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Old 08-10-2013, 11:33 PM   #1
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Question Bad O2 Sensors, I'm lost

I have been lurking around for a while and joined a while ago and was planning on my first post being a little more positive as I have been making a probably futile attempt at saving a truck from the crusher as the last 2 owners badly neglected the truck. It's a 96 F150 w/ 4.9L inline 300 5 speed, everything is stock except the rust and spray paint. I have been chasing bugs for the past few days. I went for an inspection and failed with flying colors.
One of the problems is the CEL (Check Engine Light) it is giving code PO155 (O2 sensor bank 2 sensor 1 heater circuit failure). I replaced the O2 sensor on the rear exhaust manifold, the second one back, as the O2 sensor voltage readout on the scanner was very low (less then .1 volts) even after the engine was hot. I also thought it was the B2S1 sensor, maybe I'm wrong?. The other 2 were bouncing around in voltage from about .3 volts to .790 volts with the engine running a constant 1500 rpms. Once I replaced the single o2 sensor, the other 2 sensors started reading out really low numbers, under .1 volts and sometimes 0 volts with a hot engine holding at a rough 1500 rpms. The new o2 sensor held steady, and steadily increased in voltage as I increased the RPMs, so I assume it is working normally.
I don't know if it's related, or just a bad coincidence but while I was trying to get voltage readouts from the scan tool the truck had a power surge and the red battery light is on and the voltage gauge is reading just off the front of "Normal". After the surge, the battery reads 11.9 volts and drops very slowly as the truck runs. I assume the alternator the bed, or maybe just to voltage regulator.
I cleared the codes after i put in a new O2 sensor, but before I did, I had codes: PO155, PO141, PO135, P1401, P1137.

Thanks in advance for any advice, Jon
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Old 08-11-2013, 05:10 AM   #2
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OK its late I'm tired and I don't feel like looking up codes. If no one beats me to it I'll check them out later. But let me explain a little about O2 sensors. To start with you changed the wrong one. Bank 1 is the right head bank 2 is the left. Sensor 1 is front and 2 is rear. They are sometimes listed as 11 and 21. The rear sensor primarily monitors the cat function. On OBDII it also functions for system self checks but mostly it is there yo make sure the cat works. The only thing they measured is the amount if oxygen in the exhaust compared to the ambient air. In a properly functioning vehicle the cat should store oxygen so the exhaust after the cat should contain a fairly constant amount of O2. If the reading is moving up and down the cat is likely bad. The upstream sensors are the ones that control the air fuel ratio. They switch constantly between rich and lean. When the exhaust gets lean the ecu demands more fuel from the injectors until it goes rich. Then it trims the fuel back until it goes lean. So good sensors will switch quickly from 1 to 5 volts. Hit wot and they should read lean. Let go and they will go rich the. Start switching again. If you have a heated circuit error, either the circuit to the sensor is bad, the ground is bad or the sensor itself is bad.
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Old 08-11-2013, 03:38 PM   #3
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Check the fuses for the O2 heater circuits.
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Old 08-11-2013, 04:54 PM   #4
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Quote:
Originally Posted by Nitehawkjr View Post
Check the fuses for the O2 heater circuits.
Fuses are good
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Old 08-11-2013, 05:52 PM   #5
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Originally Posted by Warlockk View Post
OK its late I'm tired and I don't feel like looking up codes. If no one beats me to it I'll check them out later. But let me explain a little about O2 sensors. To start with you changed the wrong one. Bank 1 is the right head bank 2 is the left. Sensor 1 is front and 2 is rear. They are sometimes listed as 11 and 21. The rear sensor primarily monitors the cat function. On OBDII it also functions for system self checks but mostly it is there yo make sure the cat works. The only thing they measured is the amount if oxygen in the exhaust compared to the ambient air. In a properly functioning vehicle the cat should store oxygen so the exhaust after the cat should contain a fairly constant amount of O2. If the reading is moving up and down the cat is likely bad. The upstream sensors are the ones that control the air fuel ratio. They switch constantly between rich and lean. When the exhaust gets lean the ecu demands more fuel from the injectors until it goes rich. Then it trims the fuel back until it goes lean. So good sensors will switch quickly from 1 to 5 volts. Hit wot and they should read lean. Let go and they will go rich the. Start switching again. If you have a heated circuit error, either the circuit to the sensor is bad, the ground is bad or the sensor itself is bad.
Well, I ended up replacing all of them because they were in rough shape and had 100K miles on them. On the 4.9l inline all the o2 sensors are on the same side. After replacing all the o2 sensors, some of the codes except PO155 went away. Is there any way to test the wiring? I have a multimeter. Where are the grounds located?

Last edited by Night Owl; 08-11-2013 at 06:10 PM. Reason: revision
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Old 08-25-2013, 10:01 PM   #6
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Default Problem Solved

Well, I found out the problem. I couldn't find anything obvious so I took it to a shop and it was diagnosed with a bad computer. It wasn't turning on the O2 sensor heaters and it also kept thinking the engine was running lean. I pulled the computer out and it was all rusty and covered in thick salty residue. I have no idea if that killed the computer but I wouldn't be surprised.
Once I pulled out the computer I could see right into the cab and if the computer was getting wet, guess what.... so were the floorboards which would explain why the drivers side floorboards had rotted out. Water was coming in around the computer and getting into the carpet and sitting there rusting away my floorboards. I'll have to find a better way to seal up the computer as the stock cover and foam gasket isn't doing the job when water runs down the firewall.
None of the junkyards had a computer for my truck, so I went with a remaned BWD computer, so we'll see how that works out. Anyway, thought I would pass on my findings to help anyone else.
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Old 08-26-2013, 01:33 AM   #7
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Glad you found the problem. That's usually the last thing to check as its the end of the circuit. Sorry I didn't get back to you I was in the middle of finals and lost track of the thread. I've been seeing a lot of ecu failure posts in areas that he a lot of snow and salt lately. Fortunately not a problem here in southern California. The biggest issue is the poor placement of the ecu by ford. New vehicles usually place then inside the cab. I'm not sure if a good way of sealing the unit without overheating issues. I've never really looked into it but I would recommend trying to eliminate any leaks into the area as well as possible. Then you should clean the connector and coat it with some electrical grease to help seal it. In the military we used silicone sealant but then the connector was hard to remove for testing. I'm sure there are some mudders and off roaders around that have some good tricks.
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Old 08-26-2013, 10:16 AM   #8
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The plug seals up okay. When I took the old one out, the connectors were very clean. The cover that is suppose to seal around the computer is not very good so I'll probably silicone that to the firewall to seal it up when I get the new one.
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Old 08-26-2013, 10:16 AM
 
 
 
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4.9l 300 i6 straight six, o2 sensor, po155

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