Vintagecf

My cooling fan is randomly coming on after I turn off the car. The car could be quiet for 20 minutes then the cooling fan will kick back on. It will stay on until it runs the battery all the way down. I changed the ECT sensor but it did not help. I am also getting a check engine light message. The code I am getting is P0418
Secondary Air Injection System Relay "A" Circuit Malfunction. Any idea what could be causing both issues?

2003 a4 1.8T

MikTip

under your ECU?

There are several relays under your ECU, one which is the Secondary Air Injection System Relay.

Gain access to your ECU, and have a look under it.

diagnosticator1

There is water inside the electronics compartment. The cover is damaged or not installed properly, allowing water to leak into, and accumulate inside the box. The water causes the fan relay to short out, turning the fan on. This is also the cause for the secondary air circuit fault. The secondary air pump relay is being shorted out by water. You need to remove the cover, and remove all of the water inside, replace the relays, and be sure you find and fix the cause of the water leak into the box.

Vintagecf

Thanks to both of you for your responses. I checked under the ECU and there was indeed water in there. I am replacing the realys. Hopefully, all will be well. Thanks again.

diagnosticator1

Be sure to find and fix the water leak also. Thanks for the update!