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post #8 of (permalink) Old 13th January 2007
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It's far from ignorance. I didn't know what a UCH was until I read about it on here.

I'm not sure what it stands for (it's probably something French) but it's a second computer, in addition to the ECU . The UCH controls a lot of the car's non-engine related functions that would previously have been managed by good old-fashioned switches and relays. Things like the central locking, electric windows and even the indicators are all controlled by the UCH . In fact, I imagine it's the UCH rather than the ECU that controls the airbags - it's probably that which will need replacing if it can't be reset.

To confuse matters further, a lot of dealers and mechanics like to refer to the UCH as the ECU . My guess is that they don't want to have to explain what a UCH is and assume that we'll have heard of an ECU . In its strictest sense, it is an "Electronic Control Unit" but the term ECU is generally used to refer to the computer that controls the engine. Unfortunately, this kind of 'dumbing down' can be a bit misleading at times.

Supposedly as an anti-theft measure, the UCH , ECU and keys are supplied by Renault with blank 'write once' memories. When a new computer is fitted, its memory it programmed to match the serial number programmed into the other components. The ECU is designed not to allow starting of the engine unless the serial numbers in the UCH and key match its own.

As cars have become harder to steal, the crooks have got cleverer. While 'hooking' of keys is the favourite way of stealing them these days, it's not unknown for the crooks to turn up with a spare ECU and a set of keys with the matching transponder. They'll still have to force the steering lock, but they will be able to start the car. The idea is that, by linking all the computers together, this method of theft is made that much more difficult and time consuming.

Though it could all be a blatant profiteering exercise to make sure that you have to buy a new part because a perfectly good second-hand one can't be used. The jury's out as far as I'm concerned.
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