• Troy Wierenga

How to Diagnose p0171

Updated: Jan 22, 2018

Work smarter not harder...

Finding the root cause of p0171 and/or p0174 comes down to a divide and conquer situation. These codes come with a long list of possible causes. Learn how OBDII can help narrow these down.

There is an app for that...

I have no doubt you have awesome Google skills. But..., you are going to need some actual data from your car. The good news is you can get the OBDII hardware and app that turns your phone into a scan tool. Here are a few I have and trust.

Disclaimer: How to connect to your vehicle and record live data is beyond the scope of this article. Don’t worry, it’s easy.

Fuel trim…..Know thy enemy

Let’s say your air and fuel mix is like the mix of flour and sugar in a muffin, flour being air, and sugar being fuel. Your engines computer is like Bob the baker. Bob wants his muffins to be perfect. Your oxygen (O2) sensors are his taste testers. Their job is to tell Bob if his next batch needs more or less sugar.

For each batch of muffins that Bob mixes, he writes down the percent of sugar he added or subtracted, compared to his first batch. Bob calls this number his sugar trim. If Bob’s sugar trim gets close to 25%, he throws a fit, ( your check engine light on), and screams “What's wrong with my muffins!” (your code p0171 ‘system too lean’) Easy concept, easy numbers.

‘Divide and Conquer’

Answer the following 3 questions. The first 2 can be found with your excellent Google skills, the third with your scan tool.

Do you have 1 bank or 2?

Do you have a Mass Air Flow (MAF) sensor or not? (Many Dodge/Chrysler's do not)

What are the fuel trim numbers during idle and acceleration? Your fuel trims are the added numbers of short term and long term fuel trim for a single bank.

Do you have 1 bank or 2?

If you have 1 bank and p0171 is stored.

If you have 1 bank, you still need to consider all possible causes at this point.

If you have 2 banks, but only p0171 OR p0174 is stored.

Unless your fuel trims are high on both banks, consider what would affect only one side of the engine?

If you have 2 banks and both p0171 AND p0174 are stored.

The opposite is true, now we need to consider what would only affect the engine as a whole.

Does the vehicle have a MAF sensor?

Engines that have this sensor measure the amount of air, or the bakers “flour” differently than those without.

What are the fuel trims during idle and acceleration?

It's time to record the fuel trims from your car. We need need to know these numbers when accelerating vs. idle. Your results should fall into 1 of the following 3 categories.

1. Fuel trims increase at idle but normal while accelerating.


2. Fuel trims seem normal at idle but increase while accelerating


3. Fuel trims are high at idle, and high while accelerating

Another disclaimer: Record your data, don’t watch it while driving. If you can not control yourself, check out the cost of public transportation.

If you DO have a mass air flow sensor and your:

Fuel trims increase at idle but seem normal while accelerating.

No need to suspect the fuel pump, fuel filter, or bad gas. Lets think about this. If these items can supply enough quality fuel when the engine needs the most of it, why not at an idle?

The probable cause you should consider at this point is a vacuum leak. But if you only have 1 bank, it could also be a small exhaust leak between the O2 sensor and engine, or the O2 sensor itself may be contaminated or faulty.

Why does this mean I probably have a vacuum leak?

Back to Bob again. Let’s say idling is a small batch of muffins, and accelerating is a large batch.

Imagine Bob is mixing up a small batch. He adds 1 small cup of flour at a time. When he's not looking, a little elf (or vacuum leak) is sneaking in 1 small cup of his own.

Bob then mixes a large batch and uses a large bucket to add the flour. Just like a vacuum leak, the elf is unable to change the size of his small cup.

Remember that Bob only adds the sugar needed for the flour that he measures. So the larger the batch, the better the muffins are to the taste testers.

Fuel trims seem normal at idle but increase while accelerating:

If fuel trims increase while accelerating, the next step is to answer the following question.

Does your vehicle have a ‘plan B’ if the MAF sensor goes offline? Some do and some don’t. Google “Will my (year-make-model-engine) still run with the MAF disconnected?"

If yes, then you should disconnect it and go for a drive while recording your fuel trims again. The MAF sensor is like Bob’s favorite measuring cup. If the little $%#@ elf steals his cup, he has enough experience to eyeball the flour and add the amount of sugar he thinks it needs.

If not, get the fuel pressure and flow tested. It is easier to rule out fuel supply than a MAF sensor without a plan B in place. In this case, your engine just stalls. Bob doesn't guess well, and his muffins are so bad that muffin production seems to come to a stop.

Was there a significant change with fuel trims?

If not, it is the time to have your fuel pressure and flow tested

If yes, it is likely you have a MAF reporting problem. It may be dirty, defective, a problem with wiring, or a problem with the actual airflow going through it.

If you DO NOT have a MAF sensor and your:

Fuel trims increase at idle but seem normal while accelerating:

Your most likely cause is going to be a defective o2 sensor, wiring, or an exhaust leak between the oxygen sensor and engine.

Bill, Bob's 2nd cousin and competitor, prefers to weigh his flour with a scale right before adding the sugar. So even if an elf sneaks flour into his bowl, he is going to mix the amount of sugar needed, based on the final weight of flour.

The Manifold Air Pressure sensor (MAP) is like Bill's scale. So it doesn’t matter where the air comes from. Looking for a vacuum leak in this case may prove to be a waste of time.

Fuel trims are normal at idle but increase while accelerating:

The likely culprit here is fuel delivery, such as a defective fuel pump or a restriction in the fuel lines or filter. It's time to get your fuel pressure and flow tested.

Fuel trims are high at idle and while accelerating:

We need to see if our flour or ‘air’ is getting weighed correctly. The Manifold Air Pressure (MAP) sensor is like Bill's scale. We need to make sure this scale is

calibrated, or reads zero with nothing on it.

Zero is your vehicles barometric pressure value. Find out if this is close to your local weather. Google "barometer + (Your zipcode)".

If you are at sea level, and your car's barometric pressure is 26.5 in, then your car ‘thinks’ it is at 4000’ elevation, and ‘thinks’ it's getting less air than it actually is. If the barometric pressure is too low, it is likely that you have a defective MAP sensor or wiring to it.

If the barometric pressure is close, it is time for a small chemistry experiment. Google “How to check alcohol content in gasoline”.

With or without a MAF sensor:

Sometimes without realizing it, we may fill up with bad gas, or flex fuel when we aren't supposed to. When this happens, Bob or Bill's sugar supply has been swapped with cheap artificial sweeteners. No matter what size batch they mix up, the taste testers are reporting a fairly consistent amount of "more sugar needed".

Final thoughts...

There are always exceptions to the rule. Component failures, not mentioned may also be the root cause of your p0171 and/or p0174. Faulty MAF sensors seldom cause fuel trims to only increase at idle, but it does happen. The same with fuel pumps. Common sense is to test for the common problems before chasing the “strange but true” rare occasions.


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