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Can I bring parts I bought elsewhere to you and you use them?

Servicing or repairing a car will principally involve parts and labour. When assessing a car for its need for any work to be carried out, German Motors will also define the required parts for the job. These parts would have to be in the right quantity (think spark plugs, or amount of lubricant) and the right quality.
Offering a complete package, German Motors’ work will include parts, which will be sourced in accordance with the requirements of the car’s brand. Checking online, it is well possible that one could find parts that are sold cheaper than what German Motors could offer. The idea of buying parts from other vendors and then bringing them to German Motors for their use during a service or repair has been brought forward before.
However, as a professional workshop, we would refuse to use such parts sourced by clients, which are typically sold by sources unknown to us. We do this for the benefit of our clients, for their protection and convenience. There are good reasons why we discourage this practice.
Firstly, not having sourced the parts ourselves, we cannot be sure that it is the correct part. With tens of thousands of parts in a car, there could be a chance that the part, sold by a third party, could be wrong. There may not even be any malicious intent on the side of the vendor, but a simple mistake in referencing the part may result in an incorrect part to be shipped.
Sometimes parts look extremely similar but may not be the right one. If a wrong part would be used, it could get spoiled as it may not fit properly. Using a wrong part may also damage other components of the vehicle. Besides liability issues, arising from a workshop having installed the part, such situation would surely inconvenience you, the car’s owner.
Considering that the source of the part could be questionable, a part may even be stolen or acquired in illegal ways.
Remanufacturing, the refurbishment of used parts to give them a second life, is a good way to protect resources and to offer parts at a competitive price. However, if these are sourced from unknown suppliers, the same applies as does with new parts: the quality is questionable. As remanufactured parts are by definition used, their remaining lifespan is a question mark. Installing remanufactured parts may be a short-term, cost-effective fix, however, German Motors would reject them if they are brought in as too much risk is attached.
Oftentimes, the joy over having saved a few RM will be overshadowed by the trouble one has when a cheap part fails. It is such a situation when the investment in genuine parts, sourced by German Motors would pay dividends as these would come with warranties and guarantees. Parts purchased outside may be cheaper, but there is oftentimes a good reason for it as the vendors may not be offering any support beyond free shipping (if that).
In conclusion, having German Motors offering a bundle of parts and labour offers a number of benefits and safeguards against unintended damages.