O2 Sensor Diagnosys

[swiss]

Hi everyone,

I've been visiting this forum for quite some time now and am compiling some pics/info of troubles and solutions I have had with my vehicle.

I drive a '99 Explorer Sport with a 4.0 SOHC (Vin E) engine. I've had it for three years and to be honest, even though I love my Explorer, it has given me more headaches than I care for

Lately I have been having fuel economy, hesitation in acceleration, rough idle issues. The MIL is on and the trouble codes are:

P0155 O2 Sensor Heater Circuit Malfunction (Bank2, Sensor1)
P0175 Engine Rich (Bank2)
P1152 O2 Sensor fault (Man. Code)

I took it to a smog check station (California) to get the codes. He did a smog test to check the emission results. The hydrocarbon emissions were in the sky (max avg. 34, my test 123)

I replaced the MAF (rich condition usually indicates air intake mixture failure) but the symptoms still persisted and of course the MIL came back on. Just to explain why I changed the MAF, I took it to my friend's house and swapped the MAF from his X to mine and it seemed to run way better, so I changed my MAF.

On a sidenote: the symptoms come and go.

Then I finally decided it was time to order a scantool. I got it today and hooked it up to my car. I monitored the various sensors and found that the O2 sensor Bank2S1 wasn't switching between .2V and .85V at all but rather stayed at .85V while Bank1S1 was continuously switching back and forth. If I remember correctly that indicates a constant rich mixture. Then I saw that the Bank2S2 (the sensor after the CAT on bank 2 was switching slowly as if to compensate for the first sensor not switching.

Now my question:

Does the not switching indicate a bad O2 sensor or is it just running rich constantly? I read in another post about O2 sensors that symptoms of a faulty sensor are:

Excessive fuel consumption, a faulty oxygen sensor can waste 30% of your fuel. --------------------------------------------------------YES
Driveability problems, such as engine surging or hesitation.-----YES
High hydrocarbon emissions, failing an emissions inspection.-----YES
The engine warning light may be on or service flag displayed.----YES
The ecu/computer stores a mixture-related fault code. ---------YES

So, while I know that most O2 related error codes are not the sensor itself I'm wondering if this is the 5% of times when it actually IS a faulty sensor.

Anyone got an opinion/knowledge on that?


P.S. I can post freeze frame (p0175) or graphs if needed.

Thank you all.

Stefan

 

[ExplorerDMB]

P1152 Lack of Upstream Heated Oxygen Sensor Switch - Sensor Indicates Rich - Bank No. 2

P0155 02 Sensor Heater Circuit Malfunction (Bank 2 Sensor 1)

P0175 System too Rich (Bank 2) (click HERE to read more)


Easy way to test - find out which wires are the heater circuit of the sensor - and then ohm it out ON THE SENSOR not the harness. Usually the heater circuits go bad, which cause issues and usually lean towards a bad sensor. I, hesitantly, am leaning towards a bad 02 sensor. It'd be great if you could graph out the 02, but that's ok. I'd say the sensor has finally said OL - so you are probably being safe in saying it needs a 02 sensor.

-Drew

ps - I'd also like to say that you have done a wonderful job on bring a LOT of information to the table and asking good questions with reasoning. Wish more were like you

 

[swiss]

Thank you for the help.

I'll try to put together a comprehensive graph and post it. I wanted to measure the resistance before but I couldn't find any information on the spec. values (or does it just have to be open?), plus I couldn't figure out which two of the four wires are the heater circuit (Chilton just doesn't do it for me...). I'll play around some I guess. I'm leaning towards a faulty sensor too (especially since I had a blown head gasket for a while).

 

[ExplorerDMB]

yeah a blow head gasket will cook up 02s pretty quickly

-Drew

 

[mark11]

Yes!!

this is exactly WHAT HAPPENED to my 98 FORD EX 6 CYL SOHC! Blown head gasket...finally fixed; everything's running smooth...then CHECK ENGINE LIGHT came on; got a buddy to check it with his tools and led to: O2 SENSORS ####ed now! Bought new ones and tool...replacing them tonight!

 

[alohamonte]

Just a quick question, OHV with Auto Trans has 3 O2 sensors? Upstream left/right and downstream?

 

[mark11]

2 uppers 2 lowers

i have only 2 uppers -- driver and passenger sides and 2 lowers.

 

[ZWilson07]

Yes it should have bank 1 and bank 2 front sensors before the cats, then bank 1 and bank 2 rear sensors after the cats.

OP monitor you STFT and LTFT at the same time as far as your rich condition is concerned, this will give you a much better idea rather than just trying to go by O2 sensor readings. You might also put some propane into the system will doing this at the same time and this will give you a good idea of how your O2 sensors are reacting when you add propane and take it away.

I would at this point go ahead and throw in a new O2 sensor if in fact the heater circuit tests failed with a meter, then we can go from there. I also commend you for trying to properly diagnose things instead of just being a parts changer. Many times people will come on the forums with similar problems and all kinds of ideas get thrown around which does sometimes fix the issue. Then everyone gets a, slap hands moment, and off they go. Other times people end up just throwing money at the vehicle that never actually fixes the initial problem or changing parts that were never bad in the first place.

Unfortunately as you are finding out, you really have to have a good amount of diagnostic equipment to really be able to diagnose a lot of things easily on your own. Especially when dealing with these types of things, vacuum leaks, exhaust leaks, etc that often plague these older vehicles.



Back to the problem, from the information given at this time it seems as just the heater circuit has went bad in the O2, and since it is the front bank 2 sensor, that might be causing it to give a bad reading which in turn is causing your pcm to call for more fuel. You might not actually have another problem going on, but rather that O2 sensor is just causing a rich reaction especially since it is on the same bank. Double check the O2 sensor heater function by hand, if the element is bad then replace that one O2 sensor which should clear 2 of the 3 codes, and might even clear all 3. But we will go from there once we get the other taken care of.


EDIT

This is for a 2000 SOHC but should go for the 98-01.

I am showing HO2S 2 as having 4 wires.

Orange/Red & Black/Light Blue & Orange/Yellow & Light Blue with Y&LB being the heater control. It lists them as HO2S 1, HO2S 2, HO2S 3, and HO2S 4 so I am presuming that HO2S 2 is in fact bank 2 sensor 1. If not, I can list the other 3 and let you see the colors that match up.

 

[ZWilson07]

Just a quick question, OHV with Auto Trans has 3 O2 sensors? Upstream left/right and downstream?

From what my 2000 manual is showing, only 3 oxygen sensors are shown.

For the OP, SOHC has the standard 4.

 

[ZWilson07]

What I have for 2000 4.0 SOHC. Decided to put this just in case you need the colors for the other sensors listed and sensor 2 in the diagram is not in fact bank 2 sensor 1.

 

[ZWilson07]

So dumb, didn't even realize the OP is long gone . Well maybe this will help somebody one day lol.