UK Car Repair Questions? Ask a Mechanic for Answers ASAP
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It sounds like there is a fault detected on the transmission control unit, the triangle will be indicating a problem, this also means a fault code has been stored in the memory. I suspect this is going to be a problem with one of the sensors on the gearbox but unfortunately due to the complexity of modern engine electronics it is impossible for me to give a specific cause. There are many possible causes of this and the only way to find out is to plug the car into a diagnostic scanner which will show a fault code and description of the fault. This will pinpoint the cause of the symptoms. Often it is a simple sensor problem, these Volvo transmissions often suffer from position switch failure, or speed sensor issues.The next step is to get the car scanned, any good garage can do this, it usually starts at around £40, there is no need to take it to a main dealer, however if you choose to it can costs up to £100. The garage will usually tell you exactly what caused the problem and advise on the best remedy. Occasionally faults are a little more complex such as wiring problems or mechanical issues e.g. sticking or faulty valves in the gearbox but based on the symptoms you have, it sounds like the gearbox it's self is working fine, but the car will go into limp mode which locks the gearbox into 3rd gear causing the slipping and delayed selection of gears, it also often goes with a heavy thud into gears. This is normal under limp mode conditions and is not a symptom of the fault.
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