Ok so I have reset the MIL twice and it keeps coming back. I think my cat is fine and more suspect that my primary and or secondary 02 sensor(s) has gone bad.

Looking for suggestions and comments. Here is why I think it is my 02 sensor.
-Swapped 01 header for RB due to favorable exchange.

-Bolts rusted on at triangle junction between upper mandifold and lower exhaust manifold. Bent 1/2 extensions. Had a shop loosen the muffler. Told them to hand tighten it because I was only going a short distance. They said they would fall off. I said yeah but if they do I dont need them.

-Proceed to drive to dental appointment and bolts fall off (LOL). Sound like a ricer and am getting dizzy from the fumes. With the Upper and lower sections separated the primary 02 sensor is exposed to outside air. I can smell the car start to run rich and I see a noticeable change in how quickly my fuel gauge goes down. MIL turns on.

-Leave dental office drive home swap headers. Get PB blast on to primary O2 sensor tip.

-Install but there is a leak at the RB to FM midpipe connection due to POS bolts that I bought that will not tighten all the way. (Threads are fine and matched).

-Friend reads ecu with his scanner. O2 sensors seem to oscillate fine (peak's and valleys for both o2 sensors). Clear code.


-Code returns, Borrow friends OBDII reader. Same p420. Erase it, returns.

-Tonight I erase it. But I expect it to return even after I fix the exhaust leak infront of cat.


Other reasons I suspect 02
**** I have had noticeable idle droop (my 99 has never suffered from it) in the mornings for the last 2 weeks. Goes away when car warms up (maybe heater gone bad.
My MPG has gone down over the last month. 250 miles between tanks to around 180~200.

what I have done so far and will try
-Cleaned plugs and plug wells
-Will check all header bolts and EGR in the AM
-Will fix precat exhaust leak.

Anything else I can check before I replace the primary 02 sensor? Any reason to suspect the cat vs the Primary and or 2nd 02 sensor or the exhaust leak?

 

I'm gonna go with precat exhaust leak.

 

I stopped reading at "both o2 sensors oscillate".

The precat is supposed to switch. The postcat is not. The code set criteria by definition is that the post cat signal starts to resemble the precat signal.

If this is caused by your exhaust leak I don't know, but I'd fix that and try again.

 

It is ok if the post cat O2 sensor inverts, the criteria is that the postcar O2 must do less than half of the inversions of the precar O2. If for example, the precat is doing 30 inversions per minute, and the postcat is doing 10, the system is ok. If the postcat is doing 15, it is marginal, 20 is failboat. 0 = less than half, so obviously ok.

 

dont some guys just tie the secondary o2 up under the car and plug the hole and then it wont throw the cel becaue it is getting "clean Air" or less that 15 as huslter has stated.

 

Get a couple of spark plug anti-foulers from the auto parts store and space the 2nd O2 sensor out of the exhaust stream.

 

In this case I'm assuming that it's really his pre cat that is starting to resemble the post cat signal because of his exhaust leak, making the computer think that the cat is doing nothing.

 

I have the same code, with an obvious precat leak. Fix leak.

 

LoL I fixed the exhaust leak, then I noticed the Primary 02 sensor had backed out quite a bit . I'm pretty sure that is where the problem was coming from.

would explain a lot.

 

came back on when I left my parents house. I suspect that was due to having been misreading the front 02 sensor for most of yesterday, and all this morning. Now that everything is buttoned up I will clear it and go from there.

Doesnt help my laptop crapped out on me and I'm reformatting it as we speak.

Anyone know any decent free OBDii software that maybe has gauges?

I was using this http://www.obd2crazy.com/wOBDv140/index.html but It is pretty rudimentary. I tried the scannet tools but the OBD reader my friend gave me is a cheap china Bluetooth unit and does not work well with the scannet.com tools. Would be neat if something had gauges or some sort of meter to take a look at what the 02 sensors instead of just the voltage.

 

It's your cat.

A below threshold code means that the post cat exhaust gasses, sniffed by the downstream sensor, are too similar to the upstream readings.

I don't know how old it is, but if it's older than ten years and you run rich, it's probably past its prime.

 

still do not think it is my cat. that would not explain the 50 or so mpg drop I have been seeing over the last month or so before the light came on.

 

Which would indicate that you are running rich. Running rich constantly can take out the cat. So it might still be your cat, on top of whatever is causing you to run rich.

 

correct, but you just restated what I said. I think the cat (if gone) is a symptom not the cause.

 

Fing autopartswarehouse sent two completely wrong 02 sensors. Different brand and part number from what I ordered. wont be able to fix this. No local autoparts stores have the 02 sensor I need and i do not feel like splicing.

 

It's the cat, but fix the running rich problem before you replace it and blow another one.

 

Got my 35~40 miles back after changing the front 02 sensor. check engine still giving p0420. I noticed this when i logged with an obd II scanner

is the rear o2 suppose to change voltage this drastically?
time Front Rear

9 0.72 0.15
16 0.78 0.055
23 0.13 0.145
30 0.625 0.145
36 0.53 0.78
43 0.755 0.8
50 0.735 0.64
56 0.14 0.79
63 0.12 0.825
70 0.705 0.07
76 0.08 0.2
83 0.745 0.755
90 0.78 0.74
96 0.805 0.015
103 0.7 0.655
109 0.62 0.285
116 0.63 0.33
123 0.11 0.81
129 0.205 0.825
136 0.41 0.74
143 0.08 0.79
149 0.085 0.195
156 0.695 0.495
162 0.64 0.165
169 0.215 0.07
175 0.03 0.75
182 0.01 0.19
189 0.205 0.785

 

No. You need a cat.

 

Bingo.

Time to order a new cat. I need to get measurements I might just get a metallic core.

 

Just remember it has to be a good enough cat to keep OBD2's mouth shut.