U0100: Lost Communication With ECM/PCM "A"
Other modules in the car have stopped hearing messages from the engine computer over the vehicle network.
What U0100 means
Modern cars connect their computers with a shared network, usually CAN bus, which runs on a twisted pair of wires. Each module sends regular messages and listens for others. U0100 means one or more modules, such as the transmission, ABS or instrument cluster computer, have stopped receiving messages from the engine computer. The engine computer may be dead, unpowered or disconnected from the network, or the wiring between modules may be damaged. KBB notes the car may still run, but with issues like lower fuel economy. In other cases the engine stalls or will not start, and the transmission defaults to a single gear. Low battery voltage is one of the most common triggers, because modules can drop out during cranking. Shops start by testing the battery and checking fuses, power feeds and grounds at the engine computer. Then they measure the CAN bus wires, which should show about 60 ohms between them with the battery disconnected, and check each module connector for corrosion.
Common symptoms
- Check engine light and several warning lights on
- Engine may not start or may stall
- Transmission stuck in one gear
- Gauges dropping to zero or acting erratic
- Poor fuel economy
Most common causes
- Weak battery or low system voltage
- Blown fuse or failed relay powering the engine computer
- Bad ground at the engine computer
- Damaged or corroded CAN bus wiring or connector
- Failed engine computer
- Water damage to the computer or connectors
What to check yourself first
Have the battery and alternator tested and clean the battery terminals. Check the fuses that feed the engine computer. Look for signs of water or corrosion near the engine computer connectors.
Typical fixes and what they cost
| Fix | Typical cost | Note |
|---|---|---|
| Replace battery | $456-$493 | A weak battery or low voltage is a very common cause of communication codes. |
| Network (CAN bus) electrical diagnosis | $122-$179 | Network faults are usually found by checking power, ground and the two CAN bus wires at each module. |
| Replace engine control module (ECM/PCM) | $1,122-$1,192 | Only after power, ground and network wiring check out. |
| Replace alternator | $809-$1,088 | If low charging voltage is causing modules to drop off the network. |
Typical US price ranges for parts and labor, not quotes. Your shop should confirm the cause with a diagnostic before replacing parts.
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- Hyundai Tucson ABS Module Fire Recall: Which Years Should Park Outside, and What the Free Fix Is
Related codes
- U0101 Lost Communication With TCM
- U0121 Lost Communication With Anti-Lock Brake System (ABS) Control Module
- U0140 Lost Communication With Body Control Module
- U0073 Control Module Communication Bus "A" Off
- P0562 System Voltage Low