Hi everyone!
I have a 99 Dakota with 275k miles. I failed emissions (HC 1.57, NOx 4.36) after using “Guaranteed to Pass” preemptively after failing 2 years ago. I tried adding 1 gallon of denatured alcohol to about 10 gallons of gas for the retest and failed again with similar numbers. Any ideas? As far as I know, the catalytic converter is original so I’m thinking about maybe replacing it but figured I’d ask for ideas here first.
Thank you for any ideas!

 

Any codes? You might have lazy/failing O2 sensors. Back in they day, I used to retard the timing so it would run lean and pass but nowadays it's all computer. I've got 320K and cats are still good... never failed emissions but was burning a bit of oil. Point is, I finally replaced valve stem seals and no longer burn oil, which might contribute to bad emissions.

 

Thanks for the suggestions!
I did do a head gasket job about 50k miles ago. The heads were totally reconditioned at a professional machine shop. I burn about 1 quart of oil every 3000 miles or so. That consumption has not risen much at all in the last 100k miles.
Concerning the O2 sensors...you’re saying they may be getting bad but not yet throwing a code? That’d be an easy fix if that’s the case. My neighbor has a computer of some kind (a bit better than a simple code reader but certainly no Snap-On). Maybe I’ll see if that gives any insight on O2 sensor output.

 

I replaced mine last October, in an effort to improve my mpg. https://dodgeforum.com/forum/2nd-gen...-12-5-mpg.html

Can't say it did much, but I only paid 50 buck for a pair of Denso sensors on Amazon. You might want to try there if you decide to purchase. Also, that stuff you add to the tank, you do know you should go for a long hard drive when you put that stuff in? I'd say put at least 60-70 hiway miles on it (overdrive off to bring up the revs), then bring it straight to test center.

 

Being a 99 I'm assuming it's a 3.9 or 5.2?

Replace the upstream (pre-cat) sensor(s) with genuine NTK. Don't worry about the downstream yet.

 

Spot on with NTK 02 sensors as magnethead said.
Though these trucks are picky about using only oem sensors.Using a ecm code reader / monitor via bluetooth can tell you if your ready for retest an 02 readings an avoid repeated fails.

 

Thanks guys. It's the 3.9. I'll give the sensor a try and check into the most affordable yet reliable ecm reader. Any suggestions for that?

 

I use the Ancel 310

 

Yea many high end readers tell you a lot an can do a lot of things but a really great simple one I love is this bluetooth to phone reader an you can use many great free Apps too .It tells you the basics including current running information,02 readings , upstream, downstream , fuel trim,reset ecm, TEST readiness for tags[ saves retest cost $] an more..

 

Thanks Hidden. I'll check that out. It looks basic and easy to use.
To close the loop on this thread, I ended up putting in a new cat and it passed by a large margin. Thank you for all the suggestions!