Fault code P1740 – a step-by-step guide to diagnosing and fixing it

Error code P1740 is displayed and the check engine light is on - what does this mean?

The DTC P1740 is recognized by the OBD/EOBD system when there is a problem with the torque converter lock-up clutch circuit. This manufacturer-specific fault code mainly affects the automatic transmission and the transmission control unit, although the exact definition may vary depending on the vehicle manufacturer.

What causes can this fault code have?

• Defective torque converter lock-up solenoid
• Damaged wiring to the transmission control unit
• Loose or corroded plug connections on the transmission
• Low transmission oil level or contaminated ATF
• Defective transmission control unit (TCM)
• Mechanical defect in the torque converter
• Clogged transmission oil filter
• Faulty transmission oil pressure sensors

What symptoms occur in parallel with error code P1740?

• Poor acceleration: The vehicle accelerates sluggishly or jerkily
• Increased fuel consumption: The engine operates inefficiently due to a lack of lock-up
• Unsteady idling: The engine runs out of round, especially when stationary
• Transmission shifting problems: Delayed or hard gear changes
• Overheating: The transmission becomes unusually hot
• Check engine light: The warning light is permanently illuminated

How can the problem with error code P1740 be rectified?

• The repair requires a systematic diagnosis, as several components may be affected.
• Check the transmission oil level and top up with the correct ATF specification if necessary, also checking for contamination or discoloration
• Check and clean electrical connections on the transmission control unit and torque converter solenoid for corrosion, loose contacts or damage
• Test the torque converter lock-up solenoid with a multimeter for correct resistance values and replace if there are any deviations
• Change the transmission oil filter and ATF completely if the oil is contaminated or the change intervals have been exceeded
• Read out the transmission control unit with an OBD scanner and have the TCM replaced if there are repeated faults
• Have the torque converter mechanically checked and completely replaced if there are internal defects