4EFE Oxygen Sensor

[CowboyX]

Hi, I was checking the diagnostics and realised I got a 2-1 code (main O2 sensor faulty, if I got it correctly). I've tried the resetting procedure but the code still shows. The check engine light does not light when normal driving though. Where is the main O2 sensor in the 4EFE? I sent the car to the mechanic and he told me the previous owner had probably changed the exchaust pipes with another one without the O2 sensor connector. He suggested either fixing (e.g. drilling) or fitting the original pipe that comes with the O2 connector. Is the main O2 sensor as indicated by the error code at the engine or at the pipes? How can I determine? BTW, the mechanic had lifted the car and showed me where the spot of the O2 sensor supposed to be. A connector is currently dangling unconnected near the spot he mentioned. Any suggestions? BTW, my fuel mileage is kinda lousy too. I'm gettign an average 9.5L/100km. 80% city driving. Thanks.

[bonnie*]

In all toyota corollas, regarless of engine type 7AFE or 4AFE, it uses two oxygen sensors. One is main O2 sensor and other one is sub-heated O2 sensor.

The main oxygen sensor located after exhaust manifold and before catalytic converter. You can open the hood and try to locate exhaust manifold, if you can locate main oxygen sensor mounted either directly on exhaust manifold or on a small tail pipe attached to exahust manifold before catalytic convertor. The ECU uses this feedback from main oxygen sensor to maintain proper air-to-fuel ratio. The main O2 sensor voltage should vary fairly rapidly between about 200mV and 800mV. Faulty or missing main oxygen sensor will definitely trigger "CHECK ENGINE" light.

While, sub-heated oxygen sensor is located after catalytic convertor on an exhaust pipe.
This sub-oxygen sensor is slower in response than main one and does not contribute to air-to-fuel ratio. However, ECU uses this feedback for proper functioning of catalytic converter to maintain NOx contents in exhaust gases. This sub-oxygen sensor should hold fairly steady around 500mV~700mV. Probably, missing sensor will not trigger "CHECK ENGINE" light, but, may fail your emmission / pollution test. If you can locate the unconnected electrical connector under the passenger seat besides the driver seat, definitely sub-heated oxygen sensor after catalytic convertor is missing.

If so, Do-It-Yourself, repair is easy.

You can jack up the car firmly on jack-stands, not on scissors jack supplied along with car. Lubricate the bolts properly by WD40. Remove the exhaust pipe after catalytic convertor. Buy sub-heated oxygen sensor after catalytic convertor from the part store if you can get SS nut that can fit oxygen sensor is well and good. Go to any mechanic work-shop with exhaust pipe, oxygen sensor and nut ask them to drill a hole approximately 2 to 3 inch from connection end, and tap it. Otherwise they can drill a hole and a weld a nut securely without any lekage.

Hope this is too much in detail ans may serve your purpose.

[Bitter]

not true at all, my 93 has only one 2 wire sensor, no sensor after the cat. this is perfectly normal for non OBD2 cars.

[CowboyX]

Thanks Bonnie and Bitter. Will have a look and try to locate the main sensor. My mechanic did spot an unconnected connector under the car along the pipe. He did mentioned that an O2 sensor is not there as supposed and may need to drill to insert the O2 sensor. Probably that could be the sub-sensor.

[bonnie*]

At present, without sub-heated oxygen sensor your ECU operates under fail safe mode.
When you make a provision and install this sensor precisely. Run a car for a day.
and then just reset ECU by removing battery negative termial for 2 to 3 minutes. It may reset your clock or radio settings as well, but, ECU will regain it's normal mode of operation.

This may improve you efficiency a bit-little.

WE ALWAYS TRY TO SOLVE THE PROBLEM RATHER THAN CREATE A NEW ONE

[ghost_ryder35]

Originally Posted by Bitter

not true at all, my 93 has only one 2 wire sensor, no sensor after the cat. this is perfectly normal for non OBD2 cars.

correct, most obd 1 or 0 cars only have one 02 sensor. besides, two sensors just means one more sensor to replace

[CowboyX]

Been trying to seach the main O2 at the exhaust manifold. My exhaust manifold comes with heat shield. Using a pic posted http://www.toyotanation.com/forum/t172418.html as reference. Q #1, can I just unscrew the heat shield screws to access the O2 sensor which I presume is behind it. Q #2, so far I do not see any wires or O2 sensor connector/s coming from the exhaust manifold as in the pic. So how do I fix my main O2 sensor back, if there's one? So far managed to identify the location of the sub-O2 sensor at my exhaust pipe. Probably may go with Bonnie's suggestion to drill and insert the sub-O2 there. I'm still puzzle why the check engine light don't light but the error code (2-1) persist when doing diagnostics.

[bonnie*]

You may try to locate main oxygen sensor before CATALYTIC CONVERTER & after EXHAUST MANIFOLS on exhaust pipe connecting CAT & Exhaust Manifold. IF you cant' find one.

Then remove Exhaust Manifold Shield after lubricating all bolts (soaked properly) with WD40. And try to locate same near bottom end of exhaust manifold.

Using a pic posted http://www.toyotanation.com/forum/t172418.html, The golden/brass element on immediate left of radiator cap and at bottom of radiator is main oxygen sensor mounter short flange connected to exhaust manifold by two bolts. You do not have to remove flange to remove main oxygen sensor. You may remove this sensor directly either from top by oxygen sensor slotted socket and long handed rachet, or from bottom using 7/8 or 22mm open end line wrench.

Normally Code 21 - Oxygen Sensor signal is generated, if there is any problem in this main oxygen sensor circuit. Your main oxygen sensor circuit seems OK so check engine light is not lit. But code sti 21 still persist coz it's recorded in ECU MEMORY, it seems previous owner when he worked on exhaust pipe maintenance might have replaced main oxygen sensor, but did not reset the memory resident code.

When you install sub-heated oxygen sensor and make sure both oxygen sesors are OK. You can reset memory resident code by removing negative terminal of battery for 2 to 3 minutes. and re-connect it.

NOW IT'S ENOUGH, WE WANNA SEE YOU ONCE YOU FIX SUB-HEATED OXYGEN SENSOR & RESET MEMORY RESIDENT CODE.

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