I have a BMW Z4 2003 2.5 with the exhaust emission

It would appear that there is a crack in the exhaust manifold or a faulty manifold gasket. The fault light may be the ECM warning of a "faulty" O2 sensor which is the one after the cat converter as the ECM reads the differences in voltages between the front sensor ( HO2S) and the O2 sensor. That is how it knows of such things as faulty cat converter, manifold leaks ,manifold leaks etc.
If it is not under warranty find an accredited shop that specialises in you make of car for a second opinion, but if still covered by warranty complain very loudly to the manufacturer about the service dealer.

I have a BMW X5. The engine light is on. I have read that it has to do with emissions? It will come on and stay on for a while and then go back off but then come back on next day or so. Why?

Sounds to me that the O2 sensors are sensing a problem with the emissions, or other emision controls have detected a problem that may not necessarily cause a driveability issue.

If you have emissions inspections in your area of the country, then I would have it repaired before the next inspection.

Have the OBD-II scanner check out the computer codes and descriptions by Autozone or Advance Auto Parts for free, or buy your own scanner for under $100 from Sears or Walmart.

Good luck on the repair.

Het it put on an ODB2 reader, it is likely to be an O2 sensor going wrong or something not too bad.

That sounds like you bought big problems. Oil ights indicate low oil pressure= BIG BUCKS. I would have a qualified repair shop check it out, A shop that specializes in BMW. If you are lucky, maybe the oil sender is just bad. They can also check out the Ck eng light. But the oil light is priority one.

it simply sounds like your oxy sensor(s) are bad, maybe egr system, and or clogged catalytic converter. Yes it will be a little pricey. Sorry.

HAVE IT TESTED FOR TROUBLE CODES THAT IS THE KEY TO FIND THE PROBLEM.

I bought a 2000 polo 1.4 yesterday after having an MOT. The warning lamp came on today but the car is running fine. Whether its the Lambda sensor or just the light itself is a common fault, I think garages turn off the lights to get it through the MOT then they come on again in a day or two!

it will not pass the test with the light on, and if u turn off the light they will know when they test it by testing/checking the system readiness function. Go to a Kragen or an Autozone they will run the test to see what trouble code has turned the light on, it is free, post the code back here. Oh have then reset the light after the test.

this may be your egr valve, does it want to stall when its ticking over.