The Humble Mechanic: Demystifying Code P0171 System Lean Fault

The Humble Mechanic: Demystifying Code P0171 System Lean Fault

System lean is the perception by the ECM that there’s unmetered air coming through the engine. The Humble Mechanic says “perception” because that may or may not actually be the case...


I get people asking all the time, “Hey, Charles, I’ve got a P0171 system lean fault. What do I need to replace?” or worse: “I’ve replaced this giant list of parts, and the car is still having an issue.”

This is actually one of those huge “it depends” questions. System lean issues can be a broad range of things – anything from a tiny crack in a vacuum line, a leaky gasket or a giant gaping hole.

Let’s first talk about what system lean means.

System lean is the perception by the ECM that there’s unmetered air coming through the engine. I say “perception” because that may or may not actually be the case. If we have a sensor failing, it becomes a perception. If we have a vacuum line that’s broken, then it’s actual unmetered air. But the ECM doesn’t really know the difference. It figures this by comparing the air in to the air out.

So we use a sensor like a mass air flow (MAF) meter to read the air flow coming in. The air is pulled in through the intake, the air flow meter reads the amount of air coming into the engine, through the turbocharger (if equipped), through the rest of the system, it goes through the combustion process, and then out the back end. And we’ll have an oxygen sensor on the exhaust side that’ll tell us the condition of the air coming out of the engine.

The ECM will take those two readings and say “Hey, everything’s cool, we’re running great” or “We have a problem going on.” If the ECM calculates and sees that there’s a problem, it can actually compensate for those things.

If we have a system lean fault, again, that means we have a perception of too much air or not enough fuel. The ECM will start adding fuel little by little until the oxygen sensor voltage stabilizes.

Figure 1: Using the VCDS, we can evaluate just how lean the engine is running.

On Volkswagen and Audi vehicles, technicians will check the VCDS, measure at value block 32 and use that to evaluate just how lean our engine is. See Figure 1.

Figures 2: Using the VCDS, we can evaluate just how lean the engine is running.

And, of course, we can also see this sometimes in an air flow meter reading or an oxygen sensor reading. See Figure 2.

When we’re looking at it in an oxygen sensor reading, the lower the voltage from the oxygen sensor, the more lean of a condition we’re dealing with. So if we have a .1 volt, now we know we’re dealing with a lean condition. If we have a .9 volt, that’s the opposite end of the spectrum, that’s going to be a rich condition. And these specifics depend on the engine and the manufacturer.

So now that we know a little about what system lean means, how does that help us fix the car?

Figure 3: Visually inspect the system for leaks.

One of the best things you can do with a system lean condition is a visual inspection (Figure 3). A good visual inspection will probably identify about 75% of the system lean conditions.

A system lean is unmetered air. It’s going to be the perception or actuality of an air leak between the air flow meter and the oxygen sensor. If there’s a giant vacuum leak, it’s going to be pulling in unmetered air. The ECM can only compensate so much. If it were a tiny crack, then our ECM could compensate for that.

Figure 4: The ECM may raise the amount of fuel delivered to the engine.

If there were a crack or leak before our air flow meter, we wouldn’t get a system lean fault, because that air is still getting monitored and measured as it comes into the intake. The issue we would run into would be that we would have unfiltered air coming into our engine, while it may or may not set a check engine light, we run the risk of pulling sand, dirt and debris into our intake, into our air flow meter and, worst-case scenario, pulling that into our engine.

When the O2 sensor down at the end sees a lean condition, it tells the ECM, the ECM decides “OK, we’re going to raise the amount of fuel delivered to the engine,” typically that’s done with an increase in injector pulse width. See Figure 4.

If your injector pulse width is 2 ms, it may bump it up to 2.5 or 3 ms, depending on the engine. The ECM will increase the amount of fuel to the engine. The ECM is also going to turn on the check engine light to alert the driver.

Figure 5: There could be a fuel issue causing the system lean condition.

Most of the time, from what I’ve seen with a system lean condition it’s a problem in the air system. But, remember what I said: it could also be the problem of too little fuel. It could be a fuel issue. We could be dealing with a weak fuel pump. We could be dealing with a clogged or damaged fuel filter. Anything that restricts or reduces the amount of fuel pressure or volume up to the engine can also trigger a system lean fault. See Figure 5.

Figure 6: We see the ECM correcting in the value blocks as a percentage.

Like I mentioned, the ECM will correct. We see that in value blocks as a percentage. A perfectly running engine would be a 0% correction. That rarely happens, except right after you clear a fault. As we see our number increase, that’s the engine saying “Hey, I’m seeing a lean condition on the oxygen sensor, we need to increase the amount of fuel.” That number will tick up and up. That can be anywhere from positive 1, positive 3, positive 5… the value block will actually peg out at positive 25 or on the other side, negative 25. See Figure 6.

We can use these numbers to evaluate how lean of a condition we have. If our condition is only correcting .6%, we don’t have anything to worry about. That’s normal. But if we see a correction of positive 10, now we know we have a significant issue with fuel trim. If we’re seeing a 25, which is pegged out/maxed all the way, that is typically the sign that we have a bad oxygen sensor. Oxygen sensors typically – not always, but typically – die reading lean. The ECM will see the low voltage of the oxygen sensor and it’ll dump as much fuel as it can because it’s looking for the voltage of the O2 sensor to come back up. And, because it never sees that, it keeps adding fuel and adding fuel until it’s maxed all the way out. So, that is usually the sign of a failing O2 sensor.

Remember that a lot of this is an “it depends” type of thing. So how do we find system lean faults? As I mentioned earlier, a visual inspection is going to be your best friend. Look at all the hoses, and make sure you have clamps on them. If we have a vacuum line that’s hanging off a little bit, that’s enough to cause a system lean concern. We also want to see if we have any check valves in our intake air system and that we account for those.

Typically, when we deal with system lean, a junction or curve (especially in a rubber boot) is a common spot for cracks/splits. You can also use something like an intake leak spray to help you diagnose it. As you spray it, if you hit the vacuum leak, it’s going to pull the chemical in, and typically the rpm will jump up, showing that you’re in the area of the problem. Then you can do your visual inspection to find the exact spot of the failure.

People get confused about system rich and system lean issues all the time. I’ve found what works for me is, instead of thinking as lean or rich, I like to think of what is the computer doing to compensate for the problem?

If we have a system lean condition, we’re going to see fuel being added, so our fuel correction is going to be positive – more fuel. Then ask, why would it do that? Maybe we have a vacuum leak or maybe we’re not getting enough fuel volume or pressure up to the engine. So, the ECM is going to correct for that.

On the other side of that, with a system rich condition, now we have as what’s perceived as not enough air or too much fuel. That’s going to be a negative number. That’s going to be the engine computer pulling fuel away from the engine.

To me, when you think about it that way, it helps you understand that the computer is doing something to correct for a problem and we can use our brains and our diagnostic skills to help us find where the problem is coming from.

This article was based on podcast “What Does
SYSTEM LEAN Mean?” Find it at





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