Contour Enthusiasts Group Archives Forums General Troubleshooting Bad O2 Sensor but no CEL

I think my Bank 2 O2 Sensor 2 is bad but there is no CEL. The voltage on the sensor from the OBD-II software is a relatively constant 0.03-0.04 V. All other fluctuate as expected. However this does not generate a problem code nad CEL as it is still within 'limits'. At least it is an easy one to change.
Any insight? Why doesn't a relatively constant voltage on the sensor get picked up by the computer as a problem (Not programmed for anything but a voltage out of range?).

I assume that when you refer to the "Bank 2 O2 Sensor 2", you are talking about the O2S22 value in the OBD-II software.

I believe that you will need a little more information before you can tell if the O2 sensor (O2S22) is bad. According to the Ford TSP (Technical Service Publication)(see below), the voltage that you scanned is normal if the catalyst monitor is off. On the other hand, if the catalyst monitor is on, then it seems that the O2 sensor (O2S22) is probably bad.

There is probably another value that you can read using OBD-II software (Vehicle Explorer?) that will tell you if the catalyst monitor is on or off. I'm not in front of my OBD-II software, so maybe someone else can give you the name of this value. This will help you decide if the O2S22 sensor is good or bad.

There are specs in the Ford TSP CD. (If you have the CD, the specs are located in the PCED area, under section 6A: Pinpoint Tests, and then near the bottom under your specific engine. There is a treasure trove of material in there.) The following are the O2 sensor specs from the Ford CD for my car (1996 2.5L automatic).

What car do you have (year, engine size, auto/manual transmission)? Someone else might be able to give you the specs from the Ford CD for your specific car.

Thanks for the reply. Yes it is the O2S22 value that is staying constant at 0.03 v. Note that within same time frame of monitoring the O2S12 values are fluctuating as are both upstream sensor values. I do not have the CD but my vehicle is a 1998 Contour 2.5l (of course) automatic trans with approx 165K km. Manufactured in 1997. Already changed the Bank 1 Sensor 1 last year as it went 'bad' (CEL code confirmed that one. I guess the question is: is catalyst monitoring not done synchronously for both bank 1 and 2 and if not how is that governed and what values in the OBD-II software need to monitored to know if catalyst monitoring is active in conjunction for the sensors for Banks 1 and 2.

If the Bank 2 O2 sensor isn't showing any voltage fluctuation, my guess would be the following:

A) There is a malfunction in the heater circuit for that sensor.
B) There is a bad connection on that sensor.

Might want to try cleaning the contacts for that sensor before you replace it.

The cat monitor runs really late in the scheme of things. Drive cycle page will give you info on when the different monitors run, and what conditions cause them to run. Although Cat Monitor is step 7 here, the condiions for it are generally the last to be satisfied in regular driving from my experience.

I would have expected a CEL from a constant low voltage O2 sensor - sure there isn't a "pending code" or PTC stored (wouldn't throw a CEL)? Once the heater warms the element to its operating temperature, it should begin to switch (albeit slowly for a downstream sensor). Until the element is heated, the voltage won't go above 0.05V or so (true for all the O2 sensors). Heating is supposed to occur within about a minute or so of engine on.

Hope this helps.

[QUOTE]Originally posted by PA 3L SVT:
[QB]If the Bank 2 O2 sensor isn't showing any voltage fluctuation, my guess would be the following:

A) There is a malfunction in the heater circuit for that sensor.
B) There is a bad connection on that sensor.

Thanks for the reply and advice. I will run another diagnostic check and ceck out the Sensor itself. One thing I did not mention was that the first diagnostic check was done over a 30 minute period (and that was after coming home from a 1 hour drive from work with a 30 minute downtime in between)and the voltage for the sensor did not fluctuate at all during the run. So the sensor heating cycle should have been well past initiation. I will check for a PTC but there are no pending codes. Remember that I mentioned the other bank's counterpart sensor is switching so it is either how the cat monitor works with each bank, and I missed the part of the cycle during the diagnostic or it is something wrong with the O2 sensor, wiring etc.

Keep in mind - the sensors are just reporting sensed conditions. The Cat Monitor is evaluating those conditions and analyzing them. The Cat Monitor DOES NOT effect the operation of the sensors in any way, i.e. the monitor is PASSIVE. The Cat Monitor won't turn them on and off or anything of the sort.