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George H.
George H., ASE Certified Technician
Category: Toyota
Satisfied Customers: 18390
Experience:  ASE Master Tech 15+ years
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91 toyota truck rear 02 sig wire no voltage move to front works

Customer Question

91 toyota truck rear 02 sig wire no voltage move to front works well put back to rear heater works well tested grounds both good why is there on cross count or only shows .000 volts on signal
Submitted: 1 year ago.
Category: Toyota
Expert:  George H. replied 1 year ago.
Hello I will help you with your question,
As long as the sensor is connected to ground and is heated with 12 volts and ground to the heater it should produce a voltage. Is the signal wire shorted to ground?
Is this truck in an emission state like California?
Any chance the computer was changed to a Federal computer?
Let me know so I can help
Customer: replied 1 year ago.
sig wire is good i sent 5volts from 02 with pined into sig wire at computer and voltage got there. truck is a cal truck. i have owned truck 5-6 years i have never change the computer. but rember iam testing right at 02 wire 02s produce there own voltage
Expert:  George H. replied 1 year ago.
Right, as long as the computer and the sensor are both grounded you should see a voltage at the signal wire when the sensor gets above 600F and the tip is exposed to the exhaust.
Any exhaust leaks that could allow free air to reach the tip of the sensor when mounted in the rear position?
You tried goosing the throttle to see if the sensor spikes or adding propane to the intake to drive the mixture rich?
Customer: replied 1 year ago.
you guys tryed but not helping thanks for nothing
Expert:  George H. replied 1 year ago.
What are you seeing, This is a give and take and I asked a couple of questions that you have not answered?
Customer: replied 1 year ago.
what is your question
Expert:  George H. replied 1 year ago.
As I posted above
"Any exhaust leaks that could allow free air to reach the tip of the sensor when mounted in the rear position?"
And
"You tried goosing the throttle to see if the sensor spikes or adding propane to the intake to drive the mixture rich?"
Let me know what I can do to help but this will be a process so I can know what you are seeing
Thank you
Customer: replied 1 year ago.
no exhaust leak. yes goosed and ran at about 2000 rpm for few mins
Expert:  George H. replied 1 year ago.
So when you goosed it there was no spike in the output voltage at all? Can you try running propane into the intake to drive it rich? That will run the voltage up.
If the sensor works in the front but you see 0 volts in the back then you are not getting any difference between the atmosphere it uses for reference and what is in the exhaust. Hard to believe it really is pure air in the exhaust.
Let me know what you see
Thanks
Customer: replied 1 year ago.
thank you hoping we could find the answer you tryed
Expert:  George H. replied 1 year ago.
I am sure we can find the answer with some testing, if the sensor puts out in the front position it has heat enough to produce voltage but when you change it to the rear it either does not have heat or it is seeing the same air as the reference air.
Let me know what I can do to help
Thank you
Customer: replied 1 year ago.
all you said is true and that was my question
Expert:  George H. replied 1 year ago.
So can you try propane? It will tell the tale.
Customer: replied 1 year ago.
put propane in to the point of motor labor 02 went to .003. then kick up idle more propane still no real move.
Expert:  George H. replied 1 year ago.
And it works fine in the front position?
Are you measuring at the sensor signal wire or further up to the computer?
If you are at the signal wire and the engine is hot and you have power and ground to the sensor heater then I am at a loss but if you are missing one of those it needs to be checked that way.
Let me know what you are seeing
Thank you
Customer: replied 1 year ago.
been there done that. i to am at a loss thats why iam asking the question. its just a f-ing 02 sensor
Expert:  George H. replied 1 year ago.
If it works up front that plug is wired the same as the rear. As long as you are getting exhaust to the rear of the cat and you don't have holes between the front sensor location and the rear location you should be able to deplete all the stored O2 in the cat with the propane and if the sensor is hot you should see the voltage rise.
Other than those causes I can't give you a reason why it reads 000 unless the signal wire is shorted to ground either in the computer on on its way to the computer.
Any chance the connectors for the front and rear are swapped?

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