I have a 1991 Civic Dx that recently failed the smog test in San Francisco. Here are the readings:

Low speed emission results (15mph)
-------------Max Allowed-------Measured
HC(PPM)------- 130------------21
CO (%)---------0.80-----------0.08
CO2 (%)------------------------15.2
O2 (%)--------------------------0.0
NOx (%)--------835-----------934

High speed emission results (25mph)
-------------Max Allowed-------Measured
HC(PPM)------- 105------------18
CO (%)---------0.69-----------0.09
CO2 (%)------------------------15.2
O2 (%)--------------------------0.0
NOx (%)--------774-----------750

This was a retest, I had failed it just a couple weeks before, with almost identical results, both times the low speed NOx is too high (1108ppm the first time)

I read a similar thread that suggested a new Cat.

https://honda-tech.com/zerothread/1794590

However in my case, my O2 numbers are zero, and my HC numbers are low, suggesting that the Cat is working fine, right? The low HC would suggest a LEAN mixture, right? and this would cause elevated combustion temperature and thus high NOx, right?

So whats my problem?

I do NOT have an EGR valve, so this easy fix is out.

Ive visually checked my vacuum lines for leaks or cracks.

Im sure the timing is fine, as the car does not idle rough or ping or knock at all.

The temperature seems to be fine, the gauge is always in normal range.

Im thinking of replacing the Oxygen sensor, reasoning that it might be causing too lean of a mixture if its not working right. Would the low HC and CO readings mean that the Oxygen sensor IS working correctly?

I dont really know where to go now, as there doesnt seem to be an "obvious" answer beyond an EGR valve, which this car does not have. Im zeroed in on the oxygen sensor, but not sure about this....

 

How many miles since the current CAT (not those that goes "meow" when they need to be fed.) was changed? If it has been a while then get a new CAT, it will make a huge difference.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by dude_123 &raquo;</TD></TR><TR><TD CLASS="quote">How many miles since the current CAT. If it has been a while then get a new CAT...</TD></TR></TABLE>

Yeah, new catalytic and a complete tuneup will do wonders!

 

Is nice that you did some homework on this one, what else causes NOx, elevated combustion temps, LEAN, TIMING, RADIATOR (coolant) TEMP, high compression too. What cleans up NOx, good cooling system, proper timing (SCAN IT, TIMING MARK ON PULLEY MAY BE OFF slipped), proper fuel control (DO YOU HAVE O2 ACTIVITY, SCOPE IT OR SCAN IT), Cat Convertor, it must be LIT OFF, that means it must be very hot, it must burn HC & CO if the engine is producing too much, but you dont know this without a pre cat smog test but you can do a cold engine smog test and watch the changes as the engine (and cat) warms up. Do not get a cheap cat if you have to replace it, it looks like you need another one. CAT TEST, the outlet of a hot cat should be hotter then the inlet.

A cheap cat may get you through this time but in two more years when you have to smop again it may just well fail again.

A little more theory, the cat needs O2 to burn the HC and CO, that is why the O2 sensor makes the engine rich and lean so a little fresh O2 can go back to the cat to burn HC and CO, no O2 out the back, it seems to be doing its job but the NOx cleaning stuff in the cat may be bad, again a pre cat smog check would be warrented.

Cali BAR says if you have to spend over $500 to pass they will pick up the rest or give you a waiver. Ask about it.

I know a little about smog but only enough to choke on. Did the shop use a fan in front of your car, did you drive the hell out of it to heat up the cat just before testing it.

 

A 15 year old O2 sesnor = replacement