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Oxygen sensor readings don’t have much value as a single point in time because once they warm up they switch back and forth around once per second at idle and faster at higher rpm. The majority of the diagnostic value is looking at a graph of their dynamic switching behavior. That said, the precast sensors usually switch between 0.15 and 0.85 volts or 150 to 850 millivolts. Those are not hard and fast values though so some variation is to be expected. The computer watches for the value to switch across 0.45v as the signal to change the injector pulse width value and hence the amount of fuel. I don’t know the deviation from those limit value that will cause problems, we are usually looking at switching behavior. The post cat sensor is a different story. That sensor usually floats at between 0.5 and 0.7 volts and does not change much and certainly does not switch like the upstream sensors unless something is wrong.
Your screen is cutoff so I can’t see the complete sensor labels but your truck has three sensors. The drivers side precat is bank one sensor one or B1S1. The passenger side precat is B2S1. The post cat can have two different labels but I believe yours is B1S2. Can you graph these three sensors? Also, can we straighten out the labels?
Oxygen sensor readings don’t have much value as a single point in time because once they warm up they switch back and forth around once per second at idle and faster at higher rpm. The majority of the diagnostic value is looking at a graph of their dynamic switching behavior. That said, the precast sensors usually switch between 0.15 and 0.85 volts or 150 to 850 millivolts. Those are not hard and fast values though so some variation is to be expected. The computer watches for the value to switch across 0.45v as the signal to change the injector pulse width value and hence the amount of fuel. I don’t know the deviation from those limit value that will cause problems, we are usually looking at switching behavior. The post cat sensor is a different story. That sensor usually floats at between 0.5 and 0.7 volts and does not change much and certainly does not switch like the upstream sensors unless something is wrong.
Your screen is cutoff so I can’t see the complete sensor labels but your truck has three sensors. The drivers side precat is bank one sensor one or B1S1. The passenger side precat is B2S1. The post cat can have two different labels but I believe yours is B1S2. Can you graph these three sensors? Also, can we straighten out the labels?
George
How much effect does the post cat sensor play in this? Wife says fix that junk or sell it as is so now I fully in for the fix. I’ll send you all the fuel trends later today for your input. I did just buy a cylinder leak down test kit so I’ll do that too. Compression test back in September showed good pressure on all cylinders , 260/170 for a 200k motor.
Improper post cat sensor readings from a properly functioning post cat sensor and circuit tells us that the cat is shot. Now with the exception of passing inspection, that alone does not make the truck run bad BUT a failed cat means a contaminated cat and that means something else is wrong like running rich or a coolant leak. Also, once the cat has failed it can start to dissinegrate and then start plugging up causing increased back pressure and poor performance. So once a post cat sensor either pegs high or low or starts tracking the precat behavior your next move is exhaust back pressure and fuel trims and addressing any coolant leaks.
Have a few graphs, 1st is O2 sensors Bank 1&2 at idle, 2nd is same at 2500 RPM, 3rd is Bank 1&2 fuel trends, short term. Engine was at idle and I pulled the air intake tube off the throttle intake and it immediately stalled, once removed it did crank up an run. I was hunting vacuum leaks,I kept hearing this tapping sound at a very high frequency, the hose from the PVC that goes to the fitting that goes to the intake was the source, I pulled the PVC from the valve cover and it stopped, the short piece of hose between the PVC and the hard hose was old and cracking, I replaced it and idle was good and the tapping sound was gone.
Drove blazer about 75 mile round trip today. Ran good, good acceleration, good idle at stoplights, SEC light at about usual mile marker (25 to 35 miles from house). Was still running good despite SEC light. Truck set about six hours then drove back with no issues. Scan at home only had usual P0300 code. Glanced at fuel trends and saw these, LTFT Bank 1 was 16% and back 2 was 2.0 % what gives here?
Graphs are great for O2 sensors and the only way to really see what's going on. We don't have your bank 2 precat (B2S1) so we don't have a complete picture. Your post cat is still too active.
Fuel trim graphs are not very useful especially when driving. Best is idle and then 2500 all in park, both values, both banks. Eight values total - STFT1, LTFT1, STFT2, LTFT2 - first at idle and again at 2500 in park.
05-09-2023, 09:30 AM
