Check Engine Light Help!
 

99 with 155K. Was on the freeway today and the check engine light came on and has been on since. Is the O2 sensor need replacing or could it be something else? I have noticed my gas mileage going down the past few month but I figured it was because of summer heat.

Get the code read at Autozone or ? and then figure out what it needs. No sense guessing and buying parts yet.

X2 Get the code then look it up here

Went to Advanced Auto and
 

Got the code P0171.. bank 1 to lean.

Any idea what that means, what needs to be fixed?

Check the thread above. There's a link to the diag page of the FSM. Start with the MAF, air intake system, pcv valve. How many miles do you have on the truck? Have the O2 sensors ever been replaced?

Truck has 155K and the O2 sensors have never been replaced.

Holy Cow I actually got an email from this website....?

Anyway, sounds like your A/F sensor might be a prime suspect...

Image Item # Name QTY Price Total $97.85
PSK-1-DEN2349002 Denso 234-9002 Oxygen Sensor (Air and Fuel Ratio Sensor) 1 97.85
StockWiseAuto
330 N. Midland Avenue
Saddle Brook , NJ 07663

StockWiseAuto

shipping was a few bucks but great price for denso a/f sensor imho

it worked for me cleared the p1130,1133,1135

You have a V8 in a '99 SR5 2WD???

No I also have a 2006 SR5 V8

I wouldn't suspect oxygen sensor if one cylinder is to lean. It may be a fuel injector is dirty, or the above mentioned possibilities.

It's not one cylinder that is lean, it's the A/F ratio for the entire engine, affecting the fuel trim...from the manual

-CIRCUIT DESCRIPTION
Fuel trim refers to the feedback compensation value compared to the basic injection time. Fuel trim includes
short-term fuel trim and long-term fuel trim.
Short-term fuel trim is the short-term fuel compensation used to maintain the air-fuel ratio at its ideal
theoretical value. The signal from the A/F sensor is approximately proportional to the existing air-fuel ratio,
and ECM comparing it with the ideal theoretical value, the ECM reduces fuel volume immediately if the airfuel
ratio is rich and increases fuel volume if it is lean.
Long-term fuel trim compensates for the deviation from the central value of the short-term fuel trim stored
up by each engine tolerance, and the deviation from the central value due to the passage of time and
changes of environment.
If both the short-term fuel trim and long-term fuel trim exceed a certain value, it is detected as a malfunction
and the MIL lights up.

P0171
When air fuel ratio feedback is stable after engine warming up,
fuel trim is considerably in error on RICH side
(2 trip detection logic)

Things to check for correct function...

Air induction system
Injector blockage
Mass air flow meter
Engine coolant temp. sensor
Fuel pressure
Gas leakage on exhaust system
Open or short in A/F sensor (bank 1 sensor 1) circuit
A/F sensor (bank 1 sensor 1)
PCV valve and hose
ECM

Before you buy a new O2 sensor, clean your old one! I just had the same issue on my 4Runner, but with it running too rich. The computer can adjust the ratio of gas to O2 plus or minus about 10%. When the ratio, or "read" ratio goes beyond that, the check engine light will go on as the computer can no longer properly adjust the gas to O2 ratio on it's own. In my case, the two wires that read the air flow were covered in dust and dirt, thus insulating them and causing a false read. Used electrical spray to clean them off and was right back running like a champ. This will also probably cause your decreased gas mileage. All this was from a Master Toyota Tech at my local dealer. They cleaned it up free of charge and sent me on my way. :biggrin2:

I suspect they cleaned the MAF sensor and not the O2 sensor. Whole different ball of wax. A good thing to do--but if OP really has 155K on his O2 sensor he should just change it. I bet he's 49-state, I doubt the Cali A/F sensor lasts that long without a CEL.

Isn't the O2 sensor in the exhaust system?