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Its perfectly acceptable to use a second hand ECU, they are generally very reliable and as you say a new one is not cheap so fitting a used part is the most realistic option.With fitting a used ECU, its vital that it comes from the same model, similar age and same engine size, you will need to check the part numbers on your ECU and make sure the new part matches up. No mapping is necessary however it will need to be programmed to the immobiliser system and keys. Not all garages can do this so you will need to enquire as to whether they have the kit to program your keys. The main dealer will but generally charge a bit more.The ECU its self is very easy to fit, should take a mechanic no more than an hour to remove, replace program and test. It's located under the bonnet on the far back right hand side, its a silver box with two large wiring connectors.If you wanted to avoid having the new ECU Programmed, many people sell the ECU with the immobliser control unit, ignition barrel and key, if you replace all three parts of another car it will simply plug and play and no programming will be needed as the new key is already matched to the ECU. It will involve a little more labor to fit the parts.
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