P1133 Upstream O2 Sensor Diagnostics


Feb 23, 2013
the roadie said:
WELCOME COMRADE!!!!! I'm a EE, and have been volunteering on trailvoy, then GMTN, for 7 years now, and you're the first poster to ever show scope shots!!!!! You probably don't need to have the term PWM explained, either. :wink:

Those are horribly slow transitions. O2 sensors should run at 1 Hz or faster. When I put in a new one, it was about 1.5 Hz if I remember correctly. If you have a new sensor, then something else is slowing down the control loop. A dirty throttle body would give you an erratic idle as the AC compressor cycles on and off, and you would probably have mentioned that. But depending on your mileage, it may be a good thing to do anyway. Many of us do it every 30K.

But the throttle plate isn't involved in the quick transitions, I don't think. Probably fuel injector pulse width and duty cycle. My experience is that it's usually a bad O2 sensor, but if your Delco is new, I'd start by looking at the Fuel Trim numbers to see if you have an injector issue (could benefit by running injector cleaner and/or a Seafoam treatment) or a vacuum leak. Loose hoses and especially loose intake manifold bolts are a common problem.

How did you find us, by the way, after we all left Trailvoy last December?

would you mind me asking about the reason for the exodus from the trailvoy site?

umm nevermind that question. along with the welcome message and using both sites for a little while now- it is obvious this site is superior- better info and easier to use.


Feb 23, 2013
mngopher22 said:
ok i replaced the thermostat and ECT. The ECT sensor broke off in the block and I had to use a reverse thread drill bit to back it out. Glad I am done with that. On a positive note, the thermostat and ECT change made my temp gauge read 210 dead on when im driving so I am very happy about that.

did the same damn thing. luckily they let me exchange it even though it went from round to elliptical from me throwing it against the side of my brick house.


Feb 23, 2013
Ok, obviously i dont have the accumen that a majority of you do. I have been randomly pulling the p0136 and p1133 for years now. I have replaced the precat O2 sensor a couple times. The real issue i have to add that i have not seen or overlooked in this thread is that my exaust smells strongly of gas.
These codes dont post for months and then they come back for a few days and this cycle has been repeating itself for years.
Any suggestions for someone without anything but basic handtools and a cheapo ODBII reader?
I appologiz in advance for my grammar as you all seem like you are very well read.


Apr 19, 2013
mngopher2, just summing up the 0136 says O2 voltage did not drop below 300 Mv during a lean test OR did not go above 600 Mv during the rich test. Conditions between 1000-5000 RPM , speed 20-80 MPH and air flow between 15-100 AFGS.. basically you have to be driving. The 1131 says O2 did not switch enough times (don't now the actual threshold) when tested at 1000-3500 RPM, and air flow above 25 AFGS, throttle above 5% and throttle steady with 18% for a second. Again basically running conditions also.

Outside of associated circuitry, both these types of 02 failures indicate a bad O2 zirconia sensor which I believe you have replaced 3 times if I read the thread correctly. OR a defective heater in the sensor also replaced. The O2 heater has two functions 1- get the O2 up to switching speed (600*F) ASAP to help emissions (closed loop operation) 2- to keep the O2 switching reliably when engine conditions may not be enough to keep the O2 at 600*F, such as idle and low no load RPM.

Older setups kept the O2 heater always on with the ignition, but these later setups use the PCM to control the heater by switching the ground return circuit to the heater element. This ground circuit is tested By the PCM and it must be within the .21 to 1.6 amp range or a heater DTC will post. You are not getting that DTC, so your problem is perplexing. This coupled with no on line information of when and what the PCM looks at to turn the heater on/off adds more unknowns. Your scope pictures seem to show the switching frequency slowing at certain times , would it be possible to add a trace of the heater ground circuit on channel 2 of the scope to see if the heater control changes between good an bad switching scenarios.
Does your scanner have a freeze frame display. it would be interesting to see the actual conditions when the DTC occurs, RPM, LT and ST trims, O2 voltage.. Obviously the heater would come into play more if failures were in the lower RPM range.

By the way I been planning on scoping the O2 switching on my Envoy just to see what the look like. I like those pics.. Did scope the O2's a few times on my Camaro Z28 when playing with tunes and heater testing. With the Long Tube headers in my Z28 the O2's stop switching and start look like saw tooth waves below 1000RPM without the heaters running.


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