Whats cause of high CO over 25 mph?

Question

California smog. High CO over 25 mph. No check engine light. NO codes on OBDII
Changed spark plugs,  wires, pvc value, belt, oil change.

Answer 1

If the vehicle is being tested on the dyno, at 15 mph the dyno  pressurize the engine system, this is where is the was secondary ignition leak it it show up, the transmission system itself is presurized this would test the shift solenoid, at 25 mph the dyno now heatrs up the engine, this is where fuel the fuel control is tested ny means of temperature and maintaining 210 F , What causes a high CO at 25 mph you would want to check the operation of the PCV valve and the airfilter