P0141 still being set

After going back and looking at all of the posts, some posters provided incorrect and/or confusing information, so let's start over again.

How did you check the wiring? Did you do a continuity check between the connector on the O2 sensor ande corresponding pin on the harness side of the PCM connector or did you use another method?

I

I'm not sure how GM comes up with titles to TSB's, but I doubt if the TSB you referenced above is related to your problem. TSB's related to a specific diagnostic trouble code ("DTC") generally list the DTC, and that seems to be the case in the list of TSB's for your car. In other words, I would imagine that the title would reference DTC P0141 in the title.

I see that you had a Bosch sensor installed and you changed it to an "OEM" one. Exactly what brand of O2 sensor is in the car now and where did you get it? Did you have to do any splicing for any of the sensor swaps?

Also, what is the physical location of the sensor you have been checking? Are you checking the sensor on the exhaust manifold or the one between the catalytic converter and the tailpipe?

Bad PCM's are very rare and should be the last thing that gets changed. Unfortunately, you are probably beyond any emissions warranty so it would be a customer pay situation. I wouldn't have a problem with one from a junk yard since the odds are that it will still be OK.

On Sep 3, 6:11 pm, "Ray O" wrote:

I spoke with the teacher in the auto shop that my wife works at and it turns out that that TSB is only for Prizm's manufactured in Canada but that need to pass US emissions, so it doesn't apply.

Also, here's the full story since it's probably hard to gather it all from the randomly spread out posts and I left out some info in my last post: I bought the car about a year ago and very shortly after I bought it the code P0141 came on (described as "Post-convertor O2 Sensor heater or circuit fault" in the manual). I bought a Universal Bosch O2 Sensor and installed it. The code came back on after about 30-40 minutes of driving it (which seemed odd since the manual said that it should come on within 1-2 minutes). The responses on here stated that it was probably a problem with the splicing I did or something wrong with the wiring between the sensor the PCM. I put it off until about a month or two ago when I traded in the Universal Bosch O2 Sensor for a direct fit one. Once again, the code came back on after about 30-40 minutes of driving it. I then did the tests in the Haynes manual that I have and it said that the O2 Sensor was fine, but I noticed that the output was going through a sinusoidal type of pattern instead of staying level like the manual said it should. After some investigation and phone calls it seemed that the catalytic converter was bad, so on Friday I finally got that replaced. But like usually after about 30-40 minutes of driving the code came on again. So I called up the teacher of the auto shop at my wife's work. He looked up the service for a P0141 on a 97 Geo Prizm and it was almost identical to the steps in the manual I had, but had an extra step regarding the expected voltage of the output (should be 10 V). I did this test and noticed that the output voltage wasn't even close to this. I then noticed that up near the console this output wire had been spliced, so I opened up the console to investigate. I then noticed that the wire had been split with one of the wires going to the PCM and one being grounded to the metal of the console. I believe that the wiring had a problem and they had tried to re-run the wire themselves and had grounded it for some odd reason (the wire was blue and black so maybe they thought the black meant that it was supposed to be grounded or something). But I removed this wiring and re-ran the wire like they should have. I tested the connection between terminal 19 on the PCM and the output of the O2 Sensor and it passed, so I assumed it would work and put everything back (dumb, I know). But then the code came on almost instantly after I turned the car on (more like what I expected). When I spoke with the teacher in the auto shop, he said that being grounded like that might have messed up the O2 Sensor and that I should redo the diagnostic checks to make sure that it was still OK, because it was very unlikely that the PCM was bad (just like Ray O said), so I'll redo that check and hopefully it will just be something simple like getting a replacement O2 sensor and swapping it out.

And just for completeness, here's the test that I did:

1) Disconnect the O2 Sensor and ensure that there's 12 V at the Black/ Red wire coming from the car with the ignition on. 2) Reconnect the O2 Sensor and ensure that there's 10 V at the Blue/ Black wire coming from the O2 sensor with the ignition on 3) Disconnect the O2 Sensor and ensure that there's 11.7-14.3 Ohms between those same terminals on the O2 Sensor at 68 degrees F. 4) Ensure that the output of the O2 Sensor is holding steady between 100 and 900 mV after reaching standard operating conditoin.

