[Blue Bowtie]

Start by verifying a good ignition system. Even though there may be no misfire codes, ignition could be marginal. The acceptable idle readings and poor loaded readings might indicate a weak ignition. Worn, wide spark gaps can suffer spark blow out and/or poor flame propagation and resultant poor combustion.

Verify the exhaust system, mainly to assure there are no air leaks. Include the EGR in that inspection.

Poor compression and/or leaky valves can cause generally poor combustion and the higher readings.

Of course, a clean inlet air filter is important. Make sure the MAF inlet screen is not contaminated with debris or heavy accumulations of dirt. This can skew the MAF reading and cause a leaner condition (high NOx readings).

Make sure the EGR is functioning.

Test the main O² sensor to see that it can make a minimum of 0.95V with a hot bench test (propane flame). Perform the same test of the rear O². A weak front sensor can skew the mixture rich and result in higher HC and CO readings.