2003 Holden Astra Engine Management light

The car engine management light came on. So I took it to my local Holden agent and they said it was the oxygen sensor, cost $500. It lasted 20ks and the light came back on, I took it back and was informed it was a cracked exhaust manifold, cost $480. It lasted till the next morning, when I started the car up the light stayed on. They have now changed the flow sensor, cost $540. But admitted this will not fix the problem, as I probably will need the rear sensor replacing but it might be the ECU. If not then they might need to replace the Computer. Because of the ongoing cost they have said I should consider whether to spend any more on the car, so I still have the problem and $1500 out of pocket, and by the way they then said it will not pass rego as it is a inspection report problem. Surely in these days of computerisation, they should be able to diagnose a fault rather than just start at the front and replace till the fault is found That was the way you repaired Ford Anglia's in 1950! This saga has now been going on for four months. They have had the car sometimes for a week at a time. I have asked whether the parts that have not fixed the fault could be removed and replaced but they said there could have been a combination of faults and they had to change each individual part to be sure it was not that component that was causing the problem. THOUGHTS PLEASE