Maybe im posting prematurely here but i want to see what you guys think. I have this code on my 03 eco. Haven't checked for intake or exhaust leaks yet but i will today. Also will try to put it on a scanner. Anyways, i read alot of other threads to see what other people were seeing, seems most people noticed a loss of power at higher rpm. Im actually just the opposite. Car seems to accelerate slow, until right at 3000rpm, then it feels like im adding nitrous. The increase of power is substantial. What do you guys think?

Also, i previously was battling a map sensor code, that problem was solved by removing a very poorly installed programmer, and a new map sensor. That code went away now im getting this one. Cleared it twice. First time it took about 5 miles for it to come on, second time it took about 30min to come back on. Thanks for your help everyone!

I am going to start pointing fingers at the upstream o2 sensor. Seen this exact thing before and this was the problem. The upstream is in charge of fuel trim calculations and if it "lies" then the P0171 code appears.

Is there a way to test the o2 sensor other than a tech2? I really dont wana spend that much money on one and have it not be the problem.

Really a scope should be used to check cross counts or switching. I work for a o2 company and I will warn you that 80 percent of our warranty returns are not defective. Check out all codes that could lead to this condition. This would include artificial lean or rich conditions.

Well i checked the code again this morning. Was not the same one. This time i got P0420- Catalyst System Efficiency Below Threshold (Bank 1). Plugged cat? Or is this still possibly the o2 sensor?

I found this video, did what he did in the movie with the sensor in my hand on the bench. I never seen a change in voltage at all. To me that means its bad, unless i did something wrong?
http://www.youtube.com/watch?v=vdn4Dk5PSwc

It is most likely bad. The P0420 code is still pointing me to the o2-sensor.

Well i put about 150 miles on today and P0420 came on twice. I had my scanner with me so i cleared it right away both time. Also when the light came on the freeze frame data showed that the LTFT was at around 25%, which i know is really high. STFT was at 0%. Both times it came on while idling too, not moving.

freelancer, typically the 420 code would be a catalyst failure, especially in his case where the long term fuel trim has a trim higher than the short term. This is because either the catalyst has failed, or there is something causing the long term to be higher, typically we see cracked exhaust joints or a bad gasket to 2 adjoining pipes after the upstream would do this. To test this there are a few things we can do depending on your tooling that you have. What we need to check for a catalyst would be 2 things, back pressure in the exhaust and of filtration of the catalyst.

Procedure #1 Note the car's vaccum spec at idle, attach a vaccum guage to the manifold, note the vaccum at idle, and goose the throttle to 2500rpm and immediately let off, if the vaccum doe not return to the idle spec, there is back pressure in the exhaust, and with the code that we are talking about here a catalyst is in order.

Procedure #2 Requires a 5 gas analyzer, by monitoring the output of the exhaust we can tell by the gas that is high what part of the three way catalyst has failed, if you have an analyzer, let me know and I will tell you what part vs what gas would be excessive.

Basically, the short term fuel trim is not correcting because your afr is ok out of the engine, remember the long term does not adjust fueling! However if the ltft is correcting 25 percent, there is a problem between the catalyst and the downstream o2. 10 percent trim correction is normal by the way.

That video is somewhat accurate, however there are many failure points of a o2 sensor that this video would not pick up on, which is what worries me, as I teach nationally for an o2 sensor company and this is why the video did not impress me, as it was old and outdated.

If you have any other questions please post and I will try to asnwer them.

I do not have a gas analyzer but i do have a vacuum gauge. I will try to find time this weekend to look at that. Is there a certain spot in the manifold to read the vacuum? Or an easy spot to hook into? Thanks for your help.

the cat monitor was withheld from running while the p0171 code was set. When you cleared the dtc the catalyst monitor ran and failed. I would make sure the lean condition that set the p0171 is not present before diagnosing the p0420. A false p0420 is the concern. A borderline lean codition may be present that is not setting a lean code but is influencing the cat monitor. Look at fuel trim at idle and at 2500 rpm and see if its +/- 10%

do you have a CAI on the car?

As far as i know i do not. Is there a way to tell? I haven't checked vacuum yet its been a busy weekend. Also the cel hasn't come back on yet, but, it still has the sudden increase of power at 3000rpm.

Cold air Intake CAI...do you have the stock air filter and air cleaner box? Cheaper CAI system filters (dry and wet filters) can take out maf sensors and cause fuel trim havoc. Does the car run better at first start up when cold and at wide open throttle?

Ah...i thought that maybe that meant CA emissions or something. No everything is stock on the car, and i dont have a maf sensor either. It runs really good, idles perfect too. Just has increased power at 3000prm.

the oxygen sensor input is ignored at wide open throttle and at cold start up while its in open loop. If the car is running normal during these conditons it could be an 02 sensor. best way to diagnose is to view datastream. You could try disconnecting the upstream 02 sensor connector and see if it runs better.