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Richard, Dodge Technician
Category: Dodge
Satisfied Customers: 4141
Experience:  Over 26 years of experience as lead tech and shop foreman.
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99 Dodge Dakota: V6 3.9L..check engine light..catalytic converter

Customer Question

I have a 99 Dodge Dakota, V6 3.9L, with a manual transmission. It has been idling rough, and I periodically have the check engine light come on, but go off randomly. It was only outputting the code P0420 when the check engine light was on, which says the catalytic converter isn't working properly, based on the O2 sensors. Randomly as well, when I have the clutch depressed, or just the trans. in neutral, it will idle around 1500 RPM. At other times, it will idle around 750 RPM.

Could this be a problem with an O2 sensor, or something more. Any insight is appreciated!!! Thanks for your help!
Submitted: 8 years ago.
Category: Dodge
Expert:  Richard replied 8 years ago.

Hello and welcome to Just Answer!

The O2 sensor will not cause the engine to high idle like that! The O2 sensors can however cause the P0420 code to come up.

Fault code P0420 can be caused by a faulty catalytic converter, exhaust leak, engine burning oil or faulty O2 sensors. These engines were bad for breaking exhaust manifold bolts and leaking intake pan gaskets. The first thing I would check is to see if there is an exhaust leak at both manifolds. Check for broken studs or bolts.

Then check for the intake leak by pulling out the pcv valve out of the valve cover and leave it attached to the hose and plug the valve cover hole that the pcv came out of. Then pull the breather hose off the air cleaner that goes to the left valve cover and start the engine. Then hold your thumb over the hose that came off the air cleaner for 5 seconds and then release. You should feel crankcase pressure and not vacuum. When you release your thumb you should be able to see the blow by come out. Then let me know what you find. Thanks, Richard

Customer: replied 8 years ago.
I'm sorry. I haven't had a chance to look under the hood and check the two things you stated above. However, I do have some information that I failed to mention before. Sometimes when I go to shift from first to second, when I depress the clutch, I hear a sound like a sudden rush of exhaust coming out. I read someone describe a problem with their vehicle saying it sounded like a fart, and that is the best way to describe it. Additionally, the exhaust seems to smell rich of unburnt fuel.

Could an exhaust leak prior to the cat cause the rough and periodic high idle, as well as the P0420 code?

I will check the two things as soon as I can. Thank you!
Expert:  Richard replied 8 years ago.

I have never seen an exhaust leak before the cat cause a periodic high idle. I have seen exhaust leaks cause rough idle if it is before the upstream O2 sensor because it causes it to run rich and kinda choke.

Does your truck have the same power it used to it? I have seen quite a few catalytic converters get loose inside and turn side ways which will cause a lack of power and a rush of exhaust sound, plus it will set the P0420 code. Let me know when you get a chance to check these things. The cat will usuall rattle for awhile before it is loose enough to turn sideways. Thanks, Richard

Customer: replied 8 years ago.
I just checked over the exhaust manifold, and just from a visual inspection from over the engine, and under, it appears to be okay. There are no broken bolts or studs as far as I can tell. I won't be able to check the other part until tomorrow. But, to answer your other question, it seems to have slightly less power than when I first got it (before I was getting the check engine light). However, I have only owned this vehicle for about 2.5 months.

Since I purchased it, I had to replace the exhaust from the cat back (I don't know if changing that would affect the cat, by stirring up soot or something?)

I'll let you know the results of the other test tomorrow after I perform them.
Expert:  Richard replied 8 years ago.
A good way to check the exhaust is to put 10 psi regulated air pressure in the exhaust through the tail pipe, but you need a special tool to do this or you would have to make something up. Then use a water/soap mixture and spray it on all possible leak points on a cold engine. I even have a digital borescope to insert through the O2 sensor hole to see if the converter is damaged. These are all the things that I do when I come across that fault code. Let me know when you get around to checking it. Thanks, Richard
Customer: replied 8 years ago.
Okay, I just checked the intake by removing the PCV valved and the breather hose as you specified. I held my finger over the breather hose for 5 seconds, felt pressure, and saw the blow-by as you said I should.

I haven't been able to check the exhaust for leaks with the constant air pressure since I don't have a source for a regulated 10psi.