Thanks for all the help and I'll let you know what I find out. Dave

This may indicate that the previous mechanic tried to fix a grounding problem for the Heater Return (for the lack of a better term) wire from the sensor. Which means the transistor in the ECU may be bad and not dropping the heater return line to ground when the ignition is turned ON. So the previous mechanic splices the return wire directly to ground, hoping to solve the problem which he obviously didn't do. That's why you see a splice from the return directly to ground -- it's a futile attempt to bypass that grounding transistor "switch" that turns on the heater in the ECU.

So do this check:

Since you already checked and had battery voltage at the Heater Battery

• wire, you need to check the Heater Return wire and make sure it goes to ground and therefore completes the circuit that turns on the heater.

So at the oxygen sensor's connector towards the ECU, and with the ignition switch to ON, check the continuity of the Heater Return wire to Ground. Do you see continuity to Ground? (Be careful not to connect the ohmmeter to battery voltage wire or you'll fry the meter).

may be a good way to get a half way decent ECU.

Please keep us posted.

But I

OK, next time you buy an O2 sensor, get a Denso. As johngdole mentioned, Bosch is a very good brand, but they do not always work well in Toyotas, and during my time spent as a district service manager for Toyota fixing cars that dealers had trouble fixing, I've never run across problems with Denso, while I occasionally ran into compatibility problems with Bosch. However, in this case, I suspect that the problem is with the wiring harness between the O2 sensor and the PCM and not with the sensor itself.

Jeff Strickland is saying that the O2 sensor heater is bad, but I disagree. It seems unlikely that 2 sensors are bad unless there happens to be a compatibility problem, so I would look elsewhere for the source of the problem.

I think some of the information in your Haynes manual is incorrect. DTC P0141 should have 2-trip detection logic, which means that it should take 2 trips for the MIL to illuminate unless there is a short or open in the circuit, in which case the light will come on right away.

The grounded wire is a clue that someone who is not as knowledgeable as he or she thought she is worked on the car, and it is never a good idea to cut or modify the wire harness between sensors and the PCM. I would double-check the wire in question by unplugging the harness from the O2 sensor and PCM and check continuity through the wire by using a long jumper wire so your meter leads will reach and also check continuity with ground. If the wire has any continuity with ground, then that is the likely cause of the problem - someone may have grounded the wire in more than one location. Also check the wire harness near the exhaust to make sure the insulation did not get melted or cut.

Dave is looking at a new sensor. And I'm telling him to hold the horses and diagnose the grounding path through the ECU. It's irresponsible for anyone to suggest purchasing new parts when the diagnosis isn't even complete. Besides, the splice was put in for the old sensor, presumably the OEM Denso.

The mechanic who put in the splice obviously knew about the problem in the ECU's ground path. That's why he bypassed it. The splice is a no- no, but at least the mechanic knew what connects to what. Perhaps Ray can enlighten us some more on the transistor switch in the ECU for the heater. He should know.

As far as compatibility goes: people don't use Toyota gas, fill their non-Toyota tires with Toyota air or drive on Toyota roads. Ray said no Denso problems? Perhaps he never fixed any of the Toyota TSB #EG014-03?

EG014-03 M.I.L. ON DTC P1130, P1135, P1150 OR P1155 - Under certain driving conditions, some 2002 model year Camry vehicels with V6 engines may exhibit a M.I.L. "ON" with one of the following DTCs as a result of an Air Fuel (A/F) Sensor manufacturing and installation issue

We agree, although I'm not speculationng on why the splice was put in.

As far as I know, none of the wires between the O2 sensor and the ECU should be grounded, and grounding those wires may have damaged the ECU.

That TSB refers to conditions with Camrys, while the OP has a Prism, but I get your point ;-) What I should have said is that I have never run into compatibility problems with Denso, while I have occasionally run into compatibility problems with Bosch. As I said before, Bosch is a good brand and I have seen them work perfectly fine in many cases, but when trying to fix a "problem" car, putting in Denso parts has worked in many cases. I can think of no logical reason why this is the case since the specs on the Bosch replacement parts are at least equal to the Denso parts.

The wiring diagram doesn't show any grounding and I can't imagine why something like that would be necessary. I also checked the connection between the O2 Sensor and the terminal on the ECU and it passed, so I think that's fine.

