P0740: OBD-II code P0740 flags a fault in the torque converter clutch (TCC) circuit — the transmission can't lock the converter to eliminate slip at cruise.

Symptoms

• Check engine light is on; the transmission may 'hunt' between locked and unlocked at highway cruise speeds.
• Poor fuel economy because the converter is slipping when it should be fully locked up.
• The vehicle may shudder or judder at 40-60 mph as the TCC clutch tries to engage against a fault.

Common causes

• A failed TCC solenoid inside the transmission valve body — the most common electrical cause of this code.
• Contaminated transmission fluid causing the TCC solenoid to stick or work erratically.
• A worn or damaged torque converter itself, particularly on high-mileage vehicles.

What to do?

Read live TCC command and slip in scan data during a road test. If the solenoid commands lock but slip persists, the TCC solenoid or valve body is likely at fault. Change transmission fluid and filter first (a cheap fix that resolves many cases). Replace the TCC solenoid, valve body, or torque converter as diagnostics indicate.