Auto Shift Manual Mode Circuit Intermittent
Diagnostic trouble code P0959 indicates that the powertrain control module (PCM) or transmission control module (TCM) has detected an intermittent electrical issue in the Auto Shift Manual (ASM) mode circuit. This circuit is responsible for allowing the driver to manually select gears in an automatic transmission, and an intermittent signal suggests a loose connection, damaged wire, or failing switch.
While the vehicle can typically still be driven in standard automatic mode, the manual shifting feature may be disabled or behave erratically. It should be diagnosed soon to prevent further electrical issues or unexpected transmission behavior.
Common symptoms
- Illuminated Check Engine Light
- Inability to use the manual shift mode (paddle shifters or shifter +/-)
- Transmission defaults to standard automatic mode
- Erratic or delayed shifting when manual mode is engaged
- Gear indicator on the dashboard flashes or displays incorrectly
Likely causes
- Loose, corroded, or damaged wiring and connectors in the Auto Shift Manual circuit
- Faulty upshift/downshift switch (shifter assembly or paddle shifters)
- Spilled liquid or debris inside the center console affecting the shifter switches
- Faulty transmission control module (TCM)
- Faulty powertrain control module (PCM) (rare)
What to check before replacing parts
Scan for all stored diagnostic trouble codes and record freeze frame data.
Look for: Check if other transmission or electrical codes are present, which could indicate a broader electrical issue or shared ground fault.
Visually inspect the wiring harness and connectors associated with the shifter assembly and transmission.
Look for: Look for chafed wires, loose pins, corrosion, or signs of liquid intrusion (especially in the center console area) that could cause an intermittent connection.
Test the operation of the manual shift switches (paddle shifters or console shifter) using a scan tool.
Look for: Observe the scan tool data PIDs for the upshift and downshift switches. The state should change consistently when the switches are pressed; intermittent or no change indicates a switch or circuit failure.
Perform a continuity and resistance test on the Auto Shift Manual circuit wiring between the shifter switches and the TCM/PCM.
Look for: Resistance should be within manufacturer specifications. High resistance, an open circuit, or a short to ground/power indicates a wiring fault.
Check the power and ground circuits supplying the shifter assembly and TCM.
Look for: Voltage drops or poor grounds can cause intermittent circuit codes. Ensure steady battery voltage and solid ground connections.
This page explains the generic P0959 definition and is general reference information, not a diagnosis for your specific vehicle. Actual causes, severity, and repairs vary by make, model, engine, and mileage — always confirm with a scan tool and a qualified technician before replacing parts.
Gear guide: Best OBD2 ScannersRead this code yourself next time — from $20 Bluetooth dongles to bidirectional scan tools.Rather have a pro handle it? Find a vetted local mechanic.