I am in California. I have a 1999 Honda Civic DX 4 cylinder. A couple of days before smogging the car my check engine light pops on. Checked the code and it was P0141, downstream O2 sensor. We replaced the downstream sensor and ran the car. Thirty minutes later the same code P0141 comes back. I cleared the code and drove the car for a day and the light did not come on. Took the car for a smog test and it passed with flying colors. Total HC's at both idle and higher RPM were between 9 and 13. A day later the check engine light came on again and now the code is P0420 for bad cat. The car has 340,000 miles. The cat was replaced before and has somewhere around 120,000K miles. I cleared the codes again to see what the computer spits out next time.


Any suggestions are greatly appreciated.

 

converter or non fouler trick, but since you live in commiefornia, i doubt you can use the non fouler idea.

 

If it passed the sniffer with flying colors and the sensor is new I would inspect your wiring to that sensor. The wiring could be damaged and intermittently giving improper readings.

 

Thanks. Any chance it could be the ECU? Also, how would you go about checking or fixing the wiring?

 

Once the primary O2 sensor started working, the secondary O2 sensor was able to see that the cat was dead.

 

We didn't replace the primary sensor. There was never an issue with it. It is only two years old

 

Edited:

 

Why would the code for the secondary sensor come on again after replacing the sensor? I would think if the cat is bad the readings from the smog test would not have been so clean

 

A bad secondary O2 sensor is only one of several possible causes for P0141.

Post the emission test sheet (after deleting personal information).

 

I will when I get home tonight. The max HC allowed for both idle and upper RPM is 100. The average was I think between 14 and 20. I think my car measured between 9 and 13

 

HC is only one of several different emissions gases measured.

 

I have the smog report figures in PDF format. I can't figure out how to attacfh file here.

 

Convert PDF to jpeg and upload as picture.

 

Ok, here goes

 

The engine seems to running somewhat lean. Unexpectedly for CA, NOx emissions are not shown on the sheet. I suspect your cat is dead, but this is not evident because NOx measurements were not reported.