2001 Focus

Im getting a PO171 System too lean (Bank 1) code. The engine will stall at a light periodically. I’m not sure where to start. What is Bank 1?

You can ignore the “bank 1” part since your Focus only has one bank. In engines with 6 or more cylinders “bank 1” is the side of the engine where cylinder 1 is.

Anyway…P0171 is a code sent from your oxygen sensor which is reporting too much oxygen in your exhaust (“lean” is too much air / “rich” is too much fuel).

For gosh sakes, please don’t replace the oxygen sensor (this is a common response). It is possible that there is a problem with that sensor - but so far you should assume that the sensor is doing its job exactly right - by detecting & reporting too much O2.

The first things to do are to just clean your Mass Airflow Sensor (MAF), and check for vacuum leaks.

This is a pretty good link: http://www.obd-codes.com/p0171

Bank 1 is the line of cylinders in the engine block. Since the engine in your vehicle is 4 cylinder it only has a bank 1. If it were a V6-V8 engine, Bank 1 would be the line of cylinders where cylinder #1 is located and bank 2 would be on the other line of cylinders.

As to the code and what could be causing the problem, look for a vacuum/air leaks after the Mass Air Flow sensor. Check the inlet hose to the throttle body for any leaks, and all vacuum hoses on the engine for any leaks.

The MAF sensor detects the amount of air entering the engine. The oxygen sensor compares that amount of air entering the engine to oxygen content in the exhaust gas. If extra air enters the engine from a leak that MAF sensor doesn’t detect the O2 sensor measures this extra air and sets that code. This can also cause engine to run rough at idle or stall.


I did not find any vacuum leaks. I removed and cleaned the MAF and it did help. But after a couple of days driving it stsrted trying to stall at a traffic light.

When you say that it “helped” what does that mean? Is the idle smooth? Did it smooth out and now its back to rough? Did you reset the engine light and it came back on?

You didn’t happen to recently install an oiled-type air filter such as a K&N did you? Did you check to make sure that the air filter box is sealing up properly? And that there are no leaks before the MAF? (All of that is to ask - if your MAF was dirty before cleaning, is it now maybe dirty again already).

How did you check for vacuum leaks after the MAF? - these can be really hard to find.

The idle was so smooth I thought it had stalled. Now it is eratic. 50% of the time when I stop at a light it runs very smooth and the other times it will run with a slight roughness or rough enough to feel like it’s going to stall. If it feels like it’s going to stall I will put it in neutral and excel a little until the light changes. While driving in town or the highway it runs good and has good power.
I reset the engine light and it hasn’t come back on since.
I have not installed an oiled-type air filter.
I just checked the air filter box and it looks like it’s sealing properly.
I used an hose with one end in my ear to listen for hissing sounds around the vac hoses and didn’t notice any. How do you check for leaks?