Which scan tool for testing O2 sensor?

[Vikas]

yonyon is right on the money.

If you only knew how ECM turns on converter low efficiency code, you would have not bothered to replace the rear O2 sensor. Theoretically, a bad rear O2 sensor will never turn on the low efficiency code on its own. As a matter of fact, the rear O2 sensor has to be functioning for that code to come on. There is science behind it but not many people get it.

 

[artbuc]

Originally Posted By: Vikas
yonyon is right on the money.

If you only knew how ECM turns on converter low efficiency code, you would have not bothered to replace the rear O2 sensor. Theoretically, a bad rear O2 sensor will never turn on the low efficiency code on its own. As a matter of fact, the rear O2 sensor has to be functioning for that code to come on. There is science behind it but not many people get it.


Please say a little more. I, and I bet many others, are interested. Thanks.

 

[Jimmy9190]

I bought this scanner at HF a few days ago:


http://www.harborfreight.com/can-obd-ii-deluxe-scan-tool-60693.html

It was on sale for $79.99 and I also used a 25% off coupon so it cost me about $60.00. It's a good tool, seems to be very good quality and will work fine for how I will use it. It displays live data including O2 voltages, fuel trim and a whole list of other info. The scanner told me my upstream and downstream O2's are all doing fine. I posted some of the info from the scanner in the Mechanical/Maintenance forum. Then YonYon, Trav and a couple other guys helped me figure out that one of my cat converters is bad. The HF scanner is a very good and useful diagnostic tool and I am sure it will come in handy again some time. I also hope I can return the favor to the guys who helped me some time real soon.

I returned the little Actron code reader to AAP, they took it back without any questions after I explained I had to get a better code reader that would check my O2 sensors. I also had to mention that I had bought $112.00 worth of new NTK O2 sensors in the same purchase, but they really did not give me any problems about the return. I am not returning my new O2's, at 162K my original downstream O2's needed replacing whether they still worked or not.

Tomorrow morning I am getting back under my truck again and taking measurements and studying the exhaust pipe, muffler and tail pipe. Then I will order the new cat/exhaust assembly, new muffler and tail pipe and clamps from Rock Auto. They had the best price and were about the only reputable retailer I could find that sells the cat/exhaust assembly. The muffler and pipes will add less than $100.00 to the total cost and I would rather just replace all of it and be done with it rather than wrestle 13 year old pipes apart. I will tackle the exhaust job next weekend. I am leaving my exhaust manifolds as is. If they ever develop a crack or leak I will deal with it if/when it happens.

 

[KenO]

You can't really 'test' O2 sensors. You can scope their outputs, and measure how long it's taking them to switch and see if the manufacturer listed a spec, you can have heater circuit failures that will usually throw a DTC, but thats about it.

 

[Vikas]

ECM counts the number of transitions of the O2 sensor i.e. how many switches per minute. When it finds that rear O2 sensor is switching as fast as the front one, it realizes that the catalytic converter is not doing its job.

Here is the critical piece of information. The rear O2 sensor has to be working for it to switch quickly. As a matter of fact, if the rear O2 sensor were weak, it will more likely not switch quickly and mask a bad catalytic converter.

That is why in 99 out of 100 times, replacing a rear O2 sensor to fix P0420/P0430 is futile. That code alone is sufficient to tell you that your rear sensor is switching or at least as fast as the front one. ECM is also smart to notice if your front O2 sensor is switching slowly and will give you a different code.

One of the big problem with the P0420/P0430 code is that ECM is way to aggressive on it e.g. it turns on that code when the converter efficiency is dropped even a little and there is really no need for it to do that. So, sometimes, manufacturer has a TSB to tweak that parameter but what it means is that the code will not reappear later, more likely after the federal emission warranty of 8/80 is expired!

Once you understand this, the solution to the catalytic converter efficiency low code should be obvious. Either replace the converter or convince the rear O2 sensor to switch slowly.

 

[Vikas]

You can also look at the Mode $6 data to see how close you are at failing some of the tests. Unfortunately, most of the is proprietary and difficult to decode it without finding the correct decoder ring.

For example, say you have real dual exhaust on your vehicle and you got P0430. You can look at the Mode $6 data and see if your other exhaust bank is close to failing or not. Also after you replace the converter, you can look at the data to see if you are now completely clear of that code or not. It does take a while for the ECM to run the cat converter test and declare all the tests as completed.

 

[Jimmy9190]

I am replacing both pre-cats and the main cat this coming weekend. I ordered the entire assembly from Rock Auto last night. It cost $347.00 shipped with a 5% discount. I am keeping the new downstream O2's in place. Even though they did not solve the P0432 problem, at 13 years old and 162,000 miles I feel better about replacing them. I may also replace the intermediate exhaust pipe, muffler and tail pipe while I am at it. I don't think it would be much fun trying to wrestle new pipes into 13 year old original pipes. I would rather just replace it all and be done with it.

 

[KenO]

I've learned, though, that Torque now has a nice O2 data display, it will show you live O2 sensor voltage graphs. Incredibly handy. I'd never seen this on my phone, but I did last night on my tablet.

 

[artificialist]

Originally Posted By: Jimmy9190
I am replacing both pre-cats and the main cat this coming weekend. I ordered the entire assembly from Rock Auto last night. It cost $347.00 shipped with a 5% discount. I am keeping the new downstream O2's in place. Even though they did not solve the P0432 problem, at 13 years old and 162,000 miles I feel better about replacing them. I may also replace the intermediate exhaust pipe, muffler and tail pipe while I am at it. I don't think it would be much fun trying to wrestle new pipes into 13 year old original pipes. I would rather just replace it all and be done with it.

Keep in mind that some discount cats don't have enough rare metal to pass a catalyst efficiency test, or they soon fail the efficiency test after installation.

 

[Jimmy9190]

The cat/exhaust assembly is made by Eastern Catalytic and is made here in the USA. For $347.00 shipped it had better be a quality part. I found no negative reviews of Eastern online and it was one of only a few assemblies I could find for a plain jane 2 wheel drive Dakota with a manual transmission and California emissions. I could not afford the higher-priced Magna Flow unit and the Davico assembly sold at JC Whitney was more expensive and looked really cheap-made. Eastern also has good video instructions and tutorials online. I did a lot of research and I believe I bought the best part I could afford. According to the Fed Ex site, it is scheduled for delivery tomorrow, I had it shipped to my office instead of leaving something that big at my front door when I am not home.

The trick to this whole ordeal is going to be whether I can get the rusted exhaust bolts loose without breaking anything. Once I get the old assembly out I should have no problem getting the new one in. I am putting off replacing the muffler for now, my original one is OK and I want to be sure I can get this done first before spending more money on the muffler. When I am done with the cats I will try to sell the old ones to the metal scrap guy. I understand you can get a few bucks for a cat because of the precious metal content.