Can you tell O2 sensors are going slowly bad from smog test gas results?

e9999 · 08-29-09, 04:24 PM

I imagine there are tests based on electrical resistance and all to tell if the 02 sensors are still good or not, but I was wondering whether one can also tell if the 02 sensors are going bad slowly based on the gas measurements results on a typical smog test, as in there are more HC coming out of the pipe etc. I'm not talking about bad as in throwing codes here, just slow degradation. But likely too many other factors I suppose.

(And while we're at that, is the notion that used to float around that putting new sensors in place of old would sometimes result in dramatic mpg increases still accepted by folks in the know?)

Brentbba · 08-30-09, 01:24 PM

Well Eric, I tested the resistance on both my sensors after throwing a rich fuel code 26. Steped thru all the other diagnostics myself and never threw a code 22 or 28 for O2 sensor failure, but could not get the 26 to go away, even with a thorough throttle body cleaning and tune up. Finally had the inboard sensor pulled and they tested it under load and it failed...all w/o throwing a failure code.

Must tell you that my truck has a history of eating this one sensor like f'ing candy. New sensor and she runs just great.

e9999 · 08-30-09, 04:35 PM

did you see any smog test evidence of it going bad before you starting getting the codes?

Brian894X4 · 08-30-09, 06:10 PM

I'm kind of curious about any improvements after replacing sensors that aren't obviously bad. Mine have 110,000 miles on them, but the truck seems to run great. I've heard that some manufactures, including Toyota in certain models, recommend automatic replacement of sensors around 90,000 miles.

scottm · 08-30-09, 07:15 PM

I posted graphs of my O2 sensor outputs in these threads, the last one is with new toyota sensors. The new ones are clearly more sensitive. The old ones were throwing codes for the heater in the sensor failing.

http://forum.ih8mud.com/80-series-te...look-like.html
http://forum.ih8mud.com/80-series-te...ncrease-3.html
http://forum.ih8mud.com/80-series-te...rs-better.html

dgspencer · 08-30-09, 10:59 PM

I replaced my O2 sensors before last test, and it indicates slightly MORE dirty than before.

This was replacing OEM with OEM. In my case might be more of an indication of a deteriorating CAT? Definitely NO mpg increase. What gave me 3-4 MPG better was fixing plugged EGR port and VSV.

Here are my #'s for 4 years of smog testing:

Code:

1994 FZJ80 - 1FZFE										
Smog Data										
Test	2500 RPM Test		Dilution	Idle Test		Dilution	
Year	O2	CO %	HC PPM	CO+CO2%	RPM	O2	CO %	HC PPM	CO+CO2%	RPM
2005	0.2	0.01	21	15.11	2701	0.1	0.03	36	15.13	642
2007	0.9	0.03	14	14.63	2410	1.0	0.08	41	14.40	627
2008	0.0	0.16	38	14.96	2471	0.0	0.11	78	14.81	628
2009	0.4	0.41	73	15.31	2541	0.2	0.13	107	15.14	644
										
Standard	---	<= 1.20	220	6.00	2500	---	1.20	220	6.00	775
										
Oxygen Sensors replaced with OEM parts 2 months before 2009 test.

e9999 · 08-31-09, 01:10 AM

Quote:

Originally Posted by dgspencer

I replaced my O2 sensors before last test, and it indicates slightly MORE dirty than before.

This was replacing OEM with OEM. In my case might be more of an indication of a deteriorating CAT? Definitely NO mpg increase. What gave me 3-4 MPG better was fixing plugged EGR port and VSV.

Here are my #'s for 4 years of smog testing:

Code:

1994 FZJ80 - 1FZFE										
Smog Data										
Test	2500 RPM Test		Dilution	Idle Test		Dilution	
Year	O2	CO %	HC PPM	CO+CO2%	RPM	O2	CO %	HC PPM	CO+CO2%	RPM
2005	0.2	0.01	21	15.11	2701	0.1	0.03	36	15.13	642
2007	0.9	0.03	14	14.63	2410	1.0	0.08	41	14.40	627
2008	0.0	0.16	38	14.96	2471	0.0	0.11	78	14.81	628
2009	0.4	0.41	73	15.31	2541	0.2	0.13	107	15.14	644
										
Standard	---	<= 1.20	220	6.00	2500	---	1.20	220	6.00	775
										
Oxygen Sensors replaced with OEM parts 2 months before 2009 test.

interesting, thanks.
Indeed, the new sensors did not result in better numbers. Problem is that there are too many other effects going on, so probably the emissions indeed won't be too useful for directly figuring out the state of the sensors.

Ebag333 · 08-31-09, 01:14 AM

Quote:

Originally Posted by e9999

(And while we're at that, is the notion that used to float around that putting new sensors in place of old would sometimes result in dramatic mpg increases still accepted by folks in the know?)

From what I've read from folks posting, it looks like maybe 1-2 MPG.....at the most.....for a few days/weeks/months.....

Brentbba · 08-31-09, 08:27 AM

Quote:

Originally Posted by e9999

did you see any smog test evidence of it going bad before you starting getting the codes?

No - It's been an intermitent code, but always a 28 (O2 sensor), not a 26 (rich fuel).

chiz45 · 08-31-09, 09:21 AM

Quote:

Originally Posted by Ebag333

From what I've read from folks posting, it looks like maybe 1-2 MPG.....at the most.....for a few days/weeks/months.....

I had mine replaced at 150k, and the 1-2mpg increase is what i'm seeing. I'm at 157k now, mixed city/hwy driving. Truck has stock gearing and 33" muds.

matt60/80 · 08-31-09, 08:14 PM

I'm gonna say no. I just got new cats courtesy of the #%@^ers who stole mine and took my rig thru emission today. I did not replace the O2 sensors. Readings were 0 PPM HC and 0% CO. Yes you read that right.