P278D: Kick Down Switch Circuit High

What it means

Voltage on the kick-down switch line is above the expected window. Wiring faults of this kind typically come from chafing, water ingress at connector seals, or pin push-back — a wiggle test on the harness while the code is active often confirms the location.

At a glance

• Usually lights the check-engine light

How it works

The kick-down switch tells the transmission controller when the accelerator is pressed far enough to request a lower gear for stronger acceleration. The controller monitors the switch signal and sets P278D when the line reads higher than its expected range, which can happen with an open switch or a short to battery voltage.

Common causes

Most likely first.

1. Open switch contacts
2. Switch wire shorted to B+
3. Connector pins corroded - high contact resistance

Symptoms

• Check engine or transmission warning light may be on
• Kick-down response may be unavailable or register unexpectedly
• Unintended or delayed downshifts

How to diagnose it

1. Use a scan tool to record freeze-frame data and watch the kick-down switch status while operating the pedal as directed by the vehicle’s service information.
2. Inspect the kick-down switch, its connector, and the harness for chafing, moisture, backed-out pins, or corrosion that could raise the signal.
3. With the circuit active, gently move the harness and connector while watching the signal to see whether movement changes the fault.
4. Check the switch contacts and signal wire for an open, poor connection, or short to battery voltage using the manufacturer's test procedure.
5. If the wiring and switch pass, check the transmission controller connector and input circuit according to the manufacturer's specification.

Common fixes

• Reseat the kick-down switch connector and secure the harness.
• Clean or repair corroded or pushed-back connector terminals.
• Repair a chafed signal wire or a short to battery voltage.
• Replace the kick-down switch if its contacts remain open or out of range.
• Repair the controller input circuit if testing confirms an internal fault.

Can I keep driving?

If shifting is otherwise normal, drive gently and avoid hard acceleration until the fault is repaired, since kick-down may be unavailable or trigger an unexpected downshift. Stop driving if shifts become unpredictable or the transmission warning indicates a serious fault.