Hey everyone,

My CEL came on the other day while I was driving home. I got the codes scanned and I’m getting a P0171 (Lean mixture) code.

The thing is, the car seems to be running just fine. The only possible symptom I’ve noticed is the engine seemed to hesitate for a moment the other day after I had shifted into second gear. It was very subtle, didn’t feel like a misfire (how do you coil pack go bad last year, so I know what that feels like). It was like it started to accelerate, then lost a bit of power for a fraction of a second. It was so subtle that I assumed I had just imagined it, until my light came on later that day. Aside from that one off event, everything else is fine. Fuel economy is sitting right at 39, idle is nice and smooth, and the engine produces plenty of power throughout the entire rpm range (MT). No hissing noises from the engine, so if there is a vacuum leak then it’s very small.

Considering that the car has 230k miles, my first suspicion is a gunked up fuel filter. I have a bad habit of driving an extra 20 or 30 miles once the gas light comes on (gotta save that 3¢/gallon lol), and I’ve certainly refueled at my fair share of shady dilapidated gas stations. It would seem to me that a dirty fuel filter would cause intermittent problems, but not enough to severely hinder engine performance.

My other suspicion is a bad O2 sensor. I bought the car at 114k miles, so it’s likely the original one from the factory. I have a multimeter and an oscilloscope, so if there is a way to test the sensor, then I’m certainly willing to do that.

Could also be the MAF sensor, but I’m not sure how to go about testing that.

if anyone has suggestions for what I should be testing, I’d much appreciate them. I know I could just take it to a shop, but I am very strapped for money right now, so I would prefer to do this myself if possible. Plus I just enjoy working on my car

 

Don't think the Fit has a fuel filter...

 

I had a leaky fuel like that was dripping extra gas into the intake. It gave me a P0172 code for too rich a fuel mix. Your lean so check MAP, MAF, pcv valve, fuel pressure, air filter, intake seals, so it could be one of many things that can effect fuel mix. Search Google for a more complete list. Fuel filter is checked by checking fuel pressure, if low, then maybe the pump or filter clogged (its big so probably not filter). Its under the center console in the tank. Best of luck, Clay

 

I would check the fuel pressure first, Standard is 47-54 psi at idle. You have to cut the fuel line near the fuel rail and splice in your pressure gauge. The is no valve to quickly plug into to get fuel pressure readings. Make sure you repair the line with a new piece of fuel hose. I used an old piece of fuel line that leaked into the intake and gave me a rich error code! You may need a piece of steel fuel line to reattach the long rubber line going to the firewall on its way to the tank. Use good clamps too. The kind for high pressure fuel lines. You may want to bring it to a mechanic if you're not a good wrench yourself. So, to just check the fuel pressure you need: pressure gauge (cheap at Harbor Freight), a foot of high-pressure fuel line, a piece of steel fuel line to splice the long rubber line to the firewall to the fuel rail, good high-pressure clamps to repair the line. If it's too much, call a mechanic. Don't use crappy old fuel line. Best of luck, Clay

 

One of the clips on the air filter box has been broken for a few years. It seems to make a good seal anyway, but perhaps that could cause issues if air is getting in where it shouldn't.

I'll get a pressure gauge and some high-pressure fuel line+clamps and test the fuel pressure this weekend. Might just replace the MAF sensor as well, considering how easy it is to replace. Might also disconnect the battery to clear the codes and see if the CEL comes back on. Like I said, it runs perfectly fine, so until I notice any odd behavior I think it will be fine to drive for a while.

 

If the car is giving you a code, then you have a problem. Maybe try fixing the box first, clear the codes and then see if the code reappears. Might try that before you cut the fuel line to test the fuel pressure. The replacement MAF sensor may be of questionable quality so I would test the fuel pressure before flying blind to the MAF. You can try both but you won't know what is going on right, since it seems to run good now?

 

The loose intake box may be your problem. On mine, it snaps shut tight. If yours is loose, too much air is getting into the intake causing a lean code. Really could be your problem. This is a small engine that needs precise control of fuel and air to run perfectly. It may seem like a waste of time to track down the bits needed, but it's the best way to go.

There is special spray cleaner used for MAF sensors, make sure it's cold (it heats up when ignition is on) and spray it down good, let it dry a few minutes, and then reinstall. The original should be fine.

 

Well in a twist of good fortune the CEL turned off on my way to my first jobsite this morning. I'll just chalk it up to a bad batch of gas, and keep an eye out for any unusual engine behavior going forward.

 

Well, if it comes back you know where to start now. Clay

 

So I think I finally fixed it. Replaced the intake manifold gasket, EGR valve gasket, inlet manifold gasket, and inlet gaskets (all the gaskets that come in the kit aside from the throttle body gasket which I couldn’t be bothered to replace that day). That was last August, and the light had been off for a month or so by that point anyway, as the weather was quite warm. The light remained off until mid October when it came on again, and remained on pretty much all winter. As the weather got warmer this spring, the light turned off for a day or so, but then came back on. The hesitation under acceleration was also getting a lot worse.

After a failed emissions inspection, I decided I had to get serious about addressing this. On a whim, I decided to replace the PCV valve and the rubber hose coming out of it the other day. The old valve was still rattling so it clearly wasn’t stuck, but I figured it’s a $20 part and could very well have been the original one from the factory, which by now had 275,000 miles on it. So it was overdue for replacement anyway. Well lo and behold, I took it for a test drive right after replacing it, and the power hiccups were completely gone. The light remained on as it would need a few drive cycles to clear, and this afternoon it finally went away. I won’t get my hopes up quite yet, but seeing as the engine seems to be performing better now I think I can say the PCV valve and/or hose were the culprits. I’ll report back if the light comes on again.

 

Glad you found out the problem… now remember… The PCV is recommended to be replaced once a year if I’m not mistaken…

 

Never replaced a PCV on my cars. All were kept for a long time (10-20 years) and for around 200k km.

 

Well good news and bad news. Good news is the P0171 hasn’t come back yet. Bad news is my CEL came on this morning with a P0420. So I’m thinking it’s likely the upstream O2/AFR sensor. At least, that’s what I’m hoping the issue is because I really don’t want to buy a new cat.

Can anyone tell me if the Denso 2349077 is OEM (or at least close enough to OEM)? Honda OEM suppliers are charging $550+ for the Honda part 36531-RB0-003, and I really don’t want to spend that much unless I absolutely have to. Plus if this doesn’t fix it then I’ll almost certainly need a new cat, and that’s not something I can afford if I blow half a grand on an overpriced O2 sensor. But I also want to make sure the one I install is going to work and continue to work for a reasonable amount of time. I’ve read that NTK makes the OEM sensor for the Fit, but RockAuto doesn’t have any NTK upstream sensors. The Denso one is the one that looks most similar to the OEM part.

 

Avoid Bosch O2 sensors for sure. Had problems when I used one on one of my Honda.

 

I ordered the Denso ones. General consensus seems to be NTK/NGK or Denso.

If these new sensors don’t fix it, is there a recommendation for the cat? Seems that aftermarket cats don’t last nearly as long, so I’m thinking about finding a used OEM cat from a local scrap yard or eBay.

 

Or you could go aftermarket and add an O2 spacer to foul the reading.

Did that on an older Civic; would have been crazy to put a new car on an old car like that.

But you can only do that on the downstream sensor. The upstream need to read the real values as it adjust the air-fuel ratio.