what is (po171) code on a 99 toyota 4runner ''running to lean'' what does that mean?

Question

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.

Answer 1

Fuel trim refers to the feedback compensation value compared against 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 heated oxygen sensor (bank 1 sensor 1) indicates whether the air-fuel
ratio is RICH or LEAN compared to the ideal theoretical value, triggering a reduction in fuel volume if the
air-fuel ratio is rich, and an increase in fuel volume if it is lean.
Long-term fuel trim is the overall fuel compensation carried out over the long-term to compensate for continual
deviation of the short-term fuel trim from the central value due to individual engine differences, wear over
time and changes in the usage environment.
If both the short-term fuel trim and long-term fuel trim are LEAN or RICH beyond a certain value, it is detected
as a malfunction and the MIL lights up.

Answer 2

1

Answer 3

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 completely refurbish my fuel system. MIL came back on in 2 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 was not giving the proper voltage back to the computer. Replaced that.

At the same time, the 02 sensor wasn't oscillating like it should, so I replaced that as well.

o2 sensor basically informs your computer of oxygen content in the exhaust system. It not giving the ecu a good reading would 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 likely a faulty maf sensor.

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

Answer 4

It means that the computer has seen a high percentage of lean readings from the 02 sensors in its history. This can be caused by a number of 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 actual lean running condition that needs to be addressed. Black means the engine has been 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 good idea at this point 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 need to address the actual lean running condition. Start by checking the fuel filter and verifying the fuel pressure is good. Low fuel pressure will cause lean running. A ripped intake boot (the tube going from the throttle body to the mass airflow sensor) will cause a lean condition because the computer does not "see" the incoming air. And the dirty maf will cause the same problem.

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

Answer 5

99 Toyota 4 Runner

Answer 6

Usually around 80,000 miles you will get a check engine light letting you know that it is time to change your O2 sensor(s){oxygen sensors}. This is just routine maint. If in doubt just take it by an Autozone or Advanced Auto Parts, they offer free computer diagnostics.

Answer 7

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

Answer 8

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

Answer 9

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