We recommend inspecting your Honda Accord's EGR valve. Spark plugs are responsible for igniting the air/fuel mixture in the engine. Old spark plus could cause fuel to not ignite properly, leaving behind high levels of HC, and some CO, but not normally high NO. High NO is usually caused due to defective EGR system.
EGR (Exhaust Gas Recirculation) valve recirculates burned up exhaust gases back into the combustion chambers. The inhert exhaust keeps the engine cool, lowering NO emissions. NO is produced when combustion chamber tempretures rise above 1200f. Read more below about other high NO causes.
Why did my Honda Accord fail for high NO?
