Co Too Low Or Too High

Check induction system. Check for air leaks downstream of throttle valve, using pressure tester. Pressure should be 5.8-11.6 psi (.4-.8 kg/cm² ). Visually inspect between mass airflow meter and throttle housing.
Ensure fuel pump is operating. Check fuel pressure and pressure regulator. Check for leaks or restrictions.
Ensure mass airflow meter filament is not broken and burn-off function is working.
Check coolant temperature sensor operation.
Ensure electrical harness, connectors and wires are not broken or loose. Check all input signals to ECU.
On 900 Series, check EGR valve for proper operation.