97 4 Cylinder Camry with DTC P0171

Please help.

My Check Engine Light has come on and I have determined that it was caused by Diagnostic Trouble Code P0171, System too Lean. There are no other codes and the car behaves normally in every respect.

I don't believe I have a vacuum leak as the idle speed is correct.

I don't believe the problem is low fuel pressure as the car has normal acceleration and does not bog.

My theory is that I have a lazy Air/Fuel sensor (not called O2 sensor for California emissions). It seems to me that the only feedback the emissions control computer has that would cause a lean trouble code is the Air Fuel sensor.

There is one more piece of information, the Fuel Trim indication was

-11%. I am hoping that I can get some technical advice before I spend $240 on a replacement Air/Fuel sensor.

Thanks in advance.

Marvin

Reply to
marvin shostack
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Marvin, before you spend that amount of money, get your vehicle's exhaust checked on an Exhaust gas analyser. This device (and they have been around since Adam was a boy) will tell you accurately if your engine is running the wrong mixture and what air/fuel ratio it is. You can even take it onboard (they are battery operated) and test the car on the road (the rubber/neoprene hose is attached to the exhaust tip with a clamp).

Any engine analyser rig from the last 30 yrs has one, so go to any mechanic who uses one.

Jason

Reply to
Jason James

P0171 is most likly caused by a dirty or faulty Mass Air Flows Sensor. Dirty MAF Sensors are often caused by over oiled K&N air filters. MU

Reply to
Mark Uhlenkamp

I had the same problem on my 98 4 cylinder last year. I replaced the spark plugs, and air filter... then finally took it to the dealer, they determined it was the air fuel sensor. they replaced it and then about 3 months later the light came back on... brought it back to them told them that it was replaced few months ago... tech did a test on it and said the sensor was dirty... they replaced it for free since it was under the warranty... so i got 2 sensors for the price of one... paid $431... what a rip.... $279 for the sensor, and 2 hours labor...

btw, had two codes present P0171 System too lean (fuel trim) and P1133, AF sensor circuit slow response malfunction (bank 1, sensor 1)

Reply to
grimm

Check the Mass A/F sensor as others suggested, but before you shell out

240 plus for the Toyota heated O2 sensor, Bosch makes one for about 69 bucks that will work. AutoZone has 'em.

mike

-------------------------

marv>

Reply to
MB

Toyota charge a lot for repair work,....then again the others a probably little better. They all have to pay for the huge airconditioned showrooms and offices.

Jason (got a $840 total bill for a cam-belt, idlers and oil-pump replacement ($238)

Reply to
Jason James

You are not familiar with these cars then. Go ahead and remove that huge vacuum hose going to your power brake booster. That will create a hug air leak, now hang on for a few seconds, notice how good it runs? ECU is adjusting injectro duration to compensate for it. So you probably do have a lean condition or a failed MAF sensor.

That means it was adjusting fuel. Are you sure it was negative? Freeze frame date of -11% means it was reducing fuel by 11%. Normal is around

0% and corrections to about 8% +/- is normal. But over 10 is the sign of some trouble. usually these lean codes are set when it hits closer to 16 and above.

You dont have a K&N filter in it, do you?

Reply to
MDT Tech®

BINGO!

Philip, remember the K&N lackey? He'd hate to see this!

Reply to
MDT Tech®

Ohhhhhhh... yah.

Reply to
Philip®

Reply to
Andrew

================================================================== The latest Scan indicates the following reading after idling for about

5 minutes in 85 degree outdoor temperature:

Long Term Fuel Trim: -11% Short Term Fuel Trim: 0% Trouble Code: P0171 No. 1 O2 (aka Air/Fuel) sensor varies from a low of about .4V to a high of .7V No. 2 O2 Sensor went from about a high of .6V to very near 0V when fully warmed up.

Can a normally functioning engine have its the down stream (After Catalytic Converter) O2 sensor (no. 2) ever read zero volts?

Is there a simple way to trouble shoot the MAP sensor with engine not running? I believe the reference voltage for this sensor is from the emissions control computer and requires the ignition switch to be in run. Is the MAP output inversely proportional to vacuum pressure?

If I had a vacuum leak I would expect to find a plus (+) value for long term fuel trim. I would think there is some other reason code P0171 is set.

Does anyone out there know how the emissions control computer makes use of the two O2 sensors? are they averaged and used as feedback in controling the short term fuel trim?

Can anyone recommend a good text book on trouble shooting OBDII problems?

Thanks once again for responding to my questions.

Reply to
marvin shostack

Marvin, you'll need to clear the code to clear the long term fuel trim from memory. Then make your checks. I bet it will be different.

Reply to
MDT Tech®

The on-line sensor data in the links at foot indicate the sensors must be up to full operating temp before they output meaningful voltages: O2

Yes manifold pressure rises,...output voltage rises.

Guessing, but a manifold leak would effect idle MAP output which in turn would signal ECU the engine is loaded. This may mean the inj on time increases.

The after Cat sensor appears to be a protective device to ensure a cat over-temp doesnt occur (this can destroy the catconvertor)

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These 2 provide guides to sensor characteristics: top one for O2 second for Pressure.

Jason

Reply to
Jason James

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