Fuel trim is an often overlooked piece of the puzzle when it comes to ICE operation. Every engine out there, whether gas or diesel, needs a balanced air-to-fuel ratio in order to work properly. 

When this balance is disrupted, it will cause a whole array of issues, including some that are fairly serious. In some cases, massive disruption in the fuel trim ratios will trigger a check engine light code. The one we’re looking at today is the P0170. As far as fuel trim codes go, this one falls in the medium priority category. 

Today, we’ll tell you everything you need to know about this code, how to diagnose and what steps you’ll need to take in order to fix it. 

What is the P0170 Code, and What Does It Mean? 

The P0170 is defined as “fuel trim malfunction, bank1.” This is a DTC (diagnostic trouble code) that is triggered when the fuel trim in cylinder bank 1 is incorrect. Don’t let the bank 1 part confuse you. In cars with an inline cylinder configuration, it’s the first cylinder. If you have a V engine, bank 1 is always the bank where cylinder 1 is located. 

It’s worth noting that the P0170 code is triggered when fuel trim levels are positive or negative. In other words, you’ll get this code if there’s too much air, but also if there’s too little air in the mix. 

What is Fuel Trim?

Your engine is constantly adjusting the air-fuel ratio based on its current needs, operating environment, and a bunch of other factors. Mind you, these changes in fuel/air ratio are minute. In most cars, you’re looking at a ±10% ratio change at most. Fuel trim is the response to the O2 sensor data that the PCM receives.

The types of fuel trims are split up into two categories — short-term fuel trim and long-term fuel trim. Short-term fuel trims set the O2 sensor to a closed loop, meaning that the ECU is constantly adjusting the amount of fuel based on the air intake.

Long-term fuel trims are adjustments made based on the downstream oxygen sensor and are only made at specific intervals.

What Causes a Negative Fuel Trim?

The fuel trim percentage runs on a spectrum from positive to negative readings. In the middle is 0, so no change needs to be made. Positive values mean that the system is adding more fuel, making the mixture richer. Negative values mean adjusting the mixture to be leaner.

With that said, negative fuel trim values can be an indicator of a vacuum leak. Upstream Vacuum leaks usually reduce the air intake and a richer running condition.

What Causes the P0170 Code? 

If you look at the causes of a P0170, you will encounter an extensive list of potential issues. Any issue that causes a rich or lean air-fuel mixture is likely to cause this trouble code. You must look at the related codes and symptoms to narrow down the true cause of this issue. Therefore, we will focus on the most common causes. These include:

• MAF Sensor issues
• Faulty Fuel Lines
• Clogged Fuel Injectors
• Vacuum leak

MAF Sensor Issues

A malfunctioning Mass Air Flow sensor is the most common cause of a P0170 code. This sensor monitors the amount of air that is entering the intake, which then tells the ECU how much fuel it needs to inject into each cylinder. When this crucial component fails, or when it no longer delivers precise data, the engine is essentially running blind. The reasons for MAF sensor failure are numerous. 

What often tends to happen is that the MAF sensor filament gets damaged by dirt from the air filter or that the wiring harness that connects the MAF to the ECU becomes damaged. Either way, a bad MAF sensor has to be replaced as soon as possible. 

Faulty Fuel Lines

This is mainly related to the EVAP carbon canister filling up but can also happen because of an increase in fuel pressure in the lines. The fuel tank being overfilled usually causes carbon canister saturation. A failed fuel pressure regulator often causes an increase in fuel pressure.

Clogged Fuel Injectors

A common failure in direct injection engines, clogged fuel injectors can cause a reduced amount of fuel to be fed into the cylinder and a lean running condition. In that case, the ECU may try to adjust the fuel trim, but faulty injectors will keep the actual mix outside the specified limits. 

Vacuum Leak

Any system that feeds air into the engine is vulnerable to leaks. When a leak develops, phantom air enters the system which is completely unaccounted for, especially if the leak occurs downstream from your MAF sensor. Any unaccounted-for air will usually cause the air-fuel mixture to change and a P0170 code to pop up. Leaks can occur in the exhaust, air intake, and various vacuum hoses.

What Are the Common Symptoms of P0170 Fault Code?

As this code usually only presents itself together with other codes — usually a P0300 series ones — its symptoms are fairly numerous and diverse. However, it is still important to know what they are. Symptoms of a P0170 code include :

• Check Engine Light
• Fuel Economy
• Jerky Acceleration
• Cylinder Misfires
• Bad Starting Behavior

Check Engine Light

Your check engine light or malfunction indicator light is the first signal that something is wrong in your vehicle. As the P0170 code does not point to any immediate damage, the light will only illuminate and not flash.

Fuel Economy

Any issue that affects the air-fuel mixture affects fuel economy. This is the case for most issues that involve fuel and air intake. A negative fuel trim affects starting behavior and causes stalls. A positive fuel trim will affect the fuel economy.

