Have a 2020 honda civic 1.5l no start brake light flashing. U0100 code
Feb 6, 2026 at 9:39 AM
Tcm pcm no communication?
MIKEG530
- MEMBER
- 1 POST
So the first thing to do is check the battery voltage, it needs to be above 12 volts. If not the computers will not turn on, this guide can help you with the test.
https://www.2carpros.com/articles/how-to-use-a-voltmeter
If the voltage is okay, I would next check all fuses with the systems on to help check for power as well.
https://www.2carpros.com/articles/how-to-check-a-car-fuse
U0100 - Lost Communication With ECM/PCM
Under-hood fuse box:
1. IG MAIN
2. FI MAIN
3. PCM/ECM fuse
4. Starter cut relay
I’ve seen cracked 30A FI fuses cause exactly this. Please go over these guides and get back to us.
https://www.2carpros.com/articles/how-to-use-a-voltmeter
If the voltage is okay, I would next check all fuses with the systems on to help check for power as well.
https://www.2carpros.com/articles/how-to-check-a-car-fuse
U0100 - Lost Communication With ECM/PCM
Under-hood fuse box:
1. IG MAIN
2. FI MAIN
3. PCM/ECM fuse
4. Starter cut relay
I’ve seen cracked 30A FI fuses cause exactly this. Please go over these guides and get back to us.
Feb 7, 2026 at 1:20 AM
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