My 4.6 petrol gems has just failed it's MOT. Main issue - Exhaust carbon monoxide content after 2nd fast idle exceeds default limits (8.2.1.2 (b)) The exhaust is in poor condition and been patched and bodged all over but apparently not leaking. Car not driven for a year. Where should I start? Cheap fixes first please.

Check the lambda sensors are actually working correctly would be the first step. You need to look at the output with diagnostics ideally to check that, but that should give some clue as to whats going on. Basic service may help if its been stood a while as well (oil and air filters, maybe treat it to a new set of plugs while your there if needed).

Is it on gas or petrol? And if its on gas, what fuel type has it been set to on the tester?

Many thanks, plugs seems to be most popular answer, I'll have the lambda sensors checked. Lots of people mention exhaust (I have a second hand stainless one which ultimately needs putting on). It does have LPG but tank is empty and never used. Better on gas or not?

Much, much better. When it is taken for test it is tested on the fuel it is presented for test on (although you'll probably have to tell the tester or he'll just test it assuming it is on petrol). Limits on LPG are CO 3.5%, HC 1200pmm, on petrol they are CO 0.2%, HC 200ppm and lambda between 0.97 and 1.03. Lambda isn't checked on LPG.

In saying that, it still won't be running perfectly on LPG if the petrol system is out (by how far? What actual readings did it show?), so might be worth checking it over anyway. High CO meas it is running rich so could point towards a dodgy lambda sensor while high HC is unburnt hydrocarbons so excess means incomplete combustion so points towards plugs and/or HT leads.

The time I had that fail was a leaky exhaust - maybe waggle some toilet paper near the joins of the exhaust and see if you have some kind of leak (you asked for cheap !!). Nano to see if your O2 sensors are still alive and then check the rest eg IACV/TPS - all free, if you have a nano...plugs - inspect them and replace if they look fouled (and then you have a bigger problem), bottle of redex in the fuel for the fuel injectors. What speed are you idling at?
The time I had that kind of fail a replacement rear exhaust section cleared things up

Agreed with the above advice.

I haven't been keeping up with this forum lately, occasionally checked in but as I don't own a RangeRover and there haven't been many relevant threads for me to join in on (LPG / engine fuelling / emissions / etc) I haven't had much to contribute.

I don't remember if Holland-and-Holland has diagnostics gear but at least if he doesn't, on Gems lambda signal wires are easily accessible on top of the petrol ECU next to the battery, only need a multimeter to check lambda readings on Gems.

If plugs / ignition are OK and lambda readings are OK, other things that can cause high CO / HC readings are individual cylinder mixtures. Lambda readings represent close to average mixture readings over 4 cylinders but if some cylinders are running richer/leaner than others then high HC and/or CO can result. Things that can cause that are worn injectors and/or valve gear, the former effecting petrol flow, the latter effecting airflow. I have seen many P38s run more efficiently when running on LPG than when running on petrol (e.g. lambda readings correct in both cases but the engine draw more manifold vacuum when running on LPG, which implies the engine needs less air when running on LPG, which points to an increase in efficiency at least for idle conditions) - I put this down to petrol injectors being old and worn, LPG injectors (on a new install) being new and equally balanced in terms of flow.

Different but closely related subject - On another forum (for a vehicle I've LPG converted hundreds of examples of) I've advised many owners that they don't need cats to be intact if they are converted to LPG. These vehicles are prone to failed cats, they have 4 cats (2 at each side of the V6), front cats fail then the debris clogs the entrance to the honeycomb structure of rear cats, preventing exhaust gasses escaping, causing back pressure, back pressure blows some of the (ceramic) cat dust back into engine cylinders where it acts as a grinding paste and ruins the engine. Many owners have had the front 2 cats, rear 2 cats or all 4 cats decored (4 cats only in case of LPG converted), mostly on my advice. Recently an owner who had decored all 4 cats took his vehicle for MOT but didn't tell the tester it was running on LPG, so the MOT tester failed it on emissions. This was a bit of a sticky moment for me as it could have ended with me being slated on this forum but I had faith it would work out OK in the end.. which it did. The vehicle owner then took the vehicle for MOT at another testing station, told the new tester it was running on LPG, it easily passed the emissions test. It could not have passed the MOT running on petrol with all 4 cats decored.

Other problems with V8 left standing is oil seep on valve stems and sticking hydraulic lifters either can do emissions ----- I do hope it just oxygen sensors though