On a 2005 MX5 2L runs great only symptom is fault light.

As I understand it it is catalytic converter, downstream O2 sensor or inlet manifold leak.

Would appreciate advice before splashing limited cash

Cheers!

Basically it means the injection system cannot adjust the longterm fuel trim rich enough to correct what appears to the post-cat lambda to be a lean mixture.

Check the manifolds (both inlet and exhaust) have no leaks, check all the hoses on the inlet side for vacuum leaks. If you have an induction kit remove it and put the car back to standard and ensure no leaks in trunking. Check the car is reaching full working temperature ie measured at the cylinder head at least 85c Also check the fuel filter isn't blocked.

If none of the above works suspect the MAF or Lambda sensor or on of the cats but before changing anything you really need to look at the output from the pre-cat lambda sensor to check it is "switching" between about 0.1v and 0.8v at about 2 times per second or faster.

Also look at the output from the post-cat lambda sensor it should change as the engine warms up but the should be fairly constant, if it goes up and down like the pre-cat sensor the cat isn't working.
The function of the post-cat sensor is purely to check if the cat is working. If the cat isn't working it canoften be down to the pre-cat Lambda sensor.


You really need a code reader or dongle that reads live data to look at live OBD data to check the engine is
(1) Running Closed loop
(2) The coolant is reaching full working temperature
(2) The upstream lambda sensor is "switching"
(3) The downstream lambda sensor isn't switching
(4) All he other sensors are producing sensible readings.



[Edited on 24/11/12 by britishtrident]

Ta I'll follow up tomorrow in the garage. Very comprehensive answer thanks

Cheers!