94 Honda Civic HB 1.5 High NOx

The brownish spark plugs and pinging are a clear indication of a "lean" fuel condition. You also mentioned HC is not failing but close to the max limit. We're going to assume CO is very low, probably 0.00 percent. This indicates to us there is definitely a lack of fuel and more than needed air entered the combustion chambers...  unmetered air. We highly recommend diagnosing your Honda Civic's fuel feed back system.

You want to check for any problem which could cause a lean fuel condition... less fuel being delivered to the engine combustion chambers then the amount needed for efficient combustion. This includes ensuring the oxygen sensor are working properly, the computer system is receiving the signal from the oxygen sensor, 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.

The feed back test will utilize a 5-gas analyzer (usually a smog machine) and propane. The smog technician should introduce propane to the intake system and ensure the oxygen sensor is reading this introduction as an increase in CO, and ordering the computer to decrease fuel delivery at the instant propane is added to the system.

This test should be done after the engine has sufficiently warmed up and while it is running at idle. If no change in fuel delivery is seen we know we have a feedback problem. Using a voltmeter the technician should observe the voltage output of the oxygen sensor as propane is added. Voltage should increase. If there is no increase we have a defective O2 sensor. The sensor should be replaced and the test started over. If voltage increase is present at the sensor output the technician needs to ensure voltage increase is also present at the computer input terminal. If voltage is present at the terminal as well next the technician must ensure the computer output terminal to the fuel injectors are lowering “injector pulse rates”. At the end of the feedback test it will be know whether the low CO fault is electronic or mechanical, and the technician can further diagnose the fault.