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What is po171 code on a 99 toyota 4runner ''running to lean'' what does that mean?
i have a toyota 4runner an the check engine light came on, wen I check the codes it came up po171 and also "is running to lean"
what doe's mean.

Fuel trim mentions the feedback reimbursement value compared against the basic injection time. Fuel trim
includes short term fuel trim and lasting fuel trim.
Short term fuel trim is the short term fuel reimbursement used to preserve the air fuel percentage at its ideal
theoretical value. The signal from the heated oxygen sensor bank one sensor one indicates if the air fuel
ratio is RICH or LEAN in comparison to the ideal theoretical value, triggering a reduction in fuel volume if the
air fuel percentage is rich, and a raise in fuel volume if it's lean.
Long term fuel trim is the in general fuel reimbursement carried out over the lasting to reimburse for continual
deviation of the short term fuel trim from the central value caused by individual engine differences, wear over
time and changes in the handling environment.
If both the short term fuel trim and lasting fuel trim are LEAN or RICH beyond some value, it's detected
as a malfunction and the MIL lights up.

Toyota Technical info System

running too lean typically means cheap gas, its thinned out too much for the car.

"Lean" means the percentage of gasoline to air entering the cylinders from the carburetor is low. "Rich" mean the reverse, a high gasoline to air percentage. In any way, take it to your mechanic and have it fixed.

Usually around 80,000 miles you'll get a check engine light letting you know that it's time to change your O2 sensor s {oxygen sensors}. This is just routine maint. if not sure just take it by an Autozone or Advanced Auto Parts, they offer free computer diagnostics.

It means that the computer has seen a high % of lean readings from the 02 sensors in its history. This may be caused by some things.

First, pull a spark plug. If its a v6 one from each side. If the plug is coated with a tan or brownish ash its normal, black and oily or white and crusted is bad. White means you have an real lean running condition that needs to be addressed. Black means the engine was running rich for a while. If black, change your spark plugs and the o2 sensor s that are giving a false lean reading, start from the front back. A smart idea now is to change the air filter and pcv valve if not new.

If the plugs are "normal" spray carb cleaner into the mass airflow sensor to clean it out. A dirty mass airflow will set this code.

If the plugs are white you have to address the real lean running condition. Start by checking the fuel filter and verifying the fuel pressure is good. Low fuel pressure will because lean running. A ripped intake boot the tube going from the throttle body to the mass airflow sensor will because a lean condition because the computer doesn't "see" the incoming air. And the dirty maf will because the same problem.

If youve done all that and it still recodes, have a dealer check the computer software.

When my 4Runner had a lean MIL, I quickly decided it needed a tune up. Wrong.

I decided it must be lean because the fuel system is clogged. So I spent time at work to totally refurbish my fuel system. MIL came back on in two weeks.

After $3000 in repairs all at cost mind you, I realized, it must be engine inputs or sensors.

Checked the MAF air flow sensor and that wasn't giving the correct voltage back to the computer. Replaced that.

At the same time, the 02 sensor was not oscillating like it should, so I replaced that also.

o2 sensor just informs your computer of oxygen content in the exhaust system. It not giving the ecu a good reading could be a reason why the computer sets up a MIL light.

Haven't had a problem since.

Advice: Change the 02 sensor, clean the maf sensor with brake cleaner, and clear the check light by disconnecting the negative terminal on the battery for a couple of seconds. See if it comes up again. If it does, most probably a flawed maf sensor.

Just a fast question, how's the car's drivability?

lean has nothing to do with fuel grade it means that that specific oxygen sensor is seeing too much oxygen in the exhaust content this may be a faulty O2 sensor a vacuum leak or faulty wiring or ecm