Scott Mills

Ok I got a check engine light the other day which said something like low voltage on the right rear O2 sensor as well as left and right bank lean codes. I pulled out the sensor and noticed it was dented from the clowns that did my exaust. I replaced the sensor and cleared the codes (I am sorry that I dont have the exact codes there was a PCM incident which caused replacement). My question is that the sensor doesnt have the same readings as the left rear one. It fluxuates between 95 aand 273 mv where the left reads between 143 and 812 mv. RPM range was between 1600 and 3500. The dented sensor read between 122 and 139 mv for a comparison. The truck hasent thrown any codes from the new sensor, but it seems to me the readings from the left and right sensor should be close to the same. Is it possible that an exaust leak past where the sensor is could cause this? The only mods on the truck are duel exaust, k&n filter, and UPD intake. How about an intake leak? Any ideas?

<small>[ August 29, 2002, 01:53 AM: Message edited by: Scott Mills ]</small>

jgkinsofla

Heated Oxygen Sensors Operation

FRONT:

The Heated Oxygen Sensors are mounted in the exhaust system where they can monitor the oxygen content of the exhaust gas stream. The oxygen present in the exhaust gas reacts with the sensor to produce a voltage output. This voltage should constantly fluctuate from approximately 100 mV (high oxygen content = lean mixture) to 900 mV (low oxygen content = rich mixture). The heated oxygen sensor voltage can be monitored with a scan tool. By monitoring the voltage output of the oxygen sensor, the PCM calculates the fuel pulse width command to give to the injectors (lean mixture/low HO2S voltage = rich command, rich mixture/high HO2S voltage = lean command).

REAR:

To control emissions of Hydrocarbons (HC) , Carbon Monoxide (CO) , and Oxides of Nitrogen (NOx) , a three-way catalytic converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it to nitrogen. The PCM has the ability to monitor this process using the Bank 1 HO2S 2 and the Bank 2 HO2S 2 heated oxygen sensors. The front HO2S sensors produces an output signal which indicates the amount of oxygen present in the exhaust gas entering the three-way catalytic converter. The rear HO2S sensors produces an output signal which indicates the oxygen storage capacity of the catalyst; this in turn indicates the catalysts ability to convert exhaust gases efficiently. If the catalyst is operating efficiently, the front sensors will produce a far more active signal than that produced by the rear sensors.

The catalyst monitor sensors operate the same as the fuel control sensors. Although the Bank 1 HO2S 2 and Bank 2 HO2S 2 sensors main function is catalyst monitoring they also play a limited role in fuel control. If a sensor output indicates a voltage either above or below the 450 millivolt bias voltage for an extended period of time, the PCM will make a slight adjustment to fuel trim to ensure that fuel delivery is correct for catalyst monitoring.

Having said all that, what I've been gathering is that these sensors are sensitive! To alot of different things, like fuel contamination, silicone sealant fumes, exhaust leaks, being banged around, etc.

I would think the new one will get different readings because it's not as "contaminated" as the used one - maybe you should replace both?

wrencher

iTrader: (2)

Leak upstream of sensor can cause problems. 4 o2's are youcompairing b1s1&b2s1 then b1s2&b2s2 cause post cat o2's tend to stay a bit on the lean side, but should get into upper voltages (on o.e. set-ups).You could have a scan tool that not entirely accurate. Check hot o2's with digital volt meter or see if you get the old ses light.

LS1 PWRD

Are you saying I should be able to get a reading with a DVOM when the sensor is hot? In my case I have no SES light but I am getting a low voltage code reading on Bank2 Sensor1. I have checked the resistance in the wiring between the PCM and the sensors and everything seems OK but when I monitor that particular O2 sensor it reads 600-900mv and then suddenly drops to 10-75mv and back up to the higher readings again. I don't know what to try next (he says as he pulls his hair out). I have swapped the O2's but it hasn't helped.

jimmyblue

iTrader: (11)

In all of the sensors I've looked at,
you never see a stable reading (at least,
not at idle). The gas flow is too slow
for the control loop to work on direct
readings. The output is used in an
averaged mode.

