OBD II codes P0171 & P0174

[DietDrThunder]

The "service engine soon" has been periodically coming on and turning off. Well it came on again today and I went to Autozone and they gave me the codes. P0171 and P0174.

He told me that since both fuel trim banks have detected a rich/lean condition that the most likely cause was the Mass Airflow Sensor. The other possible cause is the oxygen sensor. I went ahead and replaced the MAF sensor in the parking lot of the Autozone. The guy said it would be better if he didn't reset the codes and that it will reset on their own.

My questions are:

1) is this a correct diagnosis?
2) how many miles will I have to drive before the "service engine soon" light resets itself?

 

[LordMorpheus]

I didn't know they had OBDII on Corvairs ...

On a more serious note, my Lincoln threw lean codes on both cylinder banks a while ago (I think they were the same codes, actually). Cleaning my MAF sensor with a ~10 dollar can of aerosol MAF cleaner fixed the problem. As I understand it, the MAF is the go-to diagnosis for check-engine-codes that have to do with improper fuel/air mixtures, so the Autozone guy wasn't feeding you any bullshit (until he sold you a new MAF rather than the cleaner).

You said the light was turning on-and-off for a while, it won't take longer than one of those cycles for the code to clear.

 

[DietDrThunder]

Yea, you have to plug the obd scanner directly to the oil pressure switch for it to work.

Great, $80 down the drain instead of $10.

 

[Vette73]

You have a K&N (or similiar) filter?


Those codes mean you are running lean. That could be a vacuum leak or sensor issue. I would have done some more work before replacing the MAF.

 

[LordMorpheus]

You have a K&N (or similiar) filter?


Those codes mean you are running lean. That could be a vacuum leak or sensor issue. I would have done some more work before replacing the MAF.

Actually, this is true. We need a little more info. The lean code means you're getting more air into the engine than the computer is aware of. If the car is running fine but throwing a code then the discrepancy is pretty small and I'd suspect that the crud buildup on the MAF is causing it (or a minor vacuum leak). If the car is running like crap, then either the MAF gave up the ghost entirely (and needed replacing) or you've got a big vacuum leak letting unmetered air into the engine.

 

[DietDrThunder]

No K&N filter.

Yes, it is running fine, no idling problems or misfiring that I can detect and I do tromp on the gas pedal to get this multi ton vehicle to highway speeds quickly, so you'd think I'd notice (plus the V8 sounds great when you floor it). There has been no change in gas mileage.

This is why I hate modern vehicles. It is all guess work. I only drive the 2003 trailblazer (70,000 miles) for vacations and long trips (the Corvair is just too small for 3 people and luggage). If I didn't have to get it inspected this month (which the code didn't come on all of december, but did come on a couple of times in November). I probably wouldn't have rushed to buy the part.

 

[LordMorpheus]

No K&N filter.

Yes, it is running fine, no idling problems or misfiring that I can detect and I do tromp on the gas pedal to get this multi ton vehicle to highway speeds quickly, so you'd think I'd notice (plus the V8 sounds great when you floor it). There has been no change in gas mileage.

This is why I hate modern vehicles. It is all guess work. I only drive the 2003 trailblazer (70,000 miles) for vacations and long trips (the Corvair is just too small for 3 people and luggage). If I didn't have to get it inspected this month (which the code didn't come on all of december, but did come on a couple of times in November). I probably wouldn't have rushed to buy the part.

Well that sounds like the MAF so hopefully it'll be fixed now.

I have mixed feelings on modern cars - the ODBII stuff can make it really easy to locate problems. Maybe that's why I like my car so much, as it's firmly between two eras.