A lockup malfunction is not very obvious on a automatic transmission.

Do you know what the lockup is? It engages wien youre driving at a steady hyway speed, for example, and locks the torque converter, so you dont loose energy by using the fluid coupling of the converter.

it is supposed to make you achieve better fuel economy. When it is engaged, you can feel that the engine is more directly attached to the wheels, like on a manual transmission. Because the lockup disengages when you accelerates or decelerates, it is not very easy to know if it works.

If fuel economy did not change, maybe you can use it as is.