What readings can I expect from a diesel exhaust?

The exhaust gas readings from diesel exhausts very widely depending on the type of engine management system.

A five gas analyser will accurately measure the following gases: Oxygen, Carbon Dioxide, Carbon Monoxide and Oxides of Nitrogen (NOx).

It will also measure Hydrocarbons (HC) but because the analyser's response is optimised for hexane, the HC readings from a diesel's exhaust will be inaccurate (very low maybe only 5% or so of true HC reading) and should be discounted.

Note: To create representative levels of NOx, the engine must be under load.

Examples (not necessarily typical, just random measurements):

Normally aspirated diesel engine at idle:

Tuned Mis-Tuned
O2 1.5% 0.3%
CO2 13.5% 10.2%
CO 0.5% 6.0%

Turbo diesel engine at idle:

O2 14.9%
CO2 5.1%
CO 0.02%