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Some folks buy ECU units from us with a sense of certainty: their part is bad, and they need to replace it, period.
But, some of our customers buy a used ECU to diagnose problems with the engine not starting or running right, or at all. Given how central the ECU is to the functioning of the ignition and fuel injection, it’s a good candidate for suspicion. It also is relatively easy to remove and replace, so it’s relatively painless to install another probably-good ECU unit to help with the trouble-shooting process, by process of elimination. If, after installing the replacement ECU, the problems go away, the cause was probably the old ECU.
Internally, the ECU consists of some complex soldered circuit boards. As I understand these things, failure can be total or partial. As to the latter, it’s not as if the entire unit will catastrophically fail completely or not at all. It might well be that only one tiny electronic component is bad, and the rest of the circuitry still works well enough. Often, over the decades, the soldered joints eventually become iffy and a likely point of failure.
Some difficult-to-intimidate folks remove the ECU, remove its cover and re-solder the relevant places. This is often a relatively simple process of re-heating the bad connection just enough for the solder to melt and harden, thus re-making the connection.
As a shot in the dark specific to troubleshooting, buying a used ECU might be a good financial idea. Let’s imagine you don’t take this approach. Your $100-per-hour repair shop runs diagnostics, and an hour later you might hear “the ECU is bad.” So, you’re out the $100 that you could have saved, and you have to buy the ECU anyway. Plus, there is all the hassle of taking the car to the repair shop, waiting for it or getting a ride from someone, etc.
When you buy a used ECU to troubleshoot a problem by isolation, the odds are not 100% that this will pinpoint the problem, but they’re a fairly good bet. Worst case you end up being sent a bad ECU and then you have two problems, which makes things harder yet to troubleshoot.
More likely, you end up with a good, used ECU and the problem either fixed, or diagnosed one step further.
If the problem wasn’t the old ECU, then you have a spare ECU unit that you can use whenever next there is a problem with the engine not starting or running right, or at all.
When anyone, BMW included, makes a product, it tries to balance the quality-related aspects so as to arrive at some sort of intended-to-be optimal mix. In the case of the ECU, these aspects include fuel economy, clean air, low-end torque, high-end power, good driveability in various conditions and so on. If enough customers prefer a different mix, such as with the emphasis more on power, there are aftermarket tuners (Dinan and Turner are two, as I recall) who sell chips that plug into the circuit board of ECU and make it behave differently.