P0816: Downshift Switch Circuit

What it means

The TCM detects an electrical fault on the manual downshift request switch circuit.

At a glance

• Usually lights the check-engine light

How it works

The downshift switch sends a manual downshift request from a paddle or selector control to the transmission control module (TCM). The TCM monitors the downshift switch circuit and sets P0816 when it detects an electrical fault in that request path. The transmission may ignore manual downshift requests while other shifting functions continue to work.

Common causes

Most likely first.

1. Switch contacts failed
2. Wiring open

Symptoms

• Manual downshift request may not be recognized
• Check-engine light may be on

How to diagnose it

1. Record freeze-frame data and scan for transmission codes, then check live data to see whether the TCM recognizes a downshift request.
2. Operate the downshift paddle or selector switch while stationary and note whether the request appears consistently in live data.
3. Inspect the downshift switch, steering-wheel or selector connector, and accessible wiring for looseness, wear, or corrosion.
4. Test switch continuity and the downshift circuit for opens or shorts using the manufacturer's procedure.
5. If the switch and wiring pass, inspect the TCM connector and input circuit before considering a module fault.

Common fixes

• Repair an open or damaged downshift switch wire.
• Clean or repair a loose or corroded downshift switch connector.
• Replace the downshift switch if it fails the manufacturer's continuity or function test.
• Repair a TCM input fault only after confirming the switch and circuit are sound.

Can I keep driving?

If automatic shifting works normally, you can usually drive cautiously while avoiding situations that depend on manual downshifts. Stop if the transmission shifts unpredictably, loses drive, or becomes stuck in a gear.