What it means
Voltage on the kick-down switch line is below 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 circuit carries the signal that tells the transmission control module a full-throttle downshift is requested. P278C sets when the circuit voltage is below the expected range, commonly because the signal wire is grounded, the switch is stuck closed, or corrosion adds resistance at a connector.
Common causes
Most likely first.
- Switch wire shorted to ground
- Switch stuck closed
- Connector pins corroded - high contact resistance
Symptoms
- Check engine or transmission warning light may be on
- Kick-down response may be unavailable
- Delayed downshift during hard acceleration
How to diagnose it
- Save freeze-frame data and check the kick-down switch state and circuit voltage on a scan tool to confirm the low signal condition.
- Inspect the switch near the accelerator pedal and its connector for damage, moisture, corrosion, or a switch stuck in the activated position.
- Check the harness for chafing or a wire pinched against ground, paying attention to areas that move with pedal operation.
- Measure the circuit at the switch and then toward the transmission control module using the manufacturer's wiring diagram and specification to locate where voltage drops.
- If the circuit remains low with the switch disconnected, isolate the short to ground or high-resistance connection before replacing parts.
Common fixes
- Repair a kick-down signal wire shorted to ground.
- Clean or replace corroded connector terminals and restore a sound connection.
- Repair a chafed or pinched section of harness.
- Replace the kick-down switch if it remains closed when it should be open.
Can I keep driving?
The vehicle may still drive, but full-throttle downshift response can be unavailable or unpredictable. Avoid hard acceleration and passing maneuvers that depend on a prompt downshift until the low circuit signal is repaired.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.