Also, last night, my check engine light turned off. I noticed that it seemed to idle a little smoother. Still rough, but certainly not as rough as before. Each time the light turns on, it getts rougher, and when the light is off, it isn't quite as bad.
Expert:  Richard replied 8 years ago.
When ever your engine light turns off for that code it has to see 3 good trips in order to turn the light off. Which means it has to pass the test that it runs for cat efficiency three times. It runs the test once per key cycle. This is an intermittent problem because it passed the test. It sounds more like O2 sensors that are getting lazy but it is hard to say without replacing them and finding out. Thanks, Richard
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Customer: replied 8 years ago.
Well, I thank you very much for your help. I will go get some new O2 sensors and see if that helps tomorrow. I appreciate all your advice, and you walking me through the steps to solve this problem. I'll let you know if the O2 sensors work, or if I continue to have the problem. Just for curiosity's sake, what would be you next guess next to the O2 sensors, assuming those are fine?

Thank you,
Expert:  Richard replied 8 years ago.

Exhaust leaks can sometimes be intermittent depending on the engine temperature at the time of testing. Exhaust leaks at the manifold will sometimes seal up when hot.

When you get under the truck to change the O2 sensors bang on the catalytic converter to see if it rattles inside. Let me know how you make out! I hope the O2s solve it. Thankyou, Richard

Customer: replied 8 years ago.
Should I be changing both of the O2 sensors, or just the downstream one since that's the one that will throw the p0420 code? Are you thinking the upstream one is fading too since it's running a bit rich and rough? Also, these symptoms tend to be seen after driving for a bit, when the engine is at full, standard operating temperature. So if the leak would generally seal with the expansion, then that could somewhat be ruled out.

I tried to do the downstream sensor today, but it was all seized up. I tried doing it with the exhaust warm, and tried heating up the joint even more with no joy. I'm letting it soak tonight with PB Blaster, and hopefully tomorrow i'll have some better luck.

Seems like nothing on a vehicle is made of rust resistant material.
Expert:  Richard replied 8 years ago.

It is always recommended to change both O2 sensors since this code can set by using one new sensor and one old sensor. Both O2 sensors have to work properly or this code could still set. The signals from both sensors are used for testing the converter. When these sensors do not come out I cut them off and use my power hack saw and cut through the threads in two places oposite of each other and dig the thread out. Sometimes I have good success that way if you can get at them. Sometimes you have to break the sensor off and use a 6 point socket. When you get it out and if the threads are damaged you will have to use a O2 sensor tap to run through threads to clean them up. Good luck with replacing them! Let me know how this goes!

Customer: replied 8 years ago.
I can't seem to get the old ones off. You said you cut through the threads on the sensor. Do you drop the exhaust to get to it? I don't think there's enough room to get to them with exhaust system installed.

Either way, I clipped the wires on the downstream sensor so i could use a 6 pt. socket, which will clearly cause the check engine light to turn on. but, if I understand it correctly, a malfunctioning o2 sensor will cause the computer to use it's default air/fuel mixture instead of relying on the o2 sensor. It seems to be running much smoother than before.
Expert:  Richard replied 8 years ago.

I can't remember if I ever had to drop the exhaust on a Dakota to remove seized O2 sensors before. Sometimes using an acetylene torch to heat up the piece that the O2 screws into will help remove them but it can also cause the pipe to twist in that area. If you can't remove them you will have to drop the exhaust to be able to remove them. With the exhaust removed you can use an impact on the O2s to see if that will help. I mentioned before that sometimes I cut the threads out. If you could grind the O2 off without nicking the gasket surface and knock the center out you could use a reciprocating saw and a small metal blade to cut through the threads in 2 places and pop the threads out. I am sorry I can't be more of a help with this but when they are seized it can be very difficult to remove.

When you cut the wires the O2 sensor signal to the PCM goes to .5 volt on a 0-1 scale.

The air fuel ratio will not be fine tuned because it is already at .5 volt and that is what the PCMs goal voltage is so it doesn't do anything about it to try and correct it. Let me know how you make out! Good luck! Thanks, Richard

Customer: replied 8 years ago.
Okay, so mixed news. Sorry it took so long, but I was away this weekend and wasn't able to get those O2 sensors switched until yesterday. I got them in, cleared the codes, and it ran great! Seemed to idle much smoother. Then, today, after driving it to work an hour away, and then halfway back, the check engine light came back on, and seemed to idle rougher again. It's outputing the single P0420 code again. It does, however seem to smell less rich.

Any ideas now, or am I stuck with having to replace the cat?