Q: Heater fan comes on when engine is shut off. 2009 Mazda CX 9 AWD

If you are referring to the blower motor in the passenger cabin, a faulty (stuck on) relay is a possible cause but the exact cause would have to be determined in a circuit trace. If the circuit is powered even when it is commanded off, you have to use a voltmeter to determine exactly where power is entering the circuit. You can start with the control switch. If the switch is off, the contacts should be open. If the switch checks out, basically you have to trace back from the blower to see where it is being mistakenly powered. For obvious reasons, it is not recommended to buy any parts until you know exactly what has failed. If you request an electrical circuit diagnostic/repair, the responding certified mechanic will get this diagnosed and resolved for you, right at your location. If you have further questions or concerns, do not hesitate to re-contact YourMechanic because we want you to make the most of your repair dollars and help you to get the best possible results.