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On a 2005 Citroen C5 auto petrol it would act up in a very strange way:

Starting the car and doing several(2-5) kilometers (extra-urban) it would forcefully change to 2nd gear with a good kick and not go up or down, automatically or manually. The only thing that would 'fix' the problem is fully stopping and restarting the engine, after that it may not do it for another 2-3km or it may do it after 15km.

The issue intensified after changing the gear-box oil, before that it'd almost never do it, or if it'd do it a full stop - restart would fix it for the remainder of the trip.

Driving urban it'd not do it now or before.

Does the above described mean new gearbox ?

(no OBD codes)

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  • It is strange that there are no fault codes (if that is meant by 'no OBD codes note'), you also don't say whether any service light goes on. I am not very familiar with Citroens, but I heard, that they have "oil mileage counter" for transmission and they can also detect oil health in transmission. When you had changed the oil, shouldn't you also reset some kind of counter? This could be also just a glitch in transmission control circuits and maybe some cannibalization from other transmission would solve this problem so no new transmission would be needed.
    – honza-kasik
    Jun 3 '17 at 19:39
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    If the transmission is going into limp mode it should be storing codes that wouldn't necessarily show up in OBD2 Mode 2 or 3. Though I'd imagine a MIL request code should be stored. You may want to have a shop get a reading on the TCM.
    – Ben
    Jun 4 '17 at 2:02
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+50

I would try and use a more sophisticated scan tool, and one that specifically can look at the input and output shaft speed sensors. While I have little knowledge of Citroen (other than odd steering wheels and a shared hydraulic system) -- and even less about the C5...

The symptoms you describe suggest an intermittent connection or failure of a speed sensor, which panics the transmission control and forces a limp mode. This may or may not set a code, and even if it does the code may be a 4-digit OEM specific code that the average scan tool cannot see. I would start by monitoring "VSS" and if you have the gauges, pump pressure.

A failure induced while driving will give you a host of information, if you can borrow or use a full capability scan tool.

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    Another reason can be wrong oil amount. This is critical, one ounce less or too much and you are busted.
    – Janka
    Jun 8 '17 at 3:11
  • @Janka I'm sorry, I don't beleive this one bit. Do you have any references or citations that indicate this? 2 to 5 liters of ATF are not going to be measured to within +/- one ounce. That much escapes through the breather over 30k kilometers. Theres probably 10 times that amount that varies as residue in the torque converter and servos.I call horsefeathers . . .
    – SteveRacer
    Jun 8 '17 at 22:23
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    I've worked as an engineer on transmission control units. First, it's only about 2 litres for a small transmission suitable for a C5. Second, e.g. AW has a "no breather, no refill" policy because the oil type and amount is so sensitive. Third, the oil amount itself isn't measured but oil pressure at various places inside the transmission. When you are as little as one ounce short or too much, these pressures are wrong and because the hydraulic brakes and clutches inside won't work smoothly with the wrong pressure, the control unit forces emergency mode. Sorry if you don't believe me.
    – Janka
    Jun 9 '17 at 5:45
  • @Janka There's no need to apologize; being wrong is not a basis for a religion. Do explain how oil "amount" changes pressure in various locations? Is the pickup right at the very meniscus of the pan reserve? How does this vehicle corner and not lose intake pressure?? Is the pump psychic and moody when the oil amount isn't exact? How do the servos compensate for material wear, as the travel (and volume required) would change over time. I'll eat all the crow you want when I see a citation other than your speciuos claims... Owner's manual? Shop manual? How do you drain the converter??
    – SteveRacer
    Jun 10 '17 at 3:08
  • I'm sorry but I obviously couldn't take AW's papers with me when I quit. The same, sorry me for not forseeing this question of you 14 years later. Also sorry only reciting what my japanese colleagues told me about how to test the TCU program (because that's what I've done together with VW engineers.) Sorry again.
    – Janka
    Jun 10 '17 at 8:33

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