rebuiltSI

Well Ive had the CEL on for almost 2 months now reading Catalytic Converter below threshold (or close to that, btw its on a 99 Si). Well I decided to change the Secondary o2 sensor to see if that worked and before I did that I wanted to make sure I could get the old on off to replace it. When I took the old one off I cleaned a massive amount of carbon build up around all the little holes. So anyways autozone didnt have the o2 sensor in stock so I replace the original and unplugged the battery cable just to see if the CEL would come back on and sure enough after 20+ miles it hasnt. So my question is should I leave the original o2 sensor in? Also my mpg is really bad. I've got new spark plugs, K&N filter, and an alignment. Im getting about 220 miles to a tank.

thesmogman

Your secondary O2 sensors job is to give info on the cat. It is not for air/fuel corrections.

If you have carbon on the secondary O2 sensor I can imagine what the cat looks like along with the first O2 sensor.

Try cleaning off the first O2 sensor and see if you MPG gets better. It is probably stuck at a certain voltage which the ECU see's and think's it is OK but it is not.

There are tests that you can do to check them. I prefer using a scope and a propane bottle to richen up the mixture and watch the scope for a response.

Also a side note. When your CEL came on your car went to "Limp mode" and the ECU WILL richen up the air/fuel mixture to protect the motor and emissions equipment. You should not drive with the cel on. It's design is to get you home and not leave you stranded.

rebuiltSI

Okay well I just drove about another 8 miles and the CEL came on with no change driving conditions so Im assuming its the same CEL code (the cat problem). Tomorrow I will clean the primary o2 sensor and see if that makes any difference. Thanks for the help.

n1_egsex

just get the universal style o2 sensor and splice the wires, saves you about $100 bucks

rebuiltSI

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JJSpecTuning &raquo;</TD></TR><TR><TD CLASS="quote">just get the universal style o2 sensor and splice the wires, saves you about $100 bucks</TD></TR></TABLE>

I thought about. I already have to Secondary o2 sensor being shipped to the Autozone by my house. Its a Denso and it cost me $65, so its not to much. Should I take the time to take off the cat and see if its clogged with carbon build up?

rebuiltSI

UPDATE: I just took off the Primary o2 Sensor to see if it had carbon build up like the Secondary, but to my surprise it didnt. Basically I would just like to know if I should replace my Secondary o2 Sensor, both o2 Sensors, or both and the catalytic converter. Note P0420 CEL code is showing.

rebuiltSI

anyone?

thesmogman

That is hard to say. I would have to scope out the O2 sensor to tell you.

The reading the ECU is getting from the O2 sensor might be bad, might not. This reading would be either the cat or the o2 sensor.

Like I said it is hard to say without some diag's.

rebuiltSI

Would pictures of my o2 sensors help? I didnt think there was a way to visually inspect an o2 sensor. Anyways the gas mileage is horrible. This last tank only got me 190 miles.

rebuiltSI

Okay guys I could really use some help. I replaced the Secondary O2 sensors today with a Denso Universal 4 wire o2 sensor. I thought I had it all connected right, but then CEL code read P0141 (malfunction exists in the heater circuit for oxygen sensor 2 bank 1). Okay so I thought maybe the wires were connected properly. So I reconnected the wires (heater-heater,ground-ground,signal-signal), I crimped them together then used heat shrink. I plugged the o2 sensor back in and the CEL came on immediately. I got the codes read and here was I got this time. P0500 ( Vehicle Speed Sensor), P1298 (Electrical Load Detector Circuit Input High), and P0135 (malfunction exists in the heater circuit for exygen sensor 1 in bank 1)... Obviously it has something to do with my wiring or a connection. HELP