I’m working on a 2002 Ranger with a 3.0L Flex engine.
As I try to diagnose a misfire code in cylinder 1,
I’ve looked at all the O2 sensors.
The upstream sensors function with a normal voltage variation. The STFT for both upstream sensors varies around 0%.

The downstream O2 sensor stays around .735 -.780 mv. The STFT for this sensor is froze at 99.2%.

Shouldn’t the downstream sensor show the same function and voltage variation as the upstream sensors?
I’ve replaced the B1S2 twice, as well as the connector for it.
I have tested everything!
The voltage for the sensor heater is 12. V
The ground is intact.
The ohms between the two heater wires is 2.0 ohms.
When I apply voltage to the sensor lead wire, I get full voltage on the scanner!
I’m stumped as to why I don’t get normal voltage fluctuation in the downstream sensor!

What is the normal voltage for the downstream O2 sensor?
Thanks
Kirk

 

O2 sensors only see Oxygen
0.1v is high oxygen
0.9v is low oxygen

Cat converters burn up most of the oxygen to burn up pollutants when they are working

So Downstream O2s should run 0.7v to 0.8v, and be fairly steady
Upstream O2s should change fast and vary between 0.2v and 0.6v

You have to read up on your brand of OBD2 reader, they are not all the same, and are not always showing the data from the sensor they say or its not the correct data

The OBD2 reader "request" data using PIDs, seen here: https://en.wikipedia.org/wiki/OBD-II_PIDs

So the reader sends a Parameter ID(PID) to the computer to request the data for that device/sensor,
Computer sends back requested data, USUALLY

OBD2 reader may be sending the wrong PID or computer may misunderstand which PID it should send back

If something doesn't make sense then its most likely a communications problem not an actual system problem

 

The downstream 02 has nothing to do with fuel trims on our trucks. Some systems use the downstream as well. The voltage on the downstream should stay steady. Its only job is to make sure the catylytic convertor is working.
if you have a misfire on 1 look at cylinder 1. Is the coil pack cracked underneath? Check for spark on 1. Check compression. Check injector.

 

Thank you gentleman!
This makes complete sense!
I’ll check for spark issues and go from there.
Shouldn't the STFT for B1S2 be around 0%?
Mine is fixed at 99.2%!
An issue with the spark in cylinder 1, wouldn’t that create a high O2 reading in B1S1, due to unburnt O2 and fuel?
Kirk

 

O2s only see Oxygen, can't "see" gasoline

O2 voltage would be LOW and STFT high(+15 and up) on upstream O2 on the bank with a steady misfire, because of the extra oxygen on that bank

Downstream O2 might be slightly effected, but not much unless Cat converter was not working at all
The Cat convert CAN use the unburnt gasoline from a misfire, thats what keeps Cats HOT
In normal operation the computer constantly cycles fuel trims RICH to feed the Cats extra gasoline to keep them HOT
So in the case of a misfire the upstream O2 would see that but downstream wouldn't see much of it because the extra fuel makes the Cat hotter and it burns up even more oxygen, so downstream O2 wouldn't change much

There is no STFT % as far as downstream O2s, so ignore that
Downstream just has voltage

Just so you know how it works
The computer calculates Fuel Injector Open time based on several parameters, like RPM, Throttle position, air temp, MAF data and "pre-programmed Fuel Pressure"(there is no fuel pressure sensor)
This calculation is 0% STFT(short term fuel trim), so the % is fuel injector open time, lets say 0% = 100ms(millisecond) open time for each fuel injector

After the injectors are opened for 100ms the computer looks at UPSTREAM O2 sensor voltage, so see if injectors were opened long enough or too long
If upstream O2 shows lower then 0.4v(lean) then computer changes STFT to +2%, it increases injector open time to 102ms, a 2% increase, then STFT +4%, 104ms, ect......
Until upstream O2 settles in between .0.4-0.5v AVERAGE

So there is no relevance in % for down stream O2 since its not used in fuel injector open time
But downstream O2 is used in LTFT(long term fuel trim) calculation

As an engine system ages compression will get lower, fuel pressure as well, and there will be small air leaks and sensors can change a bit over time
So LTFT was added so the computer doesn't have to re-learn the fuel system EVERY TIME engine is started
LTFT is held in memory, its a average of STFT mostly
So if engine was running an average of +5% STFT then LTFT would be +5% at next start up, this helps engine to run better when cold when O2s can't be used, compensates for changes in the system over time, i.e. 20 years later
And +5% is also added to STFT calculation, so STFT 0% is actually +5% since LTFT was added to final calculation
So if STFT is +5% and LTFT is +5% then engine is actually running +10% at that time, AND at next start up LTFT would change to probably +8% maybe +10% as said its an average
At +15% over time the computer will usually set lean codes, but it has to be running +15% or higher for awhile
When accelerating you can go +25% easily until computer recalculates demand

The downstream O2 contributes to LTFT in regards to Cat efficiency, Cats age too, lol, no 9 lives here, so downstream O2 may increase LTFT so Cats get more gasoline to keep them hot enough to work, a richer mix

 

Thanks Ron,
I do appreciate the explanation of how the O2 sensors effect fuel delivery.

When I asked about downstream %, my question was regarding the STFT %.

Is the 99.2% an indication of the health of the Cat.? Is 99.2% STFT of the downstream entered into the LTFT along with the upstream STFT’s, or is it purely used to adjust the operating temperature of the cat?
I’m getting the typical miss when started warm, and during warmed up idling. Cold start and off idle performance is smooth. However, there is a distinct loss of power and very poor fuel mileage with perfectly normal functioning upstream O2 sensors and normal upstream STFT!

Thanks again Ron for your input. I just want to make sure I have an accurate understanding of the downstream STFT.
Kirk

 

There is no STFT for downstream O2, I guess you missed that part

STFT is OPEN TIME for fuel injectors as calculated by the computer, absolutely nothing to do with downstream O2

OBD2 readers are generic, they expect the operator to know what data they would like to view from their specific computer, and then know how to interpret that data
An OBD reader will allow you to select any data you want whether or not the vehicle is even equipped with sensors or control for that selection

There is no STFT for upstream O2s either, technically, as said that's the computers calculated open time for fuel injectors, modified by upstream O2s
i.e. STFT bank 1sensor 1 of +10% means the computer calculation was 10% light for fuel, based on feedback from upstream O2 on that bank
There is no feedback from downstream O2 and STFT

Read here: https://en.wikipedia.org/wiki/OBD-II_PIDs

As shown in the OBD PID link above, 99.2% is the default maximum when there is no valid data

 

Just a quick update to my misfire search.
I checked out the coil pack as suggested.
No obvious cracks or damage but way out of speck in Ohms resistance in the primary circuit.

I used a good old fashioned timing light to find which cylinder or how many. It was actually cylinder 2 that was misfiring, bad!!! 1 out of about 3 cycles actually fired.

swapped out the coil pack, misfire gone! Runs just as great warm as cold (unfortunately isn’t saying much, but still much better)!!
Thanks again guys!!
Kirk

 

Good work

Thanks for the update and THE FIX

 

Excellent troubleshoot & good that you solved+updated this thread to help others!