I stopped by AutoZone last night and I checked the resistance of the heater circuit of the direct fit O2 Sensor that they had in stock and it was 9.3 Ohms. The manual says that resistance of the heater circuit in the O2 Sensor should be between 11.7 and 14.3 Ohms at 68 degrees F (the resistance of the one that is current in my car was 9.2 Ohms). I also did the measurements at around 80-90 degrees so that means that the resistance at 68 degrees would be even lower (but it's hard to say by how much). But since they obviously aren't within spec and since Ray O said that he has had problems with Bosch O2 Sensors in Toyotas before, I decided to fork over the extra $20 and try out a Denso O2 Sensor. They had to order but it should get here in Friday and I'm keeping my fingers crossed that I can put it in on Saturday and that this problem will finally be solved.

Thanks again for all the help and I'll let everyone know how it turns out. Dave

Thanks for the update!

I got the Denso O2 Sensor and it still popped up the code, so I decided to rip everything apart and redo the wiring check. Here's the steps I followed

1) Disconnect the O2 Sensor. Turn ignition on. Check the voltage between ground and the O2 Sensor Terminal 1 (Black/Red wire) and ensure that it's 12 V. RESULT: It read 12.4 V. 2) Turn ignition off. Reconnect the O2 Sensor. Disconnect ECU Connectors. Turn ignition on. Check the voltage between ground and terminal 19 (Blue/Black wire) of the 26-pin ECU connector and ensure that the voltage is at least 10 V. RESULT: It read 12.4 V. 3) Turn ignition off. Disconnect the O2 Sensor. Check the resistance between O2 Sensor Terminal 1 (connects to the Black/Red wire) and O2 Sensor Terminal 2 (Connects to the Blue/Black wire) and ensure that it's between 11.7 and 14.3 Ohms at 68 degrees F. RESULT: The Bosch one is 9 Ohms and the Denso one is 16 Ohms, but I think those are both close enough and that's not the problem.

The diagnostics say that if all of these tests pass, then you should change the ECU, but from the comments on here it sounds like that's not likely.

I also did the test that John Dole suggested to check for continuity between the pin on the ECU and ground and it didn't pass. I'm not an electrician, so here's what I did and someone can tell me if it was a correct check. I hooked an alligator clip with a long cable to the ground terminal of the battery and connected one probe of the multi- meter to it. I then probed Terminal 19 on the ECU with the other probe and it didn't show a connection (didn't beep and didn't show any resistance). Is that the right way to check for continuity between ground and the ECU?

But with all of that info, does anyone have any ideas on what's wrong? I think that the O2 Sensor itself is fine and it looks like the wiring between the O2 Sensor and the ECU is fine, so is it the ECU itself that's bad? Or is there some other wiring that's wrong? And how do I find out what wiring is bad?

Thanks again for all of the help, Dave

Yes, Dave. That's precisely what I had been saying. The excellent Bosch sensors are fine. I was trying to help you avoid swapping parts trying to make it work.

And I was pointing to the ECU fault all this time.

Why? In order for the heater to turn on, the ECU has to activate a grounding transistor. Call it a switch if you will. If this switch isn't turned on, the heater WILL BE OFF. Now there are some conditions under which the heater will be turned on, so it's best to test this within minutes of a cold engine start.

All you have to measure is the heater ground that goes through the ECU, and see if it actually goes to ground. If it doesn't under the condition requiring the heater, then the ECU most likely BAD. And you should substitute a known good ECU for AN ADDITIONAL VERIFICATION TEST. In addition, there are circuits around this transistor that may be bad and resulting in the P0141 code.

Nothing in Ray O's comments even suggest testing this part. I'm disappointed that the self proclaimed Toyota tech even suggested that the ECU is not likely the problem without performing this important test. I don't think he knows how the ECU controls the oxygen sensor.

Ray, I was asking to see if you know how the ECU controls the oxygen sensor heater and check it's proper operation. It was an opportunity to demonstrate your knowledge from taking Toyota courses, if any.

I don't think you know anything about it. Otherwise you would have asked Dave to check the grounding circuitry in the ECU that turns on the oxygen sensor heater. Instead of dweeling on the sensor and telling him "OK, next time you buy an O2 sensor, get a Denso.".

.... or RayO has done enough repairs and seen enough diagnostics to know what is a likely culprit and what isn't. I would imagine Bosch is a fine brand however Bosch is not Toyota OEM and most of us have all seen instances where non-OEM parts cause strange issues. Most tend to troubleshoot from the easiest to the most difficult.

