Flyback diodes may be required with some VVT/idle solenoids to give them better control resolution.
All MaxxECU outputs have built-in flyback protection and external diodes are ONLY required in special cases with some VVT/idle solenoids.
Without diodes, the flyback voltage is limited by the MaxxECU to about 30V. Most PWM solenoids work well with this high flyback voltage.
Some solenoids don't like it and need a diode to limit the flyback voltage. This causes the solenoids to "float" more which gives better control with some solenoids.
•1n5408 - Oversize, but easier to work with than the below smaller one.
•1n4001 - Smaller than the above, but most likely enough.