Checking The O2 Sensor. How to?

[bigslow]

'96 XLT with DTC's P171 and P174 (running lean). My rig occasionally hesitates at idle and has trouble accelerating. I went to check the O2 sensors but am having trouble understanding the Chilton's manual.

The Chilton's guide says to check the HO2S wire and Signal Return (Grnd.)The voltage reading should fluctuate between .1 and 1.1 VDC. But the manual gives no picture of the wiring on the sensor. Can anyone help me out with which of the four wires is the HO2S wire?

Any tips on how to test an O2 sensor would be appreciated. Would it be easier to strip a small section of wire on the sensor to hook a lead too? I don't feel like blowing $80 to fix something that isn't bad.

 

[cjbarron5]

i used my dvom with some wire probe leads (it has a plastic "hook" over the end of the probe )and you pull the probe back and lay the wire in the "hook" and let go and it peirces the wire with the probe as far as whitch wire i had to hit and miss to find it

 

[bigslow]

thanks for the input, went ahead and checked one of the O2 sensors, the one after my cat, and it seems ok, voltage is a bit low, but with a lean condition this is what is supposed to happen I presume...

I cleaned the hell outta the MAF, and the IAC checked the voltage on my MAF and it's good, check the voltage on my TPS and it's good too, but still a rough Idle... and occasional stalling at idle, it's lacking power and I'm hoping it's not a fuel problem...

can't find any vaccum leaks either... so I guess I get to try and check the other two O2 sensors tomorrow after work... oh well, it will give me something to do...

 

[dogfriend]

This schematic is for a 97, but the wiring should be very similar.

For the 97, the signal return is Orange; the signal wire is Grey w/ Light Blue stripe for O2 sensor #1, Red w/ Black stripe for O2 sensor #2, and Red w/ Light Green stripe for O2 sensor #3.

Your colors may vary.

 

[0346 AWD]

Bigslow

I would offer my 2 cents to clarify this O2 issue. First, you can totally ignore the 'downstream' O2 sensors. Those are the ones after the cats. the only thing those sensors do is check to make sure the cats are actually in place.
You want to concern yourself with the 'upstream' sensors -- those closest to the engine (between the engine and the cats).

Second, you did not mention how many miles were on those upstream O2s. Hopefully the flaming will not get too bad, but here goes. Those upstream sensors should be replaced after 35k+ miles of use and certainly after 50k. The problem is the sensor may not have failed completely and thus failed Chilton's voltage test. What does happen after being subjected to the harsh environment of corrosive exhaust gases and high temps is that the sensor doesn't react nearly as quickly as it did when new and isn't sending information to the pcm quickly enough for the pcm to keep engine properly tuned.

The O2 sensors can be checked, but you need a scope to check how fast the sensor is reacting -- something most of us don't have. Replace those upstream sensors with Bosch sensors - about $50 each. Dealer charges about twice that much. The reason most folks don't change them is that large price tag.

You mention cleaning the mass air meter. A dirty mam can also cause tuning troubles. I assume you also have a fresh set of plugs installed. Do yourself a favor and change those upstream sensors. I just ordered a set myself for my 03 explorer.

eb

 

[Glacier991]

"You need a scope"....

Well, actually that's overstating the case. A scanner can show the voltage change, as can aftermarket O2 testers (I tried to sell one on here for $60 and had no takers). The "crossover" rate (how frequently the sensor voltage crosses over .445 volts in either direction) should be around 8x per minute, something the human eye is able to see. So you really don;t need a "scope" per se.

 

[bigslow]

a scope as in an osciloscope? even if that's not what you mean, I never even thought about it, I could take my truck to work and pull an oscope out too it... just gotta convince Sgt J, don't know if he would go for that?

then again, I'm not particualarly worried anylonger... changed the IAC, cuz after I cleaned the old one it started whinning, cuz I couldn't get that stupid little filter clean enough...

Just swapped the fuel filter, and the plugs and wires... and she's running like a champ... smoother than when I bought her a week ago. So yeah, don't flame me on the O2's I have no idea how much mileage is on em' but I will probalby change the two forward ones because they are probably hosed, just like my plugs were... and what friggin engineer burried those damn plugs anyway?

people who don't work on trucks shouldn't be allowed to design them...

 

[Kamaaina]

"people who don't work on trucks shouldn't be allowed to design them"




This is the best phrase I've heard of in a long time!!!!!!!!

 

[0346 AWD]

Bigslow

I am glad your rig is running well. I guess I should have paid better attention in high school English. I had no intention of flaming you or anyone else. What normally happens when I suggest changing O2 sensors to the guys I work with is not pretty. For example, I have a buddy who is a mechanic who religously changes his oil every 2,000 miles on his Ford Ranger. The truck has over 100,000 miles on it, and still has the original plugs, wires, etc. He won't change plugs, wires, O2 sensors, but will change his oil about every other month. The irony of this is tht he is constantly b_tching that performance and fuel economy isn't as good as he would like, but isn't willing to spend the dough for a little routine maintenance.

Again, glad your truck is running well.

If you should decide to change your upstream O2 sensors, I found it easier to remove the plastic fender liners in the wheel wells to more easily access those O2 sensors, than to try to get at them from the bottom.

eb