Thanks for the info and I'm VERY grateful for your advice, but I think that you're being a little harsh towards Ray O. I'm just glad that I have people like you, him, and everyone else here who is willing to lend a hand and give whatever advice they have. Also, I have heard from several other people that it's VERY uncommon for ECUs to go bad, so I agree with mrsteveo that Ray O was just following the natural path of investigating/fixing the easiest/most likely problem first.

But to the main point, I don't exactly have access to a "working" ECU to verify against. I could go and try and find one at a junk yard and hope that the one I get is good, but do you think it would be easier for me to just take it to the dealer and have them fix it now? If that is the best option, then what do you think I should expect to pay for something like that?

And thanks again for all of the help, Dave

I don't have a wiring diagram or manual handy, but in Step 2 above, you said that the voltage between terminal 19 and ground is 12.4 volts, which means that terminal 19 is not a ground pin. If terminal 19 was a ground pin, then the voltage between terminal 19 and ground should be 0.

Since terminal 19 is not a ground pin, then there should not be any continuity between ground or the battery negative terminal and terminal 19.

The process you used to check for continuity between ground and the ECU is correct, except that you have to find out which ECU terminal is the ground terminal.

While bad ECU's are extremely rare, they do occasionally go bad or are damaged. At this point, an ECU replacement is probably in order. The best thing to do would be to borrow a known good ECU and swap to see what happens.

John,

We have come to the same conclusion, but through a different path. You seem to be offended by the advice I am giving, and that is absolutely not my intent.

Even though I am not an expert in electronics, I have had good results by making sure technicians have followed all of the steps in the factory repair manual (unfortunately, steps get skipped fairly often by experienced technicians because their experience lets them get away with it most of the time).

I don't think that the blue-black wire that the Dave described as cut and grounded in one of his earlier posts is supposed to be grounded. In a later post, he said that proper voltage between the terminal at the end of the blue-black wire and ground is 12 volts. I don't know what grounding that wire would do the ECU - would that damage the ECU?

The reason that I did not tell Dave to check the grounding circuitry in the ECU that turns on the O2 sensor heater is that I think the grounding is inside the ECU and not testable without opening up the ECU. In a simple circuit, Toyota generally does not run ground wires all the way from a component to the ECU if the component can be grounded with a shorter wire. My assumption is that the heater return wire has to have voltage when it gets back to the ECU, and if the wire is grounded before the ECU, the ECU won't see the correct voltage and assume an open or short circuit.

I am not right 100% of the time. I only looked at cars that the dealers had problems with and can only recall 3 occasions where I had to have an engineer look at something I couldn't fix. Two of those vehicles were diesels, which I have no experience on, and we bought back the third one because we couldn't find the problem. An employee bought that third one and never experienced the problem that the original customer had. Perhaps this is another occasion where I am wrong, but in my opinion, one can't go wrong by starting with the basics.

Sorry, I wasn't clear about what I was probing. In Step 2, I was actually probing the pin on the ECU connector that connected to Terminal 19 on the ECU (and not the Terminal 19 pin). In summary, the voltage on Terminal 19 of the connector was 12 V and there was no voltage (no connection) on Terminal 19 itself.

Sorry about the ambiguity, but I'll look into testing against another ECU. Dave

If you are disconnecting the harness from the ECU and checking the pin that connects to terminal 19 of the ECU and getting 12 V, and the wire connected to that connector is the blud and black one, then it should not be connected to ground.

Good luck!

I think that I'm going to just have to buy a new ECU, because I did some quick checking and couldn't find anyone with access to one. Here's what I found with a little searching online:

This one says that it's for a 1.6L. Would that still be compatible?

This one says that it's for a 1.8L, but it has AUTO in the title. My car is a manual, so would that be a problem? I also found this site: it says that they're $300. Is that what I should expect to pay?And which one would I need to select? Is there anywhere else that you would recommend that look into gettin a new ECU from? I know that it's previously been suggested that I check a junkyard, but I just moved to Tucson, AZ a year ago and I don't know where any junkyards are here or if there are any that would even be worth checking out. Anyone familiar with junkyards in Tucson and have a suggestion?

Thanks, Dave

compatible?

problem?

site:

But it says that they're $300. Is that what I should expect to pay?> And which one would I need to select?>

I also found this one, but it doesn't give much detail about the part: