Carbon Monoxide in cabin
 

Any suggestions appreciated. I use a sensitive CO monitor in my Cessna 172XP. The monitor registers levels up to 47 ppm during climb and while stationary, but zero readings during cruise and on descent. I presume that during climb and startup exhaust finds its way into the cabin. What would be the best way of detecting where this leak may be?

I'm not familiar with the XP but generally:
Is cabin heat selected? If so, you might consider getting the exhaust muff pressure tested (double-skinned silencer that acts as a heat exchanger). Other than that, carry out a close visual inspection of the whole exhaust assembly including the heater shut-off valve and flexible SCAT hoses.

The heater shroud and scat tubing were my first thoughts though I also use the heater at cruise for extended periods with 0 reading. Is it possible that eddying air flow past the exhaust outlet could draw exhaust fumes around the upper partsof the fuselage, possibly eventhe baggage door towards the rear?

Peter: I would think it unlikely that exhaust fumes are being drawn in by the airflow as this possibility would surely have shown up during initial flight testing (wool tufts etc).
My money's on the muff passing fumes through a pinhole with the engine working hard on T/O & climb or accumulating in the engine compartment at idle because there's no dissipating flow through the cowling.
You might find this link interesting regarding domestic CO levels. I'd imagine continuous zero readings would be preferable as far as flying is concerned but aeromedical aspects are outside my field.
Perhaps you should ask your AMO when the muff pressure test is next due. It used to be on the Annual Inspection but I don't know how it stands nowadays.

Thanks Steve. I will follow that up. Cheers from Oz.

Another possibility is have you got a fresh air vent on the side of the fuselage that is open?