3rd gen 4runner P0141 code

[benXJ]

Picked up a 1996 V6 5 speed 4wd 4runner a few months ago....225K miles.

All was great for about a month, then the P0141 code popped up....rear O2 sensor heater circuit.

There are 3 things it can be, per the FSM. Bad O2, break in the heater circuit wiring, or bad ECM/ECU.

So I replaced the O2 and checked the resistance of the internal O2 heater circuit, and all is within spec.

Here is where I get confused:

Per the FSM, to check the computer, you look for 9-14V out of the respective connector with the key on.
The FSM states that the voltage is 'OK' at 9-14V when testing the ECM, but if it checks 'OK', to replace the ECM. Am I reading that right? Seems odd.....nothing is 'OK' if the ECM needs to be replaced.

So when I checked the ECM connector for the rear O2, I get 0V with key on. So just to double check, I tested the FRONT O2 connector with the key on and got a steady 12V. Odd to me, per the FSM wording.....or is that the correct function? Should the O2 heater circuit come on with the key or only when the engine is running? How long should it stay on?

To check some more, I unhooked the connector from the ECM and hooked a pos. and neg. wire straight to the battery and ran the positive wire into the connector (under the glove box) and tested the output with a multi-meter at the O2 sensor location (with the O2 unplugged) and got a steady 4.9 volts. Clipped the neg. wire of the multi-meter straight to the neg of the battery. Super odd.

I'm leaning towards a bad ECM, but a 1996 4wd manual trans ECM is rare/expensive, and no guarantees on it being good either.

COULD be a broken wire somewhere, but no other wiring or other engine work near any wiring has taken place since I've had it.

Anyone had any experience with this? I'm thinking the emissions control logic is operating fine once up to temp, so that explains the truck running/driving/smelling normal...just the rear O2 isn't heating up fast enough. Won't pass emissions testing as is, even though emissions are normal. I'm about done with 1996+ vehicles.

Thanks

 

[Chris_Keziah]

I think the FSM may have a misprint, and should have the NG and OK flipped. From the way I'm reading what you've done you tested output at the ECU and only got 4.9 VDC? Then testing at the connector for the sensor you get null? That points to a bad wire and a bad ECU output. I'd ohm the sensor wires out and go from there.

 

[benXJ]

I was confused by the FSM, but who am I to question that? I'm thinking misprint too though.

Kind of hard to explain about my wire testing....but I hooked a wire directly to the battery, and the other end of the wire to the disconected ECM connector (the ECM and connector is under the dash)....in an effort to push 12v down the wire to the O2 sensor connector to make sure the wiring was OK from the ECM connector to the O2 sensor. Could Ohm the wires to do the same thing I suppose.

If the FSM is incorrect, and I'm seeing 0 volts at the ECM with the key on....that points to the ECM being bad, as i kind of suspected.

Thanks