The P0740 code in your 2020 Infiniti QX60 indicates a malfunction with the torque converter clutch (TCC) solenoid circuit. The TCC solenoid controls when the torque converter locks up, creating a direct mechanical connection between the engine and transmission. This improves fuel economy and reduces heat during cruising.
Torque Converter Clutch Function
At highway speeds, the TCC locks the torque converter to eliminate the efficiency loss from fluid coupling. Your QX60's CVT (continuously variable transmission) uses TCC lockup to optimize efficiency. When the TCC solenoid fails, lockup may not occur properly, affecting fuel economy and transmission temperature.
Common Causes
The TCC solenoid itself may have failed electrically or mechanically. Wiring issues between the TCM and solenoid affect control signals. Valve body problems preventing proper TCC apply. Contaminated transmission fluid affecting solenoid operation. Poor electrical connections at the transmission connector.
Symptoms in Your QX60
Reduced fuel economy at highway speeds when TCC doesn't lock. Engine may feel like it's revving higher than expected during cruising. Transmission may run warmer due to continued converter slip. Shuddering or vibration if TCC is engaging improperly. Check engine light illumination with P0740.
Diagnostic Steps
Using Nissan CONSULT, monitor TCC status during highway driving. The TCC should engage above certain speeds and under specific load conditions. Check for solenoid activation and response. Test solenoid resistance against specifications. Verify wiring to the TCC solenoid. Check transmission fluid level and condition.