My 2005 Mazda 3, 128,300 miles and manual transmission, has been running well for quite a while but a couple of weeks ago it started idling rough/low once warmed up and even stalled a few times when I was stopped at a light. It would drop below about 650-700rpms and either stall completely or almost stall, and then bounce back up, only to drop again. Since then, I've:

  1. Cleaned the throttle body by disconnecting the throttle body from the engine side and wiping the carbon off with a shop rag wetted with CRC TB cleaner. No noticeable difference.
  2. Cleaned the MAF with CRC MAF cleaner, then at the same time:
  3. Cleaned the throttle body again by going at it from the intake side as my brother held the gas pedal down to keep it open (electronic TB). After this and the MAF, the car idles low and rough, around 650rpms, but is no longer trying to stall.
  4. Cleaned the throttle body again in the hopes that maybe I missed some carbon and that was still causing the problem. This time I lightly sprayed some TB cleaner down and wiped it dry with a shop rag.

After cleaning the throttle body again my car started up a little rough (I assume because some TB cleaner was still in the system) and threw a CEL. Now the idle is somewhat better, around 675-750rpms I'd say, but still rough and, I think, too low. I drove it to AutoZone to get the CEL code and it came back with:

  1. P2195: HO2S (Front, RH) Signal Stuck Lean
  2. P2195: HO2S (Front, RH) Signal Stuck Lean (yes, the same code listed a second time)
  3. P0134: HO2S-11 (Bank 1 Sensor 1) Circuit No Activity Detected

AutoZone's recommendation is to replace the PCV valve. I had them clear the CEL but when I started the car up to drive home the CEL came on again.

Since I now have error codes I can do some more research into what other Mazda 3 drivers have experienced and what their solutions were but I'm wondering if anyone here has ever had a problem like this before. Thanks!

Edit: some non-mechanical things I'm considering doing just to see if they help.

  1. Running Techron through when I fill up next
  2. Switching from Shell gas to another brand, like Exxon or even the Quik-Mart down the street
  3. Filling up with a higher octane

3 Answers 3


Given that the codes were thrown after seperating the intake piping from the throttle body for the 3rd time, I think the most logical first step is looking for a vacuum issue.

If there is any air getting in after the MAF, the engine wouldn't be putting out enough gas for the amount of air it is getting. Which would explain the rough idle and why the o2 sensor is throwing lean. The intake piping could have developed further damage when it was moved for the (probably) first time in 13 years, worsening the issue. For help detecting a vacuum leak, I recommend the answers here or here.

A lot of times, o2 sensor codes are related to other problems, however it is possible that your o2 sensor has actually failed. This answer gives a good rundown of testing one, I think.

It could also be an issue with fuel or spark. If the injectors aren't doing enough injecting, or the sparkplugs aren't doing enough sparking, you could see rough idle conditions. I would expect one of these to be accompanied by other problems (sluggish acceleration, cylinder missfiring, problems beyond idle RPM, etc.)

Could it be the PCV? yea. You can usually clean it instead of buying a new one. I'd be looking for a vacuum leak first though. Usually liquid solutions aren't solutions, but your mileage may vary.


I have the exact same problem literally identical and it threw the same code. I talked to multiple people and some said to do exactly what you did but a few people pointed out the fact that o2 sensors lifespan is 90,000-100,000 miles and their main job is to keep the mixture of gas and oxygen correct. (If that goes out it could result in bad idle.) I ended up replacing the o2 sensor and cleaned the MAF sensor it has helped so much. I rarely have problems but just the other day it threw the MAF sensor code so I will be replacing that and will probably fix the lasting effects of the problem.


I was having a check engine light on my 2005 Mazda 3, I am in the Philippines brought it used and took it to a mechanic. They said it had various codes reset it and sent me on my way. Well my wife normally drives and I work out of the country so I was in and out. Well months and the car started running rough if I accelerated on the highway too fast the car wanted to shutoff. I looked on line and brought my own OBD scanner. Codes read 2187 and another one I don't remember now. The reading were for SHTFT and LNGFT lean. LNGFT was 14 or higher and SHTFT was above 14 or higher as well. I looked on youtube they advise using Carb cleaner or brake cleaner and spray around the vacuum hoses. I did but did not find the leak. I next tried soapy water (not a good idea but I found things). I found air blowing out the #1 spark plug. Only to find all 4 spark plugs less than finger tight. Done by the dealer when the wife took it in about this problem they change the spark plugs (shitty work). Tighten all the plugs. Had bubbles blowing on the right side forward side of the engine looking at the car forward to aft. There are 2 vacuum lines that run under there could not see where they went. Big problem the car runs worse, did I cause this by tighten the spark plugs? I went back to youtube, next thing to try was to use a homemade smoker. Well I made the smoker only to find that the 2 lines that made bubbles, only 1 was bad and the intake air shutter valve connection metal tubing was broken. Looked around at local supply stores no one had it. Went to mazda and they don't carry the part number told me to buy the next higher assembly which is the air intake at 900 dollars. I did not buy it I went home to think. So I decided to take the part to a radiator shop and have them seal the tubing back on the valve. Valve fixed and reinstalled on the car. Started Still running rough and the codes changed but SHTFT and LNGFT still bad, did the smoke test again. Found 3 lines leaking replaced hoses car runs better still high. Did another smoke test and with all the other leaks fixed I got smoke blowing out of a line down behind the air intake. I removed the air intake and found the PCV hose with two, 1 inch long tears on both sides of the hose. Replaced the hose, cleaned the air intake as best I could without completely removing it and reinstalled the air intake. All codes gone and car now runs like it should.

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