O2 Sensor Upstream Diagnostics

Upstream O2 sensors should switch up and down across 450 millivolts. Above 450 millivolts or .45 volts is rich and below is lean. This should be on a .1 to .9 volt scale. I've seen oxygen sensors that were getting stuck lean. Fuel trims kept going up and up until they were maxed. At this point the O2 sensor would finally read rich and get even above 1 volt. As fuel trims decreased the graph would flatline again.

Upstream O2 sensors defined:

This is the O2 sensor which is the first to see your exhaust off the manifold. Because it's before the catalytic converter it's "upstream". Modern cars replace upstream O2 sensors with Air/Fuel sensors which can see a broader scale across versus seeing just barely up and down from the rich/lean that a catalytic converter needs to stay healthy.

I've had trouble for the last year with a running rich condition at idle. The OEM sensor I chose was reporting below 100 millivolts. While further experience with it's warranty replacement shows that oxygen sensors do indeed go below 100 millivolts, or at least it does from the manufacturer Denso, most of the time that it would operate in this range, it's before the engine has heated up and the engine has gone into closed loop operation.

Closed loop operation:

This is a condition where the PCM (Powertrain Control Module) has determined that the conditions are set that the information from the O2 sensor can now be deemed reliable (it's gotten hot enough). My vehicle would usually be in closed loop instantly. Normally, you want to be in closed loop as soon as possible. It uses less gasoline and helps with your car's mpg. This was too fast. Even though my heater circuit was 15 ohms; same as my warrantied replacement, that really isn't a complete test and I suspect that the heater core had a problem that my PCM couldn't detect.

A full test would be to have the heater in heating operation and see what the voltage drop is, but I doubt that's a procedure I could get specs on. Ohm meters use very little current. When you have more current something that looks right can suddenly test poorly. Only 1 fragile wire for example could show that a battery cable is good on an ohm meter, but it couldn't carry enough current it and it would show voltage drop when you place leads on each end of it. The voltage measured is the mount of voltage lost to that wire out of the available voltage from a battery for example.

Something I notice my PCM doing when it doubts if it has a good O2 sensor, is that it'll run rich and instead of switching back to lean like it should, it'll go even richer for a moment. This is s a test to ensure correct operation. If it keeps going richer the O2 sensor is failing the test. If it goes lean after this brief test it passed.

Realize you have to have a good testing method:

I typically rely on live scanners. These devices plug into the OBDII port, can pull trouble codes, and can show live streaming data. Except it's really not live streaming data. It shows a timeline sure, but the information isn't updated frequently to rely be "live". Such methods are fraught with difficulty when it comes to O2 sensors. O2 sensors and PCM's are communicating far faster than the live data from a scanner shows. Until an O2 sensor has gotten very bad, it can be difficult to diagnose. 

Plugging into the wiring directly from a DVOM isn't accurate either. It's sampling rate isn't bad, but it averages the data it receives and switches the screen slowly. Part of this design is intentional. You can't really see a screen and have it processed in your mind if it switched faster than you eye can see. 

An oscilloscope would be my next diagnostic tool, but they are spendy.

The catalytic converter link I put above links to my article about beating the P0420/P0430 diagnostic code. It'll also give you insight into downstream O2 sensors.

Also check out my newer article about the oxygen sensors 4 stages of death.

Check out my YouTube video for a downpipe modification that made my O2 sensor go in much easier to my 1999 Chevy Prizm (usually same as 1998-2002 Toyota Corolla). I also talk about some diagnostics that were specific to this situation.

Written and produced by AutoBravado.