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Old 09-11-2012, 17:18   #1 (permalink)
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Both o2 sensors send 0 volts!!

Hi there!! New to the forum, nice place!! My sister has a 98 Explorer with the 4.0 OHV and automatic. Long, VERY long story short, I had to overhaul the engine. I fired it back up and was greeted by a plethora of tranny codes and dead o2 sensors. (I broke the rear one during the operation, so I expected the rear one to be dead...) I cleaned the tranny harness well and replaced a damaged connector and now it shifts fine. Then I bought two new sensors. But the sensors remain dead. My scanner says 0 volts. It also displays "-100%" for the pair, I don't know what this means. I ran it to operating temp, unplugged a sensor and checked for the 17 to 87 mA they should generate. Nada. No change in engine's running. I checked the vehicle side, no resistance between ground wire and vehicle ground, infinite resistance from signal wire to ground, and 14 volts on the heater wires. I don't know where to go from here.....
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Old 09-11-2012, 18:03   #2 (permalink)
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Re: Both o2 sensors send 0 volts!!

Not sure where you got the milliamp rating from, but an easier read is in volts. >2VDC being very lean and .9 VDC being rich. It should generate .45 VDC when running under ideal conditions. Red-Black should be the signal back to the PCM, try getting a read off of it to ground while the engine is running at temperature. Some frown on it, but I'll take a pin and stick it in the wire and alligator clip it to the positive terminal on my meter and take the negative to a good body ground.
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Old 09-11-2012, 19:11   #3 (permalink)
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Re: Both o2 sensors send 0 volts!!

Yup, you're right, I meant volts, but isn't the sensor supposed to generate that voltage from nothing? Meaning with the engine running, I should be able to unplug the sensor and get .7 volts or so, right? But I got nothing. Also, someone told me that it could be a vacuum leak. Does that sound right?
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Old 09-11-2012, 21:06   #4 (permalink)
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Re: Both o2 sensors send 0 volts!!

They have to be heated up to start producing volts.
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Old 09-12-2012, 03:15   #5 (permalink)
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Re: Both o2 sensors send 0 volts!!

Yeah, I know.... I had it fully warmed and running
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Old 09-12-2012, 12:30   #6 (permalink)
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Re: Both o2 sensors send 0 volts!!

I called a mechanic today and he said to check the fuel pressure. He said the regulator may be over-pressurizing the system. What do y'all think?
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Old 09-12-2012, 14:36   #7 (permalink)
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Re: Both o2 sensors send 0 volts!!

Quote:
Originally Posted by rocketman247 View Post
I called a mechanic today and he said to check the fuel pressure. He said the regulator may be over-pressurizing the system. What do y'all think?
Well, I wouldn't tell a paid professional he's wrong, since I consider myself somewhat of an amateur in the automotive department, but I would ask him to prove that theory to me. If the regulator was over pressurizing, the engine should be burning rich which would increase the signal, not take it to zero. I would still see what signal you have while it's active and in the system by reading the signal going back to the PCM. If the heater is working and you have a solid ground and it doesn't send a signal to the PCM, I'd say you have a faulty O2 sensor. I'd take the reading as close to the sensor as possible to rule out wire breakage.
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Old 09-13-2012, 07:12   #8 (permalink)
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Re: Both o2 sensors send 0 volts!!

I checked the fuel pressure, it hovers at 28 PSI. The Haynes manual said it should be 30-40. It also said to remove the vacuum line from the regulator and it should jump to 40-50, and it came up to about 38. I plan on replacing it tonight, and see what happens. I checked for vacuum in the power brake booster hose and it read around 20 inches of mercury. Is that a good place to test?
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Old 09-13-2012, 17:50   #9 (permalink)
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Re: Both o2 sensors send 0 volts!!

That's a good vacuum and that's a good place.
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