The P0816 code in your 2020 Mazda MX-5 Miata indicates a malfunction in the downshift switch circuit. On automatic-equipped Miatas, this affects the paddle shifter or manual mode downshift control.
Miata Downshift Control
The MX-5 Miata with automatic transmission includes paddle shifters or a manual mode for driver-controlled downshifts. The downshift switch sends a signal to the TCM requesting a lower gear for engine braking or performance driving.
Common Causes of P0816
- Faulty downshift paddle switch
- Wiring damage in steering column
- Clock spring connection issues
- TCM input circuit problem
- Corroded electrical connections
- Water intrusion (convertible exposure)
Symptoms of Downshift Switch Failure
- Downshift paddle not responding
- No manual downshift capability
- Check engine light on
- Sport mode affected
- Upshift paddle may still work
- Automatic downshifts unaffected
Convertible Considerations
The Miata's convertible design can expose interior electronics to moisture. Water intrusion into the steering column or paddle connections can cause electrical faults like P0816.