Question about OBD fault codes

Just recently bought a used 2011 Mazda 3. I started driving home after buying it, and the CEL came on. I got the codes ran and it had 10 codes that came up. I looked into it and I believe most of the codes aren’t actually true or are old codes that hadn’t been cleared after repair. If there is still something wrong with the car that caused it to throw the codes in the first place will those codes reappear? That way I know what is still wrong with the car and what codes were just temporary codes from before I bought it.

The CEL didn’t come on during the test drive ?
or when you had your mechanic do a pre purchase inspection ?

If the codes are cleared and the problem still exists the CEL will light and a code will be recorded.again.

The stored codes are valuable information that can help you determine the problem. Especially since you don’t know the history of the car. Write them down before you clear them.

Hi djm:
Welcome to the site.
Based on what you wrote, it looks like the following has happened:

  1. Before being sold, the car had problems that were triggering fault codes.
  2. The seller “cleared” the codes just to keep the light off long enough for the car to be sold.
  3. During your drive home, the vehicle’s normal self-tests identified the problem(s) again and set the fault codes.

Can you reply with a list of the codes? Also provide the mileage on the car.

Thank you.


MAF causing it to run lean was the main code that tripped the CEL. I would assume that would be caused from an aftermarket cold air intake. 3 codes were for wheel speed sensors and those I would say are due to the aftermarket wheels put on the car. Another code was for battery going under the voltage threshold. And 5 codes for different modules losing comms.

After clearing codes the only codes coming back now are MAF permanent and MAF freeze frame. All the other codes have not come back yet and I have been checking regularly.

No, it didn’t come on while test driving. And I’m 18 so my parents came with me to get the car and we didn’t bring a mechanic. I live in Va and we went to Greensboro NC for the weekend to get a car. The light came on while driving back to are rental house in NC after buying the car. I then had a 2 1/2 hour drive back to VA and the car ran great no problems driving it. That Is why I believe that they may have been old codes that had been resolved/repaired prior to my buying it and they just hadn’t been cleared. Which would also explain why the codes haven’t come back yet other then the MAF code.

If the CEL was not on during your test drive but did come on later when driving, it sounds like the codes were cleared prior to the car being sold and the vehicle identified the problems again, trigging the CEL.

Our suggestions:

  1. Write down the faults codes
  2. Clear the fault codes
  3. Keep a close eye on the vehicle to see if the CEL comes back on and address from there
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Only if the aftermarket CAI had a leak in it. Just a CAI alone would not cause this. Look for a cracked or torn hose or pipe or a leaking gasket or seal. Maybe hit the wrecking yards to snag a factory intake and filter housing and ditch the useless CAI.

No, not at all. The car doesn’t care what wheels are on it.

That may have been caused by the car sitting for a long while not being driven.

Please post the actual Pxxxx codes, please, not just the description.

Will do. I think you are probably right about them being cleared but it just doesn’t make sense why they would’ve all come on, then I get them cleared off and none of them come back except for the MAF.

Will definitely look into the leak. Could it possible be a sensor with the CAI?

Ok I wasn’t sure if the sensors had to be put back on after the wheels were installed and maybe they just installed 1 of the sensors because there is only 3 speed sensor codes for different wheels not 4.

I was thinking the same thing. I also was wondering if having a low voltage code could that cause the lost module comms codes since the battery was low? I know that dead batteries can cause them to lose comms.

Will do when I get the chance. I do not have the paper with all the codes with me right now but as soon as I do I will send them.

I do know that it is 2 codes for BCM one for lost comms not sure what the 2nd BCM was. Another code for lost comms with TCM. Etc.

I believe the BCM code was u0422-00.
ECM code was u0401-00.
TCM code was a u0101-00.

Just post the MAF codes for now. The comms codes are likely the result of low voltage… and the are gone now.

The CAI likely has only 2 sensors… the MAF and maybe a temperature sensor. The MAF is the concern, you don’t mention any temp error codes.

OK , it is none of my business but curiosity makes me ask . Why would you travel 2 1/2 hours , rent a place for the weekend to buy a used vehicle ?

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MAF code was P0171
Glad that the comms are probably from the low voltage I figured it was but wasn’t sure. And since the codes haven’t come back I figured it would be that.

No there isn’t anything about temp errors. So it could possible be the MAF sensor with the CAI…Like possibly dirty or just needs to be replaced?

Because my family was going on a small vacation because we are selling my house and had to be out of the house for the weekend while it was being shown to potential buyers. So we decided to go to NC to get another car for me because I sadly totaled my last car about a month ago. And Greensboro has a ton of used dealerships for decent prices.

There I fixed for you .

My bad, thanks for fixing it.

A P0171 is NOT a MAF code, it is a lean air/fuel mixture code… Read about it here;

The possible causes are, in order of likelihood;

  • Intake air leaks
  • Faulty front heated oxygen sensor
  • Ignition misfiring
  • Faulty fuel injectors
  • Exhaust gas leaks
  • Incorrect fuel pressure
  • Lack of fuel
  • Faulty Mass Air Flow (MAF) sensor
  • Incorrect Positive Crankcase Ventilation (PCV) hose connection

Notice the MAF sensor is WAY down the list and intake leaks (between the MAF and the throttle) is at the top. Cleaning the MAF is OK, so go right ahead but I doubt it will solve it. As I posted… Do this first;

Unlikely the O2 sensor would throw only a P0171 without another code. Same for ignition misfire and no P030xx codes. Faulty injectors could be a problem… but again… a misfire code would likely be set as well. Might be incorrect fuel pressure or lack of fuel from a failing pump. If that were the case the code would not appear driving modestly around town but would appear at full throttle highway acceleration.

And then we get to MAF and PCV hose connection. Check the PCV, maybe the pervious owners CAI install buggered the PCV hose connection.

For the MAF code, cleaning the MAF may get the light to turn off, but it may take a few drive cycles. If it’s an oiled filter on the CAI, people can over oil them and the MAF sensor wire can get a little oil on it and foul up the reading. I would clean the MAF (with MAF sensor cleaner), reinstall the intake tubing, clear the codes, and see if codes come back. I’d prefer to just put the factory system back on and take the CAI out of the equation, if possible.

I cleared the p0171 code again and drove it for about 20 mins and the CEL still hasn’t come back. The permanent code is still there but not the freeze frame or pending. Last time I cleared that code it came back within 5 mins of driving. I’ll drive it around some more after work but I don’t know if the car just sitting for a bit made a bunch of codes show up or what. I think it’s kinda wierd that I haven’t fixed anything and the codes/CEL aren’t coming back. Leads me to believe that nothing is actually wrong but that might just be the fact that I don’t want anything to be wrong with it.

There is the list of codes it originally had. Also from my last post sent about clearing the remaining P0171 codes and the CEL not coming back on. The permanent code was still there for a while but no CEL and then just when I got home the permanent code is gone now. I guess after running the car some more it went away. So now there are no codes and no CEL.

I would assume that it was just the fact the car had sat the the dealership for a bit and was just throwing some codes that weren’t actually a real problem so after driving it a bit they went away.

Do you think I should still be worried about any of it?