a green check or a status word. Missing entries show as no response or blank.

4. 4 With the key on, back probe the two middle pins of the OBD port, pins 6 and 14, using a thin pin beside each meter probe. Measure each pin to pin 4, the ground, and then pin 6 to pin 14.

   Look at

   The voltage on pin 6 and pin 14 to ground, and the resistance between the two data pins with the key off.

   Fine if

   Pin 6 reads about 2.5 to 3.0 volts, pin 14 reads about 2.0 to 2.5 volts, and the resistance between them is about 60 ohms.

   Broken if

   One pin reads 0 volts, both read 0 volts, or the resistance between them reads 120 ohms or open. A dead leg means a broken wire or a dead module.

   What you'll see Two voltage numbers close to 2.5 and one resistance near 60. A steady 0 on a leg is the wire you go looking for.

5. 5 Open the hood and find the engine computer, a flat silver box about the size of a paperback, bolted at the back of the engine bay near the firewall on the passenger side. Unclip the two long connectors and look into the pin cavities.

   Look at

   The pins inside each connector for green crust, water stains, or a pushed back pin.

   Fine if

   Pins bright and dry, no water line on the plastic, no green in the cavities.

   Broken if

   Green crust, water staining, or a pin pushed back out of its row. That breaks the bus at the module and mimics a dead computer.

   What you'll see Two wide plastic plugs with rows of thin pins. Clean pins look silver, corrosion looks like green sugar in the cavities.

What the repair costs

Reference range compiled from 2 source type(s), as of 2026-11. This is not a quote — actual cost depends on your vehicle, region, and which part turns out to be the real cause.

Basis: reference · 2 source types cross-checked · Data as of 2026-11 · Confidence: medium

How to confirm it's really fixed

✓ Charge or replace the battery, clean the grounds, then clear the codes and drive 50 miles over three days with several cold starts. Re-read the module list each morning. If the engine computer answers every time and U0100 stays off, the repair held.

Common questions