High NO on your Toyota SUV can be caused either due to a lean fuel condition
(this is when the engine is not getting enough fuel to cool the combustion
chamber down), or due to a bad or defective catalytic converter.
Your 2000 Toyota 4Runner should be equipped with a Non-EGR equipped 3.4
liter engine. Once all other emissions components are inspected, and either
fixed or found to be functioning properly, you may suspect a damaged CAT
(Catalytic Converter). Typically NO is reduced on most vehicle's by the EGR
system or EGR valve. On your Toyota 4Runner however the reduction of NOx or
NO is the done by the CAT and proper fuel control; proper fuel control being
the first.
So as far a fuel control and Air/Fuel mixture, a smog check repair station
will first have to ensure your 2000 Toyota 4Runner vehicle is in proper fuel
control. This includes ensuring the oxygen sensors are working properly, the
computer system is receiving the signals from the oxygen sensors, the
computer is computing the data properly and sending the correct signals to
the fuel injectors to either increase or decrease fuel delivery to the
combustion chambers. At the end of the feedback test it will be know whether
there is a fuel control problem causing high NO, and whether it is
electronic or mechanical. The technician can further diagnose the fault.
