Using the Lambda calculator: http://www.smogsite.com/calculators.html

Here's your air/fuel ratios:

15 mph AND 25 mph: 15.54 Air/Fuel ratio, Lambda 1.057

"Lambda too lean 1.02 and above - reduction less effective (Nox high from CAT)
Lambda > 1.02 = Reduction (cleaning up NOx) Less efficient"


For the lean condition, test the O2 sensor response time, Coolant and air temp sensor readings are correct, the MAP/Baro and the mass airflow sensor are within calibration. Also verify that you don't have any misfires, which will put more O2 into the exhaust, also.

For High NOX: Verify EGR operation and flow, the ignition timing is correct, NO exhaust leaks upstream of the cat-converter, and that it is not overheating before condeming the cat-converter.