Audi A4 Engine Fan Diagnostic

In recent years we have been working tirelessly on many Audi not limited to A1, A2, A3, A4, A6, A8 and RS. Same as with any other makes and models, if the car has never been in with us before, be it just got purchased privately with little to nil service history, just came out of manufacturer warranty or simply due for an annual service, the optimal course of action would be to carry out a ‘Service A’ which comes with a comprehensive condition report as well a free e-Safety check for registration renewal. Due to the miss alignment of service schedule, a vehicle may not be due for a service or may not need a service but instead only due for an e-safety check. Such an example of this is an Audi A4 engine fan diagnostic issue we were able to solve.

We had an Audi A4 B5 came in only for an e-safety check for registration renewal one morning. The car was in good working condition and it passed the official 15+ points safety check required by the Services NSW without a problem. It also just had a basic oil service from another workshop 3 weeks prior to coming to Autoactive Mechanic Northshore.

So why do we bring this up?

It was, on the day of the e-safety inspection, our technician spotted the high speed engine fan was ‘on’ constantly. Note that the operation of engine fan is NOT a requirement for an e-safety check.

You may start wondering it’s only a fan and it passed the e-safety check so what is the concern? Let me explain. The engine fan is made to run on high speed but only when coolant temperature is high, for example: sitting in traffic with air conditioning on, or when the engine management senses a fault in the system it may turn the fan on ‘high’ as a fail safe measure, only for short periods of time.

Similar to a laptop fan it prevents the device overheating. Therefore when a fan is running continuously, it is not a healthy sign and should be diagnosed as such immediately. The owner was advised of the symptoms and was recommended that an Audi A4 engine fan diagnostic should be carried out. The owner however declined. The owner stated that the fan had been running like that for a month and the car drives…just fine, no need for an Audi car service.

Fast forward 2 weeks later, the same Audi A4 came in for overheating. Our record showed that this Audi A4 came in a couple of weeks ago and the fan was working on high speed constantly. We did notice the fan  and not surprisingly it no longer operated when it returned; not a good sign.

Diagnostics for Audi A4 Engine Fan

We were asked to diagnose the fault. Started with scanning the vehicle and 1 fault code was found in the engine control unit. The fault found in the Audi A4 engine fan diagnostic was for a Map Thermostat Fault. Tests on the Map cooling thermostat were also carried out and electronic command side was found to be ok. That gave us an indication that Map Thermostat had failed. We advised the customer there could be 2 faults: First the thermostat could be faulty and secondly the fan which was no longer operating could be a fault as well and the systematic approach was to rectify one problem at a time.

After replacement of the Map Thermostat and coolant, we found that the engine fans did not operate correctly. The small fan would operate on low speed but the large fan did not operate at all, even with the air conditioning on. We contacted the owner and advised him of more diagnostic time to diagnose the problem would be required.

We actuated the fan through our computer scanner and found the command had been sent but fan refused to operate. We started our elimination procedure by further tests on the fuses and relays. Both components were confirmed in fact operational. We then studied the wiring diagram which contains further information including Engine Control Unit (ECU) commands on high or low speeds to the Fan Control Unit located in the engine bay. The Fan Control Unit then controls and supplies the Pulse Width Modulation to the fans depending on the command of the speed required.

On further diagnosis we found that the fan control unit had failed along with the large fan motor. Our hypothesis: The Engine Control Unit (ECU) logs a fault for Map Thermostat. Within the ECU’s programming the ECU recognise faults with the thermostat associated with engine temperature, therefore as a fail safe precaution the ECU commands the fan to work on high speed to lower the temperature. High speed means more current the Fan motors need to draw, more current means more heat.

What We Found With The Audi A4 Engine Fan Diagnostic

Our conclusion was that the Fan Control Unit failed due to the constant operation of the HIGH speed command from the ECU caused by the failed thermostat. The owner was advised of our findings from our Audi A4 engine diagnostic and gave us a go ahead for the repairs. The Fan control unit which is only available from Audi needed to be replaced but the Fan motor we were able to source from Valeo, the manufacturer of the original component, at a reduced price to pass on to our customer.

The fact that the high speed fan had been operating for a month when the car last came in for e-safety check, no one, not even the most experienced mechanic, would know how much damage that would had been done to the system within such prolonged period of time but at least the fan was working, not as bad as having an overheated engine.

This ordeal would had been avoided if the Audi A4 engine fan diagnostic were carried out at the time when we first noticed the problem. The owner would of saved hundreds on the extra diagnostics time for the diagnostic of a non-operating fan, the replacement cost of the components, the Fan Motor itself and the Fan Control Unit, had he took our advice in the first place.

Here at Autoactive Mechanic Northshore, we advise our customers of our most honest findings and we work with you for the repair proposal that best suits your personal circumstances and budgets.