Oxygen sensor readings

[Falcon_LS]

I'm closing in on the 150,000 km (93,750 mi) mark on my Envoy, at which point I have a habit of replacing oxygen sensors on all my vehicles. I've got two heated oxygen sensors on this application, and was looking at the readings of both on my Tech 2 scan tool earlier this morning. Bearing in mind the readings will fluctuate between 0.1V - 0.9V every second, which is roughly 1 Hz, I noticed something peculiar about bank 1.

At idle, HO2S1 will fluctuate between frantically between 39 mV - 911 mV, whilst HO2S2 ranges from 167 mV - 747 mV. HO2S2 seems to be within range (150 - 850 mV), but the drop to 39 mV on HO2S1 got me curious. There are no pending codes stored in the PCM, the A/F ratio is 14.6:1 and the coolant temperature at the time was 191 °F.

I'm replacing them with GM Original Equipment replacements, but was wondering what would trigger the reading to go that low?

 

[Colt45ws]

Thats normal switching behavior. It never stays at Stoichiometric, always adding and pulling fuel. S2 sensor signal is squashed from S1 indicating your cat is working.

 

[tomcat27]

1 is fine. 2 is getting lazy.

 

[punisher]

Originally Posted By: tomcat27
1 is fine. 2 is getting lazy.


Yup. You will always have one O2 sensor that is slower than the other, even out of the box. If you have a scan tool that records crosscounts per second or minute, then you can really see the difference. Until you get a code, it is something that you don't need to worry about. When you get fuel trims going up, and significantly lower crosscounts, then you get the wallet out.

It also illustrates the problem of throwing parts at a vehicle based on a code. V bank engines with a primary O2 on each bank will first code one bank as lean when a MAF starts to go bad/get dirty. The MAF will lean both banks, but the weaker (but still in spec) O2 sensor will set the code first either because it is actually lean, or its crosscounts are reduced below threshold. More than once I have had a car in the shop that Autozone pulled codes on and sold a customer an O2 sensor for a lean code. It's OK for a week, then more lean codes (usually on the opposite bank now) and eventually lean on both banks.