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P0171: Engine Running Lean - Causes, Symptoms & Fix

By: CowboyBenWade

This code happens when the engine is getting too little fuel for the amount of air going into the engine - referred to as 'running lean'. The opposite is 'running rich' which means too much fuel is going into the engine. This is called air-to-fuel ratio or AFR. Fuel burns and fire needs air, and due to the combustion characteristics of gasoline, it needs a specific amount of air for a given amount of gasoline. With too much air, it may not burn at all or only partially burn. Likewise, with too much fuel, it may not ignite or it will burn very dirty.

Air-to-fuel ratio does vary depending on driving conditions and load. More power requires more fuel. When accelerating, AFR should be around 12:1 (12 parts air to 1 part fuel). When cruising at highway speed, it can go up to 22:1 to save fuel. In fuel injected engines, onboard computers measure how much air is going into the engine and electronically inject the correct amount of fuel. Conventional fuel injection systems inject the fuel into the intake right before the valves, whereas newer direct injection systems inject fuel directly into the cylinders.

Causes of P0171

P0171 is the opposite of its cousin, P0172 'running rich'. So the causes are the inverse. For example, a clogged fuel injector will throw a P0171, but a leaking fuel injector will cause P0171. Here are some common problems that would cause an engine to run lean.

Faulty MAF sensor - The computer is receiving false information about how much air is coming into the engine, so it can't reliable adjust AFR.
Bad spark plugs - The fuel will not burn completely, and excess oxygen will be left over in the exhaust.
Vacuum leaks in the air intake - The air intake must be sealed after the mass airflow so that no air is coming in that didn't flow past the MAF sensor. In this case, the MAF will lose accuracy and it will think less air is coming into the engine than is actually coming in.
Faulty oxygen sensor - The computer doesn't know the current air-fuel ratio, so it can't adjust it. Sometimes it will default to 14.7/1. If you're crusing, this is way too rich and will result in too much fuel usage.
Bad fuel pump - Not enough pressure in the fuel system.
Clogged fuel injectors - The computer knows how much fuel to supply, but it can't get though the clogged injectors.

Symptoms of P0171

Because the engine is not getting enough fuel, it may be hard to start and it may feel like it's going to shut off at any time, especially when RPMs drop. It will run like it's being chocked. The most common symptoms are:

Loss of power - the combustion process is less intense because the reaction doesn't have enough fuel.
Hesitation or skipping - A cylinder may fail to ignite and make the car sounds like it skipped a beat, and it may hesitate to get going when you push the gas.
Difficult to start - Cars increase the air-to-fuel ration at startup to get the combustion cycle going. It takes more fuel to start a car than it does to keep it going at any instant. If the fuel deliver is compromised, the engine will be hard to start.
Check engine light - the check engine light will come on, but you'll probably need an OBD2 scanner to read what it means. You can have this done for free at Advance Auto or any auto parts store.

How to Fix P0171

Anything that could block or disrupt fuel flow to the engine can cause this code. Fixing it could involve the following.

Replace the fuel pump - it's not pumping enough fuel to the engine.
Replace the fuel filter - A clogged fuel filter can reduct fuel delivery to the engine.
Replace clogged injectors - clogged fuel injectors can disrupt fuel delivery and cause the engine to run lean.
Replace oxygen sensors - if the sensor is faulty, the computer is unable to tell the injectors how much fuel to send into the engine.
Fix vacuum leaks - gaps or holes in he intake hoses or seals can throw off the accuracy of the mass airflow sensor.
Replace the mass airflow sensor - a clogged or faulty MAF can make the computer think less air is coming in than is actually coming in.

Can I drive with P0171?

A lean running engine can sometimes run but it will not run well. If this code is showing, it means the problem is big enough to make the vehicle drive poorly. Expect it to sputter, be hard to start, and it may hesitate upon pressing the gas. In heavy traffic, that could be dangerous. It could also stall randomly. It's best to take it to a mechanic as soon as possible. While you can still drive short distances with this code, extended driving can cause damage that is much more serious than the issue that likely caused the code in the first place.

Running lean can cause engine damage

Fuel, when injected into the cylinder, has a cooling effect, keeping the valves, piston crowns, and rings from getting too hot. Continuously running a lean mixture over time can cause your engine to overheat and bake the internals. This can cause burned valves, burned piston crowns, and damage to spark plugs. If the piston rings get too hot, they can begin to leak oil from the crankcase into the cylinder - called blow-by. Burned valves can leak causing a loss of compression in the cylinder.

Running just a little bit lean for a short period of time won't cause any damage at all, but its very lean and it stays that way for a long time, the damage can happen.

Does P0171 damage catalytic converters?

Catalytic converters act as capacitors for oxygen. When running lean, an engine will produce increased amounts of NOx. The catalytic converter will take that NOx and convert it to N2 + stored O. The catalyst in the catalytic converter must absorb that oxygen molecule from the NOx. With too much NOx, the catalyst will become saturated and unable to absorb additional oxygen. Therefore, the NOx will remain intact and pass through into the environment. This will result in a P0420 Catalyst Below Threshold, but it doesn't mean your catalytic converter is damaged. It just means that it can't absorb any more oxygen from NOx and has become ineffective. Getting the air-to-fuel ratio back to stoichiometric (all the fuel and all the oxygen is being burned in the combustion process), the catalytic converter will start to work again.

About the Author

CowboyBenWade does interviews and lectures on a huge variety of topics, and he is an expert in a multitude of fields. His knowledge includes but is not limited to: engineering, chemistry, aviation, cooking, and farming.

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