Check engine light: Code says O2 sensor is reading lean.

fuzzybabybunny

Moderator<br>Digital & Video Cameras
Moderator
Jan 2, 2006
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I've had this check engine light for about a year. The code says that the O2 sensor is reading lean. Apparently there are a TON of reasons why this might be the case, and no mechanics want to bother with it. O2 sensor might be messed up or the exhaust might really be lean due to something in the engine?

What should I do? My gas mileage is still incredibly high (40-45mpg, 2000 Toyota Echo 108K). My fuel filter might need replacing though. Could this fix the problem?
 

nismotigerwvu

Golden Member
May 13, 2004
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Theres a few things to try. I'd start by cleaning the airflow sensor (they have sprays designed just for this, its a quick, 5 minute job). You can also try replacing the oxygen sensor itself, which typically is a plug and play deal. I say all this becuase in most cases, fuel filters are a PAIN to get to and tend not to get clogged this soon (if ever). Also, I should point out running lean means you aren't getting enough fuel in, which would mean if anything, better gas milage (but not something you want to be doing, predetonation will KILL your motor.
 

Zenmervolt

Elite member
Oct 22, 2000
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Very first thing: Check the intake clamps. A lot of times the cause of a lean reading from the O2 sensor is air leaking into the intake after the airflow sensor. It might be just a simple fix like that.

ZV
 

exdeath

Lifer
Jan 29, 2004
13,684
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1) MAF or O2 sensor or their connections
2) true lean condition; unmetered air entering the engine, via a clamp as per above, or less obvious places like a PCV or EGR or vacuum hose.
3) true lean condition; fuel supply problem, bad regulator, dirty injectors, fuel filter, pump issues, etc. less fuel being delivered than what the computer demands.

If you can't find anything obvious right away, it might be easiest to have a sniffer test done with calibrated equipment to see if in fact it's actually lean and go from there before you start replacing costly parts for no reason. Then start testing sensors on a scope to verify (most shops have specialized hand held scopes with ROM cartridges designed for testing a wide variety of sensor outputs).

2000 seems kind of new for this, but check the plastic intake pipe/box, those are prone to cracking to the point of separating into two pieces, particularly if there is a ribbed accordion elbow.
 

XZeroII

Lifer
Jun 30, 2001
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if you offer them enough money, mechanices will replace anything. Whether it needs to be replaced or not.
 

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