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O2 sensor (Const. Reading)

22K views 15 replies 9 participants last post by  papajetta 
#1 · (Edited)
hi

i was in a shop last week to check my o2 and Maf sensors because of high gas consumption , using GT1 .

anyway the maf reading was normal , but all 4 o2 sensors post and pre cat, gave a constant 0.4v even if u rev the car the readings dont change , which is not normal .

i want to change them but how can i know its not the ECU or grounding of the sensor because all 4 of them give the same reading , is that odd ???

and i know there is a way to test the votage directly at the sensor but the question is why the 4 of them give the same reading ????

by the way the shop re-programmed my DME ( re-flashed) and its still constant

and if there was wrong with wiring,dme it should register (close-loop) instead of const. reading

finally could these be caused by connecting bank 1 connector to sensor 2 and vise versa ?
 
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#5 ·
You can measure the voltage from the O2 sensors using a multi-meter. Find a place either at the connector or somewhere else where you can tap on to the wires. The voltage should fluctuate constantly. If it does not then its bad.

If the voltage is fluctuation I would guess something is up with the ECU or cables/grounds
 
#8 ·
Do you have any codes?
Do you have headers?

Maybe the Gt1 doesn't update the displayed info often enough to see the voltage changes?

For about the first minute after a warm start I see about .4 volts on all 4 O2 sensors (using AutoEnginuity Software)

Code:
 Time	         B1S1    B1S2    B2S1    B2S2 
                 O2 V    O2 V    O2 V    O2 V
				
  10:30:47.494	0.435	0.43	0.43	0.435
  10:30:55.076	0.44	0.43	0.445	0.435
  10:31:02.689	0.55	0.44	0.36	0.44
  10:31:10.317	0.285	0.485	0.185	0.495
  10:31:17.868	0.73	0.565	0.515	0.58
  10:31:25.434	0.105	0.63	0.765	0.65
  10:31:33.015	0.71	0.675	0.055	0.695
  10:31:40.628	0.72	0.69	0.76	0.72

Closed loop mode would indicate the ECM is using the O2 sensor voltages to control the air / fuel mixture. Open loop does not use the O2 sensors (usually during warm up or during full throttle)

If the post cat O2 sensors are showing the same voltages as the pre cat sensors, it may indicate your cats are not functioning properly. (if you have cats)
The post cat O2 sensors normally show a constant ~ 0.7 volts when working properly.


More info:
http://www.beisansystems.com/misc/SEIMENS_MS_420_ENGINE_CONTROL_SYSTEM.pdf
See page 20-

OXYGEN SENSOR SIGNAL INFLUENCE ON INJECTOR “OPEN” TIME
The ECM monitors the:
• Amplitude of the signal (highest voltage or range sensor is producing)
• Switching time of the signal (how fast from lean to rich)
• Frequency of complete cycles (how many within a period of time)
These characteristics provide info to the ECM that reflect the overall condition of the sensor.

POST CATALYTIC CONVERTER SENSOR SIGNAL
The post catalyst O2 sensors monitor the efficiency of the catalyst as a requirement of OBD
II. This signal also provides feedback of the pre-catalyst sensors efficiency and can cause
the ECM to “trim” the ms injection time to correct for slight deviations.
Bosh Systems:
• If the catalyst is operating efficiently, most of the
remaining oxygen in the exhaust gas is burned
(lack of O2 - “constant lean signal”).
The sensor signal fluctuates slightly in the higher
end of the voltage scale.
• If the post sensor shows excessive fluctuations
(which echo the scope pattern of the pre sensor),
this indicates that the catalytic converter is not
functioning correctly and cannot consume the O2
(fault set).
If the post sensor fluctuations move out of the normal voltage “window”, this indicates
that the pre sensor is not performing properly due to slight deterioration. These systems
can also “trim” the ms injection time to compensate for this.


The constantly changing oxygen sensor input to the ECM is needed to correct the ms
injection time to ensure that the ideal air/fuel ratio is maintained.
 
#9 ·
well, the car is stock , i don't have any aftermarket headers, i dont get any codes,

and no strange lights on the dash(check enginer or eml ) !!!

and the gt1 updates fast because the maf reading changes fast i can see it changing constantly but the 4 o2 sensors are showing a constant 0.4v it doesn't change by a 0.01 !!
 
#10 ·
Having all four show the same output is troubling. I would not suspect the sensors themselves for all failing with the same symptom at the same time. I'd have to look at a common cause for all of them to do the same thing at the same time.

I hooked up my DVM to the O2 sensor on my '94 and got an error reading, but the reading changed with conditions. I cannot imagine the GT1 not following the sensors as the conditions change. When I had my '00 plugged into a GT1, the data changed very rapidly. I don't recall specifically looking at the O2 Sensors at the time, but whatever we looked at -- at least the MAF, and more -- the data was very dynamic; it was changing constantly with a decimal point or two.

Do you have a Check Engine light? What are the codes?
 
#11 · (Edited)
the car has almost 140K miles on it and never changed any o2 sensors , so maybe the have gone bad one by one but the previous owner ignored it ,,

the car is stock , i don't have any aftermarket headers, i dont get any codes,

and no strange lights on the dash(check enginer or eml ) !!!

and the gt1 updates fast because the maf reading changes fast i can see it changing constantly but the 4 o2 sensors are showing a constant 0.4v it doesn't change by a 0.01 !!
 
#15 ·
What mileage have you got?
It's not unheard of O2 sensors to lasts past 140K miles. Mine still work alright at 162K miles. If your mileage is not too low and you've got no codes, I suspect that the voltage wasn't read properly. Having constant 0.4V at all four sensor and no codes sounds very unlikely.
 
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