It looks like things haven't changed much,
and the idle O2 output still "hunts" on
the LS-1. The less it "crashes", or the
"tighter" its swing, the better. A happy
loop won't be banging around to 50mV off
the rails (0, 1.0). That says that the
engine is bouncing between soot and sunshine.
Yeh, the average may be right.

You should replace sensors pairwise.
Although it looks like maybe this EFI
tries to manage by bank, you may still
be making a compromise between readings
from stale & fresh sensors. Their time
constants are the first thing that varies.
The more faithfully they track gas content,
the more responsive the loop will be.

Scott Mills

Ok the truck threw a code at me again P0157 (o2 Sensor circut low voltage Bank 2 sensor 2). Just to troubleshoot a little I switched the two rear sensors with eachother and reset the code. After a short while the same code returned B2S2. So I know it is not a bad sensor. What else shoud I look at? Oh one other note my gas milage has been in the crapper since the truck was new never better than 11mpg <img border="0" alt="[barf]" title="" src="graemlins/gr_barf.gif" /> . Maybe this problem is affecting my mpg.

<small>[ September 03, 2002, 10:34 AM: Message edited by: Scott Mills ]</small>

wrencher

iTrader: (2)

B2s2 low voltage when does it set code at idle, at cruise? Could be a few different things. Exhaust leaks, bad cat, wiring & pcm.

Scott Mills

It sets the code at cruise. There was no exaust work done in front of the sensor so I would think that a leak wouldnt be the culprit, but how do you tell if the cat is bad? The PCM was just replaced the other day. I took the truck to the dealer today because it is starting to bug me that a truck with 3000 miles on it is allready running poorly. The dealer said it was a bad o2 sensor. I explained to them that I had switched the sensors from left to right and still got the same code, but they seemed convinced that it is the sensor. I suspect they are just giving me the brush off.

<small>[ September 04, 2002, 12:28 AM: Message edited by: Scott Mills ]</small>

Scott Mills

So the dealer changed the sensor and the problem returned within 5 miles from the dealership. Say seem convinced that it is a bad O2 sensor despite the fact that you can switch the sensor from left to right , but the the problem remains on the right. The dealer also said that because of my aftermarket exaust that it is causing the problem and will void any other warranty work. I think I will try another dealer. I stopped of at Sears on the way home and bought a thermomator so I could check the Cats temps. The left Cat is reading about 200 at the entry and 380 at the exit, the right Cat is reading about 200 at the enteranct and 320 at the exit. Could a bad cat be the problem? Other than that it has to be the wiring. One other thing the dealer today said that they stuffed a rag up one of the pipes to create more back pressure and the Sensor function fine, but when I performed this test at home I saw no differance on the O2 readings for the right sensor.

<small>[ September 05, 2002, 05:30 PM: Message edited by: Scott Mills ]</small>

joecar

A bad cat can cause the problem you're looking at;
you swapped B1S2 and B2S2 sensors (swapped sides), and the problem did not swap sides;
you say it happens at cruise;
check your wiring again while the vehicle is hot
(check that the wiring is not fouled upon by anything mechanical or fluid);
sensor voltage reads low, meaning high O2 level,
possibly due to the following:
* upstream air leak (check at the heads and at any joints; check for holes);
* bad cat (if you had an overly rich condition the cat's insides may have partly melted together rendering the catalyst material inoperable; or other reason for bad cat).

Scott Mills

I have suggested all the thing mentioned to the dealer to look at. They said that they have check the wiring and it checks good, no large voltage drops from the PCM to the sensor. I asked about the Cat being faulty, but they didnt think so because it would throw off a code. I didnt realy agree with what he said, but this service center seems to be takeing a genuine interest to find the problem.