[Suterman]

It's worth bearing in mind that prolonged running rich may cause premature wear on the engine as the excess fuel can wash the oil off the cylinder wall (bore wash) but only in very rich conditions (I Think)

Glen, if you are interested I will tell you what I know about lh2.4

All this is from memory so read up to be certain!

The computers in the car control the amount of fuel injected and at what time and when the sparks fire.

They rely on different sensors to tell them whats going on, like the Air mass meter, the crank position sensor and oxygen meter.

The computers are programmed with a chart or "base map" to tell the ballpark figures for how much fuel (F) they need to inject relative to the amount of air going into the engine (A), revs and throttle position to achieve the best air/fuel ratio (AFR) the ideal AFR for a petrol engine is 14.7(A) to 1(F) or 14.7:1 Don't forget that these cars have to work all over the worlds at extremes of altitude and temperature (air density) and the maps have to accommodate this, the boffins at Volvo don't want the car to break down at all if possible so they make the "base maps" run a richer rather than a leaner mixture (because it's safer) just to be on the safe side.

The o2 sensor tells the ECU how much o2 is in the exhaust mixture, this information expressed as a voltage to the ECU, more oxygen means the mixture is lean so the ECU will add more fuel and less oxygen means the mixture is too rich and so the ECU will lean out the mixture and fine tuning the "base map" to suit exactly the requirements of the engine at any time.

When everything is working fine the system is is closed loop mode (normal), meaning that it's reading the maps and adjusting the mixture according to the reading from the o2 sensor and other sensors hundreds of times a second.

When the Lambda light is on it means the car is using open loop or "base map" fueling settings, so in other words there is a fault that has caused the ECU to switch to open loop mode (mostly known as limp home mode)

So lambda light faults are fueling related.

Hope this helps, its only the basics, sorry if I'm rambling.

This is why I suggest that the cat may be bad,