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Old 05 Feb 2013, 02:47 am
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Posts: 454
Default Complex Oxygen Sensor Issue, Analytical Minds Needed, Electrical Experts Appreciated?

2003 Isuzu Rodeo 4WD

Automatic Transmission

3.2L V6 "W" VIN

P0155 - Oxygen Sensor Heater Bank 2 Sensor 1

simple enough, right?


I had two codes. Both upstream heaters were supposedly out when they had both been fine a few days earlier. They

are plugged up, checked the fuse (10A, spot 4 in underhood relay/junction box) and it's good, and busted out the DMM.

I hadn't gotten any codes for the 02 sensor not working, just the heater malfunction, but when I tested resistance (Ohms)

on the sensors both of them were OL (out of limits) for both circuits, heater and 02 sensor. Well, checked DC voltage at

the fuse, had 12 volts, so I checked power down at the harness where sensors connect. Good to go, 12 volts. Hooked up scan

tool, showed about half a volt action on the 02 sensors, but no reading of before and after, like the upstreams were bad and

post cat was fine.

So, being that my tests showed OL I assumed the circuits were burnt out and it was just a high failure item and coincidence they

both went out at the same time.

I replaced them with two known good ones from the exact same vehicle, that ran and had no MIL or CEL so I took them from it

since it's a parts car. Checked resistance, it was about 15 ohms on both of them, and all four circuits had continuity, (two from each)

so I installed them. Erased codes and went driving around to reset drive cycle. CEL came on. WTF. Went back to the shop.

Hooked up scan tool and it was a P0155, B2S1 bad heater WTF

Took that 02 sensor that I knew was good a few minutes earlier, and got the DMM, checked resistance, mother loving OL, WTF

Okay, this is where it really gets tricky;

Went to other side and disconnected that upstream sensor (B1S1) and checked resistance. Slightly different reading, because of the

temperature difference after driving, but continuity remained intact. Grounded DMM to frame, with light scratching for an awesome ground,

and probed heater circuit hookup on the hot wire (+) of the harness where sensor plugs in. It was zero because it was supposed to be zero. The

sensor should be grounded by the PCM when the vehicle is turned on, or even to accessory, but it was completely off, no key in ignition. Zero, good.

Hooked sensor back up and went to other side. Unhooked that O2 sensor (S1B2) and ran same test, grounded to frame and checked heater power source,

on Ohms setting. Nearly 18ohms, 18 freaking ohms of resistance running through heater circuit wiring.

I am dumbfounded and stuplified. WTF'

And also, correct me if I'm wrong here, but if the theory is that (it takes 1 amp to push 1 volt through 1 ohm)

and I have 18ohms of resistance, then wouldn't I logically need about 20 to 30 amps to allow O2 sensor power?

Note, that this is a 10amp system, 10 amp fuse, not popping, I need someone who is either a god damned professional or seriously thinks outside the box.

I'm seriously hoping someone on here will have some idea what I am talking about, and can help me out a little bit.

Thanks for reading guys, I really appreciate it, I know it was F@#$ing diatribe
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Old 05 Feb 2013, 03:01 am
Senior Member
Join Date: Aug 2007
Posts: 1,032

A hefty predicament, well... here's thinking out of the box for you. Try and bypass the wires on the O2 sensor. As in put a jumper wire from the O2 sensor to wherever how far you're able to trace and tap the wire. Might be a loose connection or a break inside the wire because its quite an old car already. Seems to me you're doing everything right with regards to troubleshooting.
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