P1686 Automatic transmission FI signal B low input
What does P1686 mean on a Honda?
P1686. Automatic transmission FI signal B low input. This Honda-enhanced code applies to 1999 to 2003 TL, CL and Accord V6 with automatic.
Common causes of P1686
- Short to ground on signal B wire
- Damaged harness near the transmission
- Corroded TCM plug
- Failed signal B driver
- Weak charging voltage
How to fix P1686
- Check charging voltage first
- Inspect harness near hot exhaust parts
- Test signal B wire for shorts
- Clean the TCM connector pins
- Replace the failed module if wire tests good
Sources and review
- Last updated 2026-09-24.