I’m currently trying to help my brother smog his 1995 2WD 22RE truck. It has failed smog a couple of times due to high hydrocarbon emissions at 15MPH (85ppm, 66ppm max). At the same time, CO is quite low (.26%, .53% max). And NOx is basically nonexistant (6ppm, 386ppm max)! Also, CO2 is about as high as can be (15.4%). And O2 is literally ZERO.

The truck passed at 25mph. Hydrocarbons were elevated, but still within compliance (35ppm, 42ppm max). CO was fairly low (.15%, .63% max). And much like at 15mph, NOx was basically nonexistant (5ppm, 564ppm max). Also, like at 15MPH, CO2 was 15.4%, and O2 was 0%.

The truck runs just fine and doesn’t burn excessive gas. And these numbers just don’t seem to match up with typical emissions problems. Any suggestions? Bad O2 sensor? CO, O2, and CO2 don’t suggest it, but maybe? Worn cat after 200000 miles?

 

Your CO numbers really aren't that low; the AVERAGE CO numbers for cars that pass smog in California is 0.02% at 15mph (0.04% at 25mph).

Take a look at this article: http://www.cygnusx1.net/Media/Supra/...taTech/h56.pdf Probably the most common cause of high HC is misfire. You say it runs fine, but it doesn't take much of a misfire to mess up HC. You could try putting the inductive lead of your timing light on each plug lead to see if one isn't flashing "on the beat," though this will all be more difficult without load on the engine. Replacing parts "on the blind" is always a desperate ploy, but new plugs and wires wouldn't cost too much, isn't difficult to do, and it might work.

An intake leak (excessive lean) can also cause high HC, but as you point out your decent O2 numbers suggest that's not the problem.

An easy test is to put a voltmeter on the OX1 terminal in the diagnostic port. That's a direct connection to your O2 sensor; you're looking for it to flop from about 0.9v to about 0.4v at least 8 times in 10 seconds (this is easier to see with a needle-type voltmeter, but one that will read that low is hard to find. A "better" modern multimeter often has a bargraph scale for just this purpose.) Even easier is to jumper E1 to TE1 and measure the voltage on VF. http://web.archive.org/web/201003261...01heatedox.pdf This is "conditioned" O2 sensor, and it flops from about 5v to about 0v, making it easier to read with ordinary meters.

What is your location? The limits you quote are close to what I know about in California, but not quite.

 

It might be worth considering replacing the O2 sensor if it's never been done. Mine was still working according to the tests @scope103 lists above, but the response seemed somewhat sluggish, and my emission numbers were creeping up from year to year. I replaced the O2 sensor a year ago, and my HC numbers dropped by 4x, and CO and Nox dropped by 2x. I think the response time of the sensor increases with age, and eventually it does not do as good a job of closed loop control of the mixture.

 

I am in California. As far as CO, the ‘average’ numbers are actually .08% and .09% for CO on a 1995 22RE truck. So the CO results (.26ppm@15MPH and .15ppm@25MPH) are somewhat elevated, but not to the point of ‘barely squeaking by’.

As for ignition components, we actually replaced the spark plugs, cap, and rotor. This (along with using a couple of bottles of Techron) actually brought the HCs at 15MPH down from 91ppm to 85ppm. They also brought CO down from .44% to .26%, and NOx from 20ppm to 6ppm. Changing wires COULD help, but I would think that a misfire would send lots of unused O2 into the exhaust. On the other hand, maybe wires should be changed just as a matter of keeping things ‘tuned up’.

As far as testing the oxygen sensor, I don’t know that my DMM is up for the job. And buying a better one would probably cost AT LEAST as much as a new Denso sensor shipped from Rock Auto. Anyway, we replaced the oxygen sensor back in 2012 since the old one was making the truck run bad at the time. When I asked my brother to get an O2 sensor, he went out and got the cheapest thing he could find - a universal Bosch sensor. You know, the kind that comes with the splicing block. This has worked as far as getting rid of the stutter. But smog numbers have grown steadily worse over the years. Given its age and mileage (~60000 miles), it may have seen better days. Perhaps a new direct fit Denso might not be a bad idea?

Another concern of mine is the EGR system. Those NOx numbers just look TOO low. I’m wondering whether there is somehow too much EGR, perhaps due to faulty EGR controls or a valve that is slightly sticking open. The engine doesn’t seem to run bad. But I’m wondering if there is just enough to drop NOx, but not enough to cause stumbling during the testing.

 

I have seen people have bad luck with the Bosch universal oxygen sensors. Now, like Scope103 and RJR, I am not one to just replace parts based on a wild guess, but getting a direct fit Denso oxygen sensor should give you a reliable baseline. If this were my truck, I would go ahead and get a new Denso oxygen sensor.

 

First things first. Your Denso O2 sensor 2344151 from RockAuto is about $46 plus shipping. A very good deal. This meter https://www.aliexpress.com/item/33004794265.html is really quite good; True RMS, 9999 count, non-contact voltage detection, temperature sensor, and a bar graph. $24 delivered.

But, the purpose of "testing" the O2 sensor is not just to tell if the sensor is bad. If you have a leak in the intake system, you may find that O2 sensor tends to indicate "lean" and have trouble getting the system to recover. Or if you have a sticky injector, you may see the O2 sensor swing to rich and back; while the system is trying to catch up your tailpipe emissions could increase.

Having said all that, and with the usual caveats about throwing parts, getting your direct-fit O2 sensor might help. And for the money, at some point, you can consider it a maintenance item.

Not to me. My last smog check was about 1 1/2 years ago, but my NOx was 6 and 4ppm. Yeah, I'm bragging (a little), but I think you just have a correctly-working EGR system. Kudos to you.

 

We got the truck to smog last week. Turns out the oxygen sensor was the problem, and it passed fine with a new Denso.

 

Congrats! And thanks for the report back.

 

Glad you got it to pass! While I don't normally support just replacing parts without a good diagnosis, sometimes, if the part is relatively cheap and easy to replace, the best diagnostic technique is a parts replacement.

 

This is particularly true if the part is known to be problematic (ie Bosch universal oxygen sensor, particularly if it is old and has lots of miles). This can also be the case if the proper equipment to test the part either exceeds the cost of the part and/or is difficult to obtain (like waiting weeks to get that aliexpress meter above shipped from China).