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My 2003 Toyota Corolla's Check Engine Light came on. I read the codes using an OBD code reader and got P0171 - System Too Lean (Bank 1) and P0133 - 02 Circuit Slow Response (Bank 1, Sensor 1). The light is on solid, not blinking.

From what I can research online this seems like the most probable causes are the intake gaskets needing to be replaced or needing a new Mass Airflow Sensor. Though I haven't done any smoke diagnostics so I can't rule out another cause.

My question is: is it okay to drive my car while it shows these codes? I don't want to do permanent damage running too lean. If I drive to the mechanic's or to pick up parts will this cause issues? Or should I get it towed?

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This should not cause you an issue, but this sounds more like a problem with the O2 sensors than with the intake gaskets. When was the last time you had them changed? If you are over 100k miles without new ones, I'd highly suggest this first.

One way to check your theory for the intake gasket is by using a spray bottle with water (on jet, not spray). With the engine running at normal operating temps, spray the area of the intake manifold gasket. You are wanting to hear a change in tone of how the engine is running. When you find a hole (or leaking area), there will be a distinct difference in how the engine runs. The O2 response being slow is what is inclining me to think of this. As O2 sensors get older, the don't normally go bad per-se, but rather they get lazy. They will continue to get more lazy as time goes on. This will cause you to get increasingly worse fuel mileage.

Other things you can do is to ensure you do not have any leaks in your intake tract after the Mass Airflow Sensor. This could be anywhere between it and the head, where the air/fuel enters the cylinder head. Check all vacuum lines/hoses attached to the intake manifold for cracking or breakage. Check to ensure the intake tract is firmly and correctly attached to the throttle body.

As I said, running this to the store to pick up parts will not cause you issue, but get this fixed as soon as possible, you darn sure aren't helping your engine any.

  • Thanks Paulster! Yes this car has about 90k miles on it. Bought it used so I'll have to check the service records to see if O2 sensors were ever replaced. – culix Feb 12 '14 at 6:12
  • Thanks for all of the advice! I will probably take it to a mechanic who knows more than me, but this is a fantastic answer! – culix Feb 12 '14 at 6:13
  • I am going to defer to your expertise, but I just want to ask if you've considered that lean-running engines might end up melting pistons? – Captain Kenpachi Feb 12 '14 at 8:40
  • To solve the mystery - the MAF sensor was dirty. Cleaned that, and replaced the intake gasket for good measure. Things are now running good as new! Thanks again Paulster! – culix Feb 14 '14 at 5:41
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Sounds like the Mass Airflow Sensor needs cleaning. There is a write-up on how to do this with pictures at this link:

https://manicmechanics.wordpress.com/2017/10/23/p0171-or-p0174-error-how-to-clean-a-mass-airflow-sensor-maf/

  • Welcome to Motor Vehicle Maintenance & Repair! Whilst this may theoretically answer the question, it would be preferable to include the essential parts of the answer here, and provide the link for reference. – Pᴀᴜʟsᴛᴇʀ2 Oct 24 '17 at 13:34

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