Hi folks,

It's been a long time I haven't popped a CEL. This time around it's P0135 - Heater Circuit Bank 1 Sensor 1, and it's held constant. I've already done some reading on diagnosing P0135 for example measuring the voltage+current to the heater circuit of the O2 harness, such as using a multimeter on the plug and connecting a headlight bulb to it. What I want to know is even before I jack the car up and start diagnostics from the bottom, how far can I diagnose a faulty O2 sensor from something else, such as wiring, using HP tuners? I can check my O2 activity and compare the mV values of my Bank 1 and 2 front sensors, but what are the values I should be seeing at different operating temps, that could help me narrow down if it's actually a bad O2 sensor or not?

PS: I have a wideband O2 gauge and with it I retuned my engine currently running MAF only in open loop, where I found best and consistent drivability, so worse comes to worst, I can always disable P0135 since the O2 sensors aren't correcting anything at the moment.

 

As far as the O2 sensor it should read 450mv (a bias voltage) on a cold engine key ON...the only thing is when you turn the key to ON the heater circuit becomes energized and the millivoltage should drop below 300mv after 2 minutes or so I believe if the heater circuit is functioning...so pull the O2 sensor heater fuse before turning the key on and then see if you have 450mv...if you do the sensor circuit is good...if you plug the fuse back in and the millivoltage does not drop you have a problem with the heater circuit...you then get under the car and start checking power and grounds at the O2 sensor...the heater circuit on my Bank 1 sensor 1 draws about 1.55 amps key on !!

 

Ok, let me see if I understood you. All this can and should be done with engine off, key on, on a cold engine? Fuse removed, key on, 450mv should be reading? Then, pop fuse back in, key on, wait 2 mins and my readings should drop below 200mv in 2 mins or so of just key on, engine off?

 

YES !!...KOEO on a fairly cold engine with O2 heater fuse pulled you should see 450mv or close to it !!...you can turn the key OFF then pop in the fuse and I don't know if HP Tuners needs the key ON to connect like most scan tools and then key ON and the millivoltage should be dropping by the time you look at the O2 voltage !!

 

Yes once warmed and in CLOSED LOOP your front 02 voltages should have an aveage from between .400 and ,550 millivolts Both sides should be very close
If you have LT headers this opens up a whole can of worms as with LT's the 02 bungs can be move aft as much as 2-3FT making then not read correctly There are tuning tricks to compensate for LT headers but they ar never vack to 100% as the readings were as stock
If you do not have LT headers and have a huge average mismatch in 02 voltages that may point to something else
The most common culprit is an exhaust leak Even a small one makes a huge difference
You could have a bad 02
Your fuel pressure regulator if your car has one may be bad -Typically the pass side will be lean or have low 02 averages beacuse of lack of fuel
Check everyhting Look at your Injector duty cycles They should be almoct equal If not the smaller duty cycle side may have an issue
EVEN a misfire or a bad plug or wire will give mismatched left and right o2 averages

 

With a car this old, odds are that if the fuse is good, you have power to the connector, and the connector isn't damaged, the heater circuit in the sensor went out. What was probably read about using a bulb was to do a voltage drop test and look for the 12V at the connector with the bulb illuminated.

In the OP original question, he is also asking about reading the sensor voltage (the 450mv discussion), which I believe will confuse the issue as they are separate circuits in the same sensor housing.

 

Yes !!...the heater and O2 are 2 separate circuits but must be tested differently !!

 

Well, well, well. Things got more interesting.

First off, C5 Diag thanks for the excellent diagnostic procedure. I was able to do it on a cold motor this morning, with engine off. The good thing everything went well; the bad thing I didn't find anything wrong. Code hasn't popped again either since yesterday.

1. Cold motor engine off, I removed Oxy-sense fuse, and then keyON. My front O2 voltages for both banks were steady at 438mv - 443mv. Fuse was good by the way too.
2. Cold motor engine off, put back Oxy-sense fuse, and then keyON. My front O2 voltages for both banks slowly ramped down, from initially at 438-443mv down to 248-253mv for both banks at about the 2min mark. Both O2's hovered and plateaued at 248-253mV even after 2mins.
3. Once I warmed up driving around, my tune is set so I'm idling somewhere between 14.3-14.7 AFR (it's jumpy because of cam) in open loop (remember, I retuned my car for impeccable driveability and left it as a MAF only tune and running in OPEN LOOP, so no O2 feedback for short term fueling correction). So idling in between 14.3-14.7 AFR my front O2's register somewhere between 800-860mV. The higher the mV value, the richer AFR. Typically, I only see 450-550mV when my wideband AFR reads 15.0 or slightly higher.

I'm stumped. Could be an intermittent issue I think at this point.

trublu92, I have LT headers and I'm running OPEN LOOP. That said though, I've had my LT header setup for almost a year now (about 12k miles already) and I haven't popped this code until now.



O2 fuse removed, engine cold and motor off. Key ON.



O2 fuse back in, engine cold and motor off. 2 mins after Key ON.



Engine warm, idling at about 14.3-14.7AFR (wideband reading, running in open loop)

 

With the P0135 the car will stay in OL...did you clear the DTC and see if it comes back ??...the last pic with the engine idling what is the value of the last PID...ambient air temp ??...looks like a minus something but hard to read...is it -40 F?????...you in Alaska ?????...is that the IAT from the MAF ????...also what does your ECT look like ??

 

I had cleared the P0135 yesterday morning. It hasn't come back since yesterday morning, and I've done about 3-4 drives already since clearing. LOL at the -40F remark!! I'll check but I don't think it was from the intake. I think it's from the cabin air temp sensor which is absent in my car. I'll check all the temps later and give you values, but I'm pretty sure everything is normal with the exception of that -40F which I'm almost positive is from that absent cabin air temp sensor.

 

Some of the data PIDS I like to look at even before starting the car (KOEO) is ECT (should be close to outside air temp on cold engine), BARO or MAP (psi or voltage), and IAT (from MAF)