Check Engine Light After Air Filter Replacement

I decided to try a K&N filter after hearing my friends rave about them. I forked out the nearly $50.00 and installed the pre-oiled filter with hopes that I would gain at least a few mpg’s. After a few weeks I noticed my Check Engine light turned on although the truck seemed to be running fine. Since the dealer where I purchased the truck was simply too busy to look at the problem, I purchased a code scanner and discovered that both oxygen sensors in front of the catalytic converter were faulting (I had 33k miles on my truck at the time). This seemed rather strange, and I suspected that something upstream could be causing the problem. After a little searching on Google, I discovered that several other folks had experienced issues with the check engine light after installing a K&N filter. Some recommend cleaning the air flow sensor, which could be covered with oil. I didn’t find any evidence of oil but decided to clean it with a spray for this purpose anyways. I’ve also replaced the filter with a conventional air filter. This didn’t really change the symptoms. Would anyone have any recommendation on where I should look next? There has been a huge debate in several forums as to whether the K&N filter actually caused the problem, but I’ll try to keep an open mind.

What codes came up after installing that so called air filter?


First, put the stock setup back on.
Does the problem go away? If so then drive on.
K&N will let a wee bit more air in; and more dirt to go with it.
Oiled cotton gauze is obsolete, IMHO.
You don’t say what year your vehicle is, but most vehicles of the last 20+ years have a pretty well designed intake system; no significant restriction.Modify the rest of the engine to suck in a lot more air and an aftermarket setup starts to make sense.

You do need to report the very exact and specific codes in the “P0123” format.

You first reported no symptoms other than the check engine light/codes. As you said “although the truck seemed to be running fine” So by “symptoms” do you just mean that the light is still on? Many codes don’t just clear that the specific moment something is fixed. The computer waits a certain number of drive cycles without seeing anomaly. So clear the codes and see if they come back.

If they do, then report the specific codes. And like circuitsmith said, you won’t get anything out of a K&N so you might as well just go back to stock.

While it is true that reusable air filters contaminate mass air flow sensors, it usually takes awhile like three months or more.

It is possible that the problems are not related to the air filter and just a coincidence.

Catalytic converter fault is dealt with last. Start by replacing oxygen sensors that are indicating fault. Proceed one repair at a time, don’t try to fix everything at once in case the codes are false for some reason.

Tell us the year, make, model and engine size of your truck.

I’ve used a K&N filter for about 90,000 miles now on my truck. I just recently got the out-of-range code common to over-oiled filters. I was towing a heavy trailer and was ingesting lots of air so the code wasn’t surprising. The MAF will burn off the contaminants if they aren’t too bad (mine did).

The excess filter oil will not make it through to the O2 sensors. There can’t be enough on the filter to load up the O2 sensors.

Is it a failed O2 sensor or a failed HEATER for the O2 sensor? The heater portion fails way more often that the O2 part. Since both failed at the same time, I’d suspect an electrical problem rather than a sensor but I don’t know the exact code you saw. If they both really did fail, you should have a significant oil consumption problem or they were defective as manufactured. Either way both are still under warranty as they are emission related equipment.

If both o2 manifold sensors are bad than u may have a vacuum leak. Did u put in a k&n filter element and reuse stock housing? I assume so for $50. I could see a torn intake plenum after maf causing bad o2 issues. IF u have code for lean condition. There are many o2 codes.

I’ve heard of K&N air filters doing this but I’ve never experienced it myself. It’s probably because I have never believed that the cost associated with a K&N filter would be worth it in the long run.

As far as I can tell, the only advantage to K&N filters is that they have a better marketing department.

My guess is the O2 sensors are ok, but their readings are confusing the ECM b/c the new filter has changed the airflow characteristics to the MAF. Suggest to replace things as they were, buy a new air filter of the same type as the original if necessary. The drive the truck normally & see if this fixes the check engine light problem. It may take several weeks of driving before the computer realizes the MAF and O2 sensors are reporting consistent data again, and for it to turn off the CEL.

Thanks for all of the responses, and sorry it took so long for me to write back. Somehow I failed to mention that this is a Ford F-150 with a 4.6L V8. The codes are P0130 and P0150. After about 3 drive cycles the check engine light will go away. I have noticed that it does take longer for the light to come on again.

I also failed to mention that I’ve noticed this seems to happen when I am slowing to a stop. That is when the check engine light displays. The codes have been the same.