Hyundai Repair: Poor Fuel economy and loss of power, poor fuel economy, hyundai santa fe

Question
Hi there,
My 2003 beloved Hyundai Santa fe is causing me a few problems which is extremely costly at the fuel pump!  She is the 2.7l V6 but won't go over 60 mph on the motorway and certainly there is no pick up when I 'hit the gas'!  I have noticed that the £55.00 it takes to fill her up (which usually lasts approximately 250 miles is now only lasting 100 miles!)  I have done a bit of research online which looks like it could be the oxygen sensor which needs replacing...?  Is this expensive and can I damage the engine in the meantime by driving it?  And is this a common problem with a 3 year old car?
Any help gratefully received,
Sarah

Answer
Although the 2.7 has a fair amount of oxygen sensor issues and oxygen sensor is a possibility, but that wouldn't be my prime suspect if your mileage is low *and* your vehicle is running poorly.  Oxygen sensors tend to do little more than turn on a check engine lamp and sometimes cause increased fuel consumption.

I would typically expect a problem with a misfire.  In the U.S., these would typically trigger a check engine lamp.  But I'm not sure if you have the same or a similar fuel management diagnostic system in Britain.  If your check engine lamp is on, you should absolutely start there.  Check the trouble codes and investigate the cause.  If you have a misfire, you should begin by checking the secondary ignition system (plugs, plug wires, coil).