Where does the E46 store its mileage data?
The BMW E46 has important data stored within the car at two primary locations. The instrument cluster and light switch module. This ensures that data cannot be lost if one device goes bad or if one becomes destroyed in a crash. It is similar to why aircraft have redundant flight controls if one set goes out there is another system in place as a backup. The three types of data stored redundantly in the BMW E46 are: the vehicle identification number (VIN), current mileage and service interval data.
How do I know if my E46 mileage is accurate?
Each time the "accessory switch" is engaged on the ignition the data compares itself to each other. If the computers find conflicting data there will be a manipulation / tamper dot that appears on the gauge cluster next to the trip mileage. If the VIN does not match, the manipulation dot will appear. Also, if the two mileages stored vary greater than 120 miles but the VINs are the same, the cluster will continue to accumulate mileage and a fault will be stored in the cluster.
Once the manipulation / tamper dot has appeared, there is no removing it. You must go to the dealer to order a new cluster and have it programmed. This can be very costly. However, when buying a new BMW E46 this should be something all prospective buyers should look out for. If you see the tamper dot you know either the mileage, VIN or service data is not matching. Reference the image below for what the tamper dot looks like so you know what to look for.
What does the tamper dot look like?
Reference the image below with the arrow pointing to it.
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