O2 sensor replaced and I still have the check engine light on! Please help

I replaced my upstream o2 sensor on Saturday, and put a little over 200 miles on it. I figured that would be more then enuf to cycle the ECU, but still nothing. I took it down to autozone and its still reading as the upstream o2 sensor.

 

Anyone else have that happen? Or have any advice? Thanks in advance.

Remove the negative battery cable for 60 seconds and push on the brake several times while it's removed. That should clear the ECM. If it doesn't, then perhaps your sensor was bad out of the box. It happens.

Or just have autozone clear the code, not just read it. If it comes back, you'll know...

Did you get the oem sensor? I had bought an aftermarket sensor and that didn't work. Bought the denso oem sensor and the engine light went away. This was the air/ratio sensor, which is the upstream...

What was the code that made you think that was the problem ?

 

I have a aftermarket in my rear O2 and it works fine.

I just took it back to autozone and they did the scan again. They said the code was

 

P0420

 

Catalyst efficiency below threshold-bank 1

 

Problem case

-rich a/f ratio

-ignition system malfunction

-faulty cat

 

I'm not sure where I should even start. I just got the car a few weeks ago and the guy said the resonator was removed, just after the turbo there's something that looks like a cat, so I assumed the resonator was the precat. Is that right? And could that have something to do with it?

 

Oh and the sensor I got is a OE bosh. Idk, I'll keep digging thanks again guys

It could be a few things. The 420 code is a tell tale sign that you might have a cat issue.

 

BUT...there are other things it can be. If you had an issue with the front O2, it might have thrown the A/F off some, and caused the rear O2 to falsely throw the code. Before you dig too much, reset the codes, if you havent already. See if the light comes back on.

 

It could be a bad O2 sensor, and that will probably be cheaper and easier than replacing the cat. Worth the gamble in my opinion...Unless you are planning to change your DP anyway.

 

Resonator usually refers to a pre-muffler muffler. At least in my experience. Cats are cats. Resonators usually reduce rasp and do some quieting before the muffler(s). A popular thing, even back in the day, to get a little more sound out of the car is to remove the resonator, and leave the muffler. What you see after the turbo is in fact the cat.

 

What you are dealing with is a pretty universal issue. Not really Subi LGT specific. I fought with my 2000 Jetta with this code for the longest time. In my instance, the cat was bad. Motor was using oil, and killed the cat, twice. Before the first cat went bad, it was a bad sensor at 120k miles.

 

So clear the code/reset the ECU, see if she pops back up. If it does, double check that no wires are melted or cut (since you said prev owner modified the exhaust some). If all looks good, Ide just pop a new sensor in. Downstream sensors can be had a good bit cheaper than the upstream in most cases, and Rockauto had good prices last I checked.

 

J.

Bank 1 is the front O2 sensor. You say you replaced it. It still could be a bad sensor.

 

Like ^ he said, check the wires first.

 

The Cat's are in the up pipe and down pipe on 05-06's. Under the car is the resonator in the mid pipe.

 

A lot of us remove the cat inthe up pipe, it has been knnown to come loose and take out the turbo.

 

I believe it's better to use a OEM O2 sensor for the front, and aftermarket for the rear.

 

That's what I have in my wagon

I agree with the choice of O2. I would NEVER use a questionable O2 brand on the upstream. You can get a Denso on Amazon for $85 shipped, and for that price, there is no reason not to.

 

For the rear, if you cant find a good price on an OE fit one, you could use a lesser brand or a universal. Just make sure you are comfortable working with wires. The rear O2 only reports Cat efficency (at least in all the cars Ive owned in the past). It should have no impact on how your car runs, and for that fact, I have never had a problem using a non-OE sensor in the back. On my Jetta I used the universal and it worked great and was 1/4 the cost of the OE downstream.

 

J.

^^^What they said. Also, if it's telling you that your cat is bad, there has to be a reason for it. Catalytic converter media will run for millions of miles without ever going bad on it's own but introduce some foreign matter (oil, coolant, transmission fluid etc.) and it will cause it to go bad quickly. Does the car use a lot of oil? Does it use any coolant between oil changes? Those things can ruin the cat and if you change it without fixing the cause of the failure, then you'll ruin a new cat in a short time and that's EXPENSIVE.

 

Good luck.

I'm not sure if its using oil or coolant, I just got it and only put a few hundred mils on it. I did change the oil when I got it, and the oil looked fine, just old. It was a little low in coolant, but nothing crazy.

 

I'm in the middle of doing my plugs now, and the two I've pulled out have oil on the threads. Bad rings? And possibly the oil burning killing the cat? I was going to do a compression check but don't know if I even want to bother with it now. It didn't seem to run ruff or smoke at all, but I don't see the oil coming from anywhere els. What are your thoughts? Thanks

Yeah it's on all four! Someone please tell me this is normal. I really don't feal like dropping the engine already. I've had the car a month.. Lmfao!

And all four

I was even thinking it could be the valve cover gaskets, but it doesn't seem like there's oil anywhere els besides the threads.

Do all the electrodes look the same-same color and dry? If so, MAYBE you are lucky and the PO used oil on the threads rather than anti seize when he last changed the plugs. There is really no way that oil could end up on the threads like that unless it was put there by someone, or you have a straight 4 cylinder and the plug holes have oil down in them and got on the threads when you took the plugs out.

 

Use a mirror and a flashlight and look down in the plug tube/hole area and see if there is oil down there. If there is, that could possibly be a valve cover leak around the seals but to have all 4 do it all the same, is a bit rare.

I've seen a little oil on the threads before on other cars. A little blow by mixed with high cylinder pressures can push the oil up into the threads. It just kind of seeps, the way oil likes to move. Then it really cant go anywhere. Doesn't burn, doesn't push out, just sits. Heck on an H4 you don't even have gravity helping to pull it back out haha.

 

I wouldn't get too panicked yet. If you want a piece of mind, do a compression and leak down. I plan to check mine, and my car has no issues. Its good to have a bench mark so if you do start having issues, you can track a worsening problem.

 

But stick to the basics for now. Compression/leakdown (while plugs are out), put your new plugs in, maybe clean the MAF, reset the ECu and see what you get.

 

Code for O2 pops back up, or if you see bad compression numbers, go from there. Just don't get overwhelmed. Its nothing magical, just an engine

Ok, thanks. I'll have to give that a try sometime this week. Thanks

FYI- The rear 02 is NOT strictly for cat efficiency. There is a feedback loop that tweaks the A/F ratios.. Running without a rear 02 can cause A/F ratio fluctuations and can be a pain to tune. Also, the car will tend to run rich.

Is this something Subaru included in their ECU map? Ive never seen a rear have any effect on fueling. Not saying it doesnt...or that it doesnt on other cars as well.

 

Interesting if it does! If that is in fact the case, my previous statement about using a cheapie on the rear wouldnt hold water to me. I would use OEM replacements, or at least good replacements, for both.

 

J.

Is this something Subaru included in their ECU map? Ive never seen a rear have any effect on fueling. Not saying it doesnt...or that it doesnt on other cars as well.

 

Interesting if it does! If that is in fact the case, my previous statement about using a cheapie on the rear wouldnt hold water to me. I would use OEM replacements, or at least good replacements, for both.

 

J.

 

From what I understand, its a Subaru thing. My tuner has been experimenting with the map trying to eliminate the rear 02 feedback. The car was running consistently rich and the fuel trims were erratic until this last revision where he was actually able to eliminate the rear 02.

OP are you sure the Front O2 sensor is a good one ?

 

Not a cheap on name brand