What Does P2780 Mean?
The TCM is detecting an abnormally low voltage on the downshift switch circuit. This indicates a short to ground condition or complete circuit failure preventing proper downshift switch operation.
Common Causes
35%
Wiring harness short to ground or damaged insulation
30%
Downshift switch internal short circuit failure
20%
Damaged or corroded connector causing ground contact
15%
Transmission control module internal circuit fault
Diagnostic Steps
1
Step 1: Disconnect downshift switch and measure voltage at TCM side of harness - should read reference voltage if circuit is intact
2
Step 2: Check resistance from signal wire to ground with switch disconnected - should read infinite resistance (open circuit)
3
Step 3: Inspect wiring harness for pinched, cut, or abraded wires along entire length from switch to TCM
4
Step 4: Test downshift switch with ohmmeter in all positions to verify no internal short to ground exists
5
Step 5: If wiring and switch test good, check TCM supply voltage and ground circuits for proper operation
Estimated Repair Cost
$100 - $650
Parts + labor, varies by vehicle and location
The TCM is detecting an abnormally low voltage on the downshift switch circuit. This indicates a short to ground condition or complete circuit failure preventing proper downshift switch operation.
The most common cause of P2780 (Downshift Switch Circuit Low) is: Wiring harness short to ground or damaged insulation
Typical repair costs for P2780 range from $100 to $650, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Safe to operate vehicle though manual downshift control will not function. Transmission continues automatic operation normally.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P2780 to identify the root cause.
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Quick Info
Category
Powertrain
System
Transmission Control System
Difficulty
Type
Generic (SAE)
Recommended Tools
OBD2 Scanner
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