I've had some problems with my 2008 (08) Volvo V50 D5 Geartronic, which I bought from Johnsons Volvo two years ago. At that point the car had done 35k miles and had had seven Volvo services. It has now done 55k and I am slightly overdue a service, but the car is out of warranty.

A few months ago, an airbag warning appeared on the dashboard and - the local Volvo dealership being very busy and not very close - I took the car to a local independent garage, with whom I have had a great relationship for many years.

The local garage were able to clear the error, but when it occurred a month later read the code as "Resistance too high - rear O/S pretensioner". The pretensioner was replaced with a part sourced from Johnsons Volvo for a total cost of £200.

A month later the error reoccured and could not be cleared, so I took the car to Volvo for a "while you wait" diagnostic. After an hour, I was told that they would need the car in over Christmas, and in due course they had it for a few days. Unfortunately, despite being given a detailed rundown of the history, when I collected the car all Volvo had done was cleared the code and by the time I had driven 12 miles, the warning light was back on.

I returned to Volvo for the third time, and after two days was told that the pretensioner error was a red herring and the airbag control unit was faulty, at a cost of £656.12 (after taking into account the £45 diagnostic charge paid earlier). I paid and drove over the road to pick up some lunch (approx 200 yards), only to find the airbag warning was back on.

Volvo has now had my car for another week, and today I have had a call to say that they found that a section of wire under the back seat had been "replaced", and it was this that has caused all the issues. Volvo are suggesting that my local garage did this (as it's approx 12" from the pretensioner), but they are saying they didn't (and I believe them - why would they replace a bit of wire a foot from the indicated problem?). If the local garage did damage the wire, what caused the original warning? If it was not my garage who damaged the wire, then the damage was present when I bought the car from Johnsons Volvo.

Apparently the original control unit is now "unresponsive", and they are suggesting it has been burnt out by the dodgy wiring.

I feel that the damage to the airbag control unit could have been avoided had Volvo properly diagnosed the issue during the initial diagnosis or the subsequent two bookings. Each time I collected the "fixed" car from Volvo there was - for at least some time - no error, so the control unit was obviously working fine until the most recent diagnosis.

Volvo have also apparently only done "a better job" on the wire (their words) by replacing the dodgy part and soldering it properly. They have warned that it might not last, but I'm not happy with that as it is our family car and we have a three-month-old son.

What is my best course of action moving forwards? Am I being unreasonable to expect at least some goodwill on the cost of the control unit?