My car was misfiring and i took it to R.W.Autos who seemed to know what my car and said it was probably a faulty coil. They said they put it on their computer and diagnosed the problem which was a catalytic converter and a sensor. They said i would need 2 new converters and a set of sensors, and i should get these. New they cost about £3,000, so get reconditioned. The labour cost would be about half days work, 3-4 hours.
Relatives advised caution and i took my car to my usual garage which diagnosed on their computer, whilst i watched, a faulty coil, changed it while i waited in about 15 minutes, and charged me £40. The car drives perfectly again. The mechanic here also said that if the catalytic converters and sensors had been changed, it would have made no difference to the misfiring.