'95 Toyota T100 P0135 cannot figure this one out.......

Hey guys and girls, been working on this 95 T100 for a few months now with no luck of fixing the issue. BTW its a One tone DLX 3.4 V6 with manual trans.....

So the only issue this truck has is the P0135. It runs great, starts up every time has has no driving issues minus the CEL for the code. Since CA requires there to be no CEL for smogging the truck, it obviously needs to be fixed otherwise the owner has been driving the like that for over a year like that with no concerns since it drives well. The code will come up in very quickly when the ECU is cleared. We are talking about 5-10 mins with a few key cycles.

When I received the car, another shop has already worked on it and replaced the O2 sensor. The customer also gave me another new O2 and requested to replaced it because someone else told him to do so so I did it anyway. Still no fix. As the code says, if there is an amp draw of greater than 5 amps or less than .25 amps, the CEL will come one. What I did was check the wiring. The heater circuit has as 12v ignition switched source and that wire and plug is good. The other wire in the circuit is a ground wire switched by the ecu and again, the wire and plug is good. I checked for continuity from the O2 sensor all the way to the ECU terminal and the circuit is good. So what I did next is cut the wires going to the heater circuit (one the grounded side) and hooked up my multi meter to the wires to measure the amp draw. With the ecu cleared, I started the car and looked at my meter. Nothing. So right there I new my ecu was not grounding out and I verified this my grounding out the wire that comes from the ECU (pink and green wire) and sure enough the meter was showing and amp draw between 1.6-2.0 amps. I did this with the wire disconnected from the ECU in case it back fed to the ECU and damaged it or something.

With this diagnoses I figured the ecu was bad, it wasn't grounding the O2 heater so something was up with the ECU. So I located a rebuilt ECU on ebay from a seller that does nothing but rebuild ECUs. I messaged them telling my issue and stressed that I want to make sure the O2 heater circuit was good. They replied back that they test their rebuilds and the heater circuit is one of the circuits they test. The model numbers were matching with my ECU and the description matched too so I figured its the same parts and purchased it. When it came in I disconnected the battery, swapped the ECUs and drove the truck. Truck drives the same, no issues but the CEL came on and its the SAME CODE!!!!!! At this point i'm furious. All this work and still the same issue.

I noticed that in the wiring diagram it shows that the wire that the ECU grounds out is switched by a transistor. So I figured that maybe the transistor is bad in both ECUs and not switching but the likeliness of that is very low but anyways I took apart the original ECU to see if I could test the transistor. So one leg of the transistor goes to the wire that grounds the O2 heater. The other leg goes to a brown wire on the same ECU plug and it gets its ground from that wire to feed the heater circuit wire. I went ahead tested that brown wire for ground, its a good ground. So the last and 3rd lead of the transistor is switched by the ECU. I tried to follow the trace but had no luck. I went ahead and made a makeshift bench top power supply and grounded the leg of the transistor that gets the ground from the brown wire and then gave 5 volts to the leg that is switched the ECU and the transistor switches on and I have continuity between the ground wire pin and the heater circuit pin, thus verifying my issue is the ECU is not getting the signal to switch the transistor on and complete the heater circuit to ground.

But WHY?!?! why isn't it getting the signal? Originally I thought it was the ECU bad because it wouldn't switch the transistor but I replaced it with a "good" rebuilt ECU and it has the exact same problem. It could be that the rebuilt ECU has the same issue but what are that chances that they both work perfectly fine minus that one circuit is messed up on both of them?? I'm leaning more to a wiring issue somewhere on the truck. I think that there is another circuit in the wiring that provides a power or ground to the ECU that then allows that transistor to be switched by the ECU but it's only a theory. Also I checked all the grounds going to the ECU at the ECU and they are all good.

Im just so stuck and cant figure out what the next move is. Thanks for making this far while I rant about the issues with the truck and hopefully we can fix this! Thanks so much!

Attached here is the wiring diagram I keep referring to.

That's the thing, I checked all the grounds at the ecu connector and they are all good. I printed out a pin out of every wire that goes to the ECU, hooked up my power probe to the battery, and back probed every ground I found and they all they all checked out good. Also checked for ground on the actual case of the ECU and that has a good ground as well.

kris.sondors wrote: hooked up my power probe to the battery, and back probed every ground I found and they all they all checked out good.

Just because the LED on the power probe turns green does not mean the ground is good. You have to load the circuit. An incandescent test light connected to battery positive would be a better test of the ground circuit. Make sure the bulb is nice and bright.

Good idea!
I went ahead and redid exactly what you said, I got an incandescent light that was connected to the battery 12v and when I touch it to a good ground it lights up nice and bright. I went ahead and tested all the grounds at the ECU both while unplugged and plugged in and every ground resulted in a nice bright light same as a good ground. Do I still need to do a physical ground check on all the grounds?

I did a physical check of the grounds, they and intact and clean. Also checked with the light and they had a nice solid light.....
Any other ideas??
Thanks for all the support and help!

Correct, both O2s are heated and the second sensor has the same issue, the wire that is supposed to ground the heater from the ECU is not getting the switched ground from the ECU, also there hasn't been a code for the 2nd sensor if there even is one

Thanks for the diagram! But unfortunately it just shows the ground the sensor is needed to read the signal to the ECU, it doesn't have to do with the heater circuit. If you look at this diagram I just provided, the ground for the O2 is going to be brown wire (4). The white wire (3) goes to the ECU and that's the wire that give the ECU the voltage range. Now if you go to the white/red wire (2), its an ignition switched 12v source. This is where the O2 gets the power source for the heater. Then if you go to pink/green wire (1), it also goes to the ECU but the only job of the ECU is to provide a ground to this wire. Once it gives the ground, the heater circuit is complete and draws about 1.7amps, thus heating the O2. But here is the problem, the pink/green wire NEVER get the ground from the ECU (same issue with the red/green wire for O2 sensor 2). I even cut the pink/green wire, then grounded the wire and the O2 heater circuit works exactly how it should but no matter what, the ECU will not provide the ground.

It is fused, yes but it shares the same fuse as the ECU. I blew the fused by accidentally grounding the wire during one of the tests and the engine wouldn't start as well as not have power for the heater circuit so I found that out real quick that the fuse is good but also not the issue because when I ground the heater it can sustain a nice current draw.

One of the fault setting criterias - apart from incorrect current flow - is an excessive voltage drop. Your incandescent test light may draw only some 200 or 300 mA. You may want to check for voltage drop on the feed and ground side with an adequate substitute load, just to make sure.....

Another point of interest is the fact that the B1S1 sensors for automatic and standard transmissions are DIFFERENT.
SI states at 68 deg F (20 deg C) the sensor used with AT should be around 11-16 ohms where as the injectors on MT equipped vehicles should measure 5 -7 ohms.

If you yet have not checked then just make sure your sensor is the right one for the application!