Figuring Out Car Emissions Level

Hello Everyone,

I am a resident of Georgia who will soon be moving back to Tennessee. I have an older car that still runs great but always spits out System Too Lean and other emissions related codes. Years ago, I swapped out o2 Sensor, then Mass Flow, then the Caty, but the Check Engine Light always came back on, within a 100 miles or so of re-setting. (It now comes back on within 10 miles.)

The car gets 30+ mpg and barely loses any oil. I ended up asking within another post about an easy way to truly measure emissions but didn’t get much reply, so I thought a dedicated thread would be worth it. (FYI, in Georgia, they only pull codes at inspection stations. They never put something directly on the tail pipe. And in TN, they’ve ended inspections altogether.)

Someone has already suggested the Georgia Clean Air Force site, but again, that’s just for getting you to an inspection location that only pulls codes. Someone else has also suggested buying a personal reading device and researching to find emissions levels across various vehicles. I’m looking for something more stream-lined, like a chain that charges $50 to 100 fee then gives you a read-out. Does that just not exist?



Those sites used to exist when tailpipe emissions were tested and cars had fewer sensors. There is no need anymore because a Check Engine Light tells you when the emissions are excessive.

You should be able to find a shop with a rolling road dynomometer with a tailpipe sniffer. Expect this to run $150 or more with a 1 hour minimum charge. But why do you want this? What would you do with this information?

You can get codes read free at AutoZone or O’Reilly’s. You can borrow readers from both places and read out the error codes as well as take snapshots of various parameters while running. The car is TELLING you its problem symptoms and you just threw parts at it failing to fix it because you don’t know how to interpret that data nor diagnose the problems based on the codes. That won’t lead to a solution. Knowing the tailpipe emissions won’t either

Take it to a shop that can diagnose the problems based on the error codes. Expect to pay for their time. $150 to $200 would be typical. Then you can decide what you need to do.


With out reading everything, just letting you know that Tennessee NO LONGER has Marta (emissions testing) as of 2022

As of January 14, 2022, residents in Hamilton, Williamson, Rutherford, Sumner, and Wilson counties may now register or renew their registration without an emissions testing certificate. Davidson County’s requirement for vehicle emissions testing ended on February 4, 2022.

Besides that, as long as your check engine light was off and all readiness monitors were all set and your gas cap passed then you passed Marta (emissions testing)… But again, NO MORE Marta!!!..
“celebrate good times come on”… lol

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Thanks, I should have been clear. I didn’t just swap the parts out. Those were replaced by my mechanic at the time, who I trusted to make the right choice. I have since moved on, based largely on their failure to diagnose the problem correctly.

Some auto parts stores will read the error codes from your car for free.
Try to do that and post them here. Then we can help.

The PCM fault code for fuel system lean is P0171.

The PCM should have also recorded freeze frame data for that fault: input and output values at the time the fault occurred. A review of this data will help to identify the cause of the lean condition. Lean mixture at idle can be caused by a vacuum leak. A lean condition under load can be caused by a weak fuel pump. Proper analysis is where to start.

A tailpipe emission test won’t show much, the PCM is compensating for the lean condition, the tailpipe emissions may not be affected.


Might help if you told us whether your car is obd 1 or obd 2 (1995 model years +). It’s going to be difficult and/or expensive to get a tailpipe emissions reading I expect. If your car has obd 2 technology, your best bet to assess the severity of the problem is a fuel trim test. The car’s computer can make the necessary adjustments to the fuel-control to accommodate problems resulting in fuel trims in the +/-10% range. So if less than 10%, no worries. Beyond 10%, some diagnostic & repair work is probably gonna be required.

Common causes of too lean

  • vacuum hose connection is broken and leaking air
  • pcv system malfunction
  • brake booster diaphragm is leaking air
  • fuel pressure is too low
  • fuel injectors are clogged
  • intake air system leak is confusing maf sensor. ripped intake boot for example.
  • exhaust leak is confusing o2 sensor.

Feeling lucky? If your engine sports a fuel pressure regulator located at the fuel rail, remove its vacuum hose. If there is any gasoline inside the vacuum hose, the fpr has sprung a leak & must be replaced.

Moving to Tennessee? I recently had occassion to Google-drive through one of the smaller towns there. Looked like pretty nice place to live. Had everything a person could want, McDonalds, other fast foods, car repair shops, etc etc. The main drag had a two-story building on the left side of the road with the sign “Smoke House” on it, presumably a fancy restaurant. Opposite side of the road, a small strip mall with store called “The Smoke Shop”. So you can dine on smoked meats, and buy your cigarette smoking products at the same time, just by walking across the street! … lol …

Thanks for this…I just read up on the System Too Lean/ Rich codes. I ALWAYS thought it was just an issue of worse emissions with such codes, but now I see if it’s really out of whack, it can do some real damage to your engine.

That being said, these Lean codes have been spitting out for at least 50K miles, and I’ve never had an issue with power loss or fuel efficiency (by way of reminder, it is still running great today.) Doesn’t that mean that it must be in that 10% or less range? And that OBD is just letting me know it’s having to make that correction?

Or (and forgive me if this is blasphemous to suggest) the 23 year old computer is giving an error message…in error? A replaced senor never was correctly recognized or communicating properly with OBD? If that’s an impossibility, my apologies for even suggesting it.

If the light is on, you have a problem.

If the sensors you installed were not working properly, you would set more codes.

If the computer was bad, there would be something not working properly, or the engine would not work at all.

Post the codes here if you want help. If you want justification for your lack of maintenance, try another forum.

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You could have the same vaccum leak for 50k miles.

The only time I’ve had a lean code was from a new, non oem maf sensor. But a good diagnostic computer in the right hands should be able to discover what is out of wack.

Possible, but unlikely. If there’s an error involved, most likely it would be one of the sensor readings is incorrect. The reason your engine seems to run ok is b/c the computer is able to compensate, up to a certain limit. When it gets to the point it thinks it is no longer able to completely compensate, it will post the lean code. A fuel trim test takes little time & therefore is is relatively inexpensive shop procedure, & will give you solid measured data where your engine is in relationship to the limit.

Overly lean can cause internal engine parts to overheat and eventually be damaged, so best to address this with due alacrity.

So glad you shared this…they are not OEM sensors, and someone once suggested that might be the issue. I mean, half the car’s life it’s been spitting out these codes. If there were a real emissions issue, it sounds like (and correct me if I’m wrong) it would have affected performance at some point. But it’s at 260K still chugging along just fine.

Thanks, I think I’ll do that if I choose to keep or sell instead of scrap. But I did want to make clear that it’s ran these codes for half of it’s long life (260K), so I must ask if it could truly run lean (or have any real emissions issue for that matter) for that long without any effects.

But, for X number of years, you haven’t had this problem professionally diagnosed?

I did! I let a shop put in new 02 sensors, the Mass Flow, then new Caty. At that point, I figured, as someone suggested the idea above, a non-OEM part can lead to a code failing to go away, even if that part is new.

At some point, you have to stop spending and wait for it to act up ; it just never did.

If the computer senses a lean condition it will try its little heart out to richen the fuel mixture until it just can’t compensate enough. Same thing if it were rich. Why it is lean who knows? Maybe fuel pressure. Can’t get enough gas or too much air leaning it out.

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