I just took my 92 Civic LX Sedan (auto tranny, stock motor/tranny) to get smogged and it failed the 25mph dyno test becaose of high NO (PPM) reading. The max is 798, average is 187, and mine was 990. Every other test passed. I just took the car into the local Honda dealer for an oil change and an inspection. They told me the car was fine and didn't need anything changed, minus some brake work and some suspension work. Any ideas on what could be causing the high reading? Here are my results

PCV - Pass
Catalytic Converter - Pass
Wiring to sensors - Pass
Fuel cap visual test - pass
Fuel cap functional test - pass
Fuel EVAP Test - pass
Spark Controls - Pass
Vacuum Lines to sensors/switches - Pass
Ignition Timing: B15 - Pass
Fuel Evaporative Controls - Pass
Oxygen Sensor - Pass
Carb./Fuel Injection - Pass
Other emission related components - Pass
System Malfunction Light - Pass
Liquid Fuel Leaks - Pass

Thanks in advance for any help.


Test
15mph
25mph

RPM
1983
2618

%C02
14.60
14.50

%02
0.11
0.37

HC (PPM)
72 MAX; 35 AVE; 67 MEAS
44 MAX; 22 AVE; 23 MEAS

C0 (%)
0.59 MAX; 0.12 AVE; 0.20 MEAS
0.59 MAX; 0.10 AVE; 0.03 MEAS

NO (PPM)
731 MAX; 216 AVE; 458 MEAS
798 MAX; 187 AVE; 990 MEAS

 

Does the car have an EGR? Those are there to reduce NOx.

 

Also, in general, high NOx is caused by the engine running too hot or too lean. Check the cooling system and the fuel pressure (fuel filter and pressure regulator).