Thanks for sharing that code. Out of curiosity, is that the only fault code that your car is displaying? Are there any dashboard light indicators, e.g., Check Engine", etc? Does the "aftermarket supercharger heat exchanger with separate reservoir" on your car have any temperature sensors or electrical connections on it? Perhaps a call to the manufacturer (or reseller) of that unit can confirm whether the use of it on your vehicle can cause the erratic or occasional fan run on condition you're experiencing, or any unusual temperature gauge readings you're seeing. You also mentioned "thermostat issues" in your post. Do you know if the thermostat has ever been changed? Also, since you got the car used, after an accident, do you know whether the coolant has ever been changed, and whether the appropriate Audi-approved coolant was used (e.g., G12++, G13)?
A search on the web for Fault Code P0481 (Cooling Fan 2 Control Circuit) yielded
this page of info. Here's what it says (and recognize this may not be specific to Audi S4s):
P0481 code definition:
The P0481 code identifies a problem with the control circuit for cooling fan 2. It is often associated with the P0480 code.
What the P0481 code means:
P0481 is the OBD-II generic code for a problem with the control circuit for cooling fan 2. The cooling fan should be commanded on under certain conditions where the engine requires more cooling, and this is controlled via the cooling fan relay. If this does not take place, the Check Engine light will illuminate on the dashboard.
What causes the P0481 code?
The cooling fan 2 control circuit malfunction code may have several causes:
The cooling fan itself is faulty
The cooling fan relay is faulty
There is damage or a short in the harness connecting the cooling fan relay causing a poor connection.
The wiring connecting the cooling fan to the harness has a short or other damage causing a poor connection.
What are the symptoms of the P0481 code?
Other than illuminating the Check Engine light on the dashboard, the P0481 code may cause the engine to overheat as a symptom. This is because the cooling fan, meant to help regulate the coolant temperature, did not function properly when commanded on by the ECM (engine control module).
How does a mechanic diagnose the P0481 code?
P0481 is diagnosed with an OBD-II scanner. A qualified technician will use a scanner that is capable of scanning the factory sensor readings (more advanced than one from the auto parts store) to read the code. The code will then be reset.
The technician will then observe data on the scanner while driving. If the same code comes back, then both cooling fan 2 and the associated relay will be inspected for damage or a short in the wiring. If the code does not come back, then the technician will perform further testing to diagnose the issue.
Common mistakes when diagnosing the P0481 code:
Often when the P0481 code is observed, it is immediately assumed that either the cooling fan itself needs to be replaced or the cooling fan relay is faulty. There are a number of sensors the ECM (engine control module) uses to control the cooling fan relay. These include:
The air intake temperature sensor
The engine coolant temperature sensor
The A/C refrigerant pressure sensor
The vehicle speed sensor
These components should be inspected along with with the cooling fan relay and wiring to ensure that the issue is taken care of without needlessly replacing parts.
How serious is the P0481 code?
The P0481 code will not prevent the vehicle from starting and driving, but all the same it can be a serious issue. In conditions where the engine is experiencing excess heat, serious damage could occur if the cooling system cannot work properly. Overheating in the engine can damage a number of crucial components and lead to very expensive repairs.
What repairs can fix the P0481 code?
The most common potential repairs to address the P0481 code are as follows:
Inspect the electrical connections to the cooling fan and relay to insure they are attached.
Disconnect it, inspect the connections for corrosion or other issues, and then reinstall to insure a fresh electrical connection.
Visually inspect for frayed or broken wiring on the connector or harness. Repair or replace the wiring as necessary.
If the wiring is not the issue, then the cooling fan relay itself needs to be inspected and possibly replaced.
If none of the above steps solve the problem and make the code go away, then the cooling fan itself may be to blame.
The cooling fan may need replacement. In some cases, the cooling fan and the cooling fan relay need to both be replaced.
Additional comments for consideration regarding the P0481 code
When there is a trouble code present that can be set off by a sensor in the drivetrain, there are a number of possible causes. Sometimes small electrical issues or malfunctions can falsely set off codes, so the first step in diagnosis is always to reset the code and see if it activates again when the vehicle is running. Sometimes the code occurs constantly as soon as the engine is turned on.
Simple things like a bad wire or corrosion around the connectors can cause trouble codes to appear. This is why it is crucial to have a qualified technician perform a diagnosis before any parts are replaced.
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