J-Rod074

Hello, this is my first post and I hope to get a little help with my emission problem...

I have a 1999 RS I bought from a bumb who pulled the "Check Engine Light".
Unfortuately, I waited to long before getting a sticker. This is when I diagnosed the bulb was removed causing the emissions to fail (System not ready).. I waited to long to figure this out and have to deal with it myself.

I have an OBD II scan tool which is pulling a code (P0135 o2 sensor heater ciruit malfunction)..

My question is: Where is the "o2 sensor heater circuit" and how do I test / replace it ? :?:

CODE4

1999 Mitsubishi Eclipse RS

Most new oxygen sensors are 4-wire design. Two for the voltage signal that fluctuates with gas content. The other two wires are the heater. The ECU feeds a decent amount of current thru the sensor to heat it up to operating temperature quickly rather than rely on the exhaust gases to do so. A hot sensor reads more accurately quicker than a cold one.

If the prevous tuner did a resitor/capacitor mod on the signal circuit but left the heater circuit open, the ECU will throw the code you are referring to. That circuit either needs to be repaired, or you need a new sensor, or the wiring is hacked and needs to be repaired.

Fyrehydrant

While this ^^^ was an awesome reply, i will add a few things.

P0135 is the upstream o2 sensor heater circuit malfunction. The information posted above is right on, but i highly doubt anyone would place a resistor/capacitor in the upstream o2 sensor harness. Usually that is done on the rear(downstream) o2 sensor, to rid the CEL after someone replaces the cat with a test pipe.

Most likely, your front o2 sensor took a shit. Buy a new one, install it, and clear the CEL.