Jerky Acceleration

When not enough fuel is being pumped into the engine, vehicle performance can be affected. Jerky acceleration is a known symptom of the P0170 trouble code.

Cylinder Misfires

Cylinder misfires occur more often than not when the engine is running rich or lean. Too much or too little fuel will alter the air/fuel mix to a point where it will impact the ignition cycle of that particular cylinder. Although there are specific codes for cylinder misfires, a P0170 may also present when you’re experiencing this problem.

Bad Starting Behavior

Similar to the fuel economy issue, when there is a lean running condition, your car could have trouble starting in the morning. Engines are often set up to add more fuel to the mix on cold starts as the engine has not reached operating temperature yet. If that fuel is missing, you could experience starting issues. 

How to Diagnose and Fix the Cause of P0170 Code

Now that you know some causes and symptoms of this trouble code, it’s time to diagnose the root cause of the problem. Here’s what you can do. 

Run a Full OBD-II Scan

An OBD-II reader or scan tool allows you to interface with your vehicle’s computer to read trouble codes. Whenever you see a check engine light on the dash, the very first thing you need to do as far as troubleshooting goes is to run a full scan using an OBD-II scanner. 

Not only will doing so tell you exactly what has caused the CEL, but you’ll also get a more elaborate picture of the problem. In the case of P0170 codes, they rarely appear alone. Therefore, you need to run a scan to see what else is logged along with this code. Only then will you know which cylinder(s), or which part of the engine you should focus on. 

Visual Inspection

Once you have a rough idea of what’s going on, it’s time to take a closer look with your Mk1 eyeball. Any melting or damage on the wiring harness, damage to the O2 sensor or fuel injectors and excess carbon can cause fuel trim issues.

Checking your hoses is also a crucial step in diagnosing this trouble code. Any visible cracks or damage are a plausible point of failure. Although it is less common in modern vehicles, PCV (positive crankcase ventilation) system failure is a known flaw of certain 2000s vehicles, so that is also worth checking out.

Another external sign to look out for is black smoke coming out of the exhaust. This is not a problem when the vehicle is starting, but only if it is happening for an extended period. Of course, excess fuel dumping will let off a strong smell, so that’s another indicator to look out for.

Vacuum Pressure Issues

Once you’ve made sure that there are no external sources of damage, it’s important to check for invisible signs. Old age causes the hose clamps and fittings, in particular, to degrade and reduce vacuum pressure.

A vacuum tester or smoke machine will help diagnose and identify this issue quickly. However, most DIY car enthusiasts do not have access to these. A spray bottle filled with soapy water will work just as well in identifying small leaks.

Start your vehicle and leave it idle. Use a spray mister bottle to spray water on the hose fittings. If there’s a leak, you’ll see bubbles forming around that area.

If you have a vacuum tester, you can connect it to your car in an idle state. It should be between 14HG and 22HG and should remain consistent throughout the testing.

Fuel Pressure Regulator

The fuel pressure regulator is one of the chief causes of a P0170 trouble code. It’s responsible for regulating pressure in the fuel lines and when it fails, two things can happen.

Excess pressure in the fuel lines causes damage to the fuel regulator and gasoline leaks out of the regulator or fittings. A lack of pressure will cause gasoline to flow slower than normal and reduce performance.

Of course, if there is evidence that gasoline is leaking from the fuel pressure regulator, it needs to be replaced. Normal fuel pressure depends on the car you drive, but it is usually around 60 PSI when the engine is running. You can test this by starting your vehicle and applying throttle or even leaving your car idle. You need an OBD II reader to obtain this live data.

Fixing the Cause of a P0170 Code

A P0170 doesn’t have a quick fix. Most of the solutions involve finding what has failed and replacing those parts with new units. Cleaning your injectors and O2 sensors can sometimes fix the issue but more often than not you’ll have to pull out the parts cannon. 

Does P0170 Clear Itself?

The code will clear itself within a drive cycle. That being said, it’s always better to manually clear any codes after you apply a fix. That way you’ll know if you’ve solved the issue as soon as possible. 

Is It OK to Drive with P0170?

A P0170 comes with several related codes. More often than not, though, you will be able to drive with an engine that runs richer than normal. However, it’s definitely recommended that you don’t drive your car in this condition unless you absolutely have to. 

Prolonged fuel trim issues can do damage to the engine or other parts of the car, such as the catalytic converter. To avoid long-term damage, fix this issue as soon as possible.

Fix Your P0170 Code with Quality Parts

Fixing a P0170 can be tedious, especially if you’re dealing with a hard-to-find vacuum leak or similar vague causes. Once you accurately diagnose the issue, the only way to make sure it doesn’t come back is to use quality parts. Here at eEuroparts.com, we offer a wide range of genuine, OEM, and quality aftermarket parts for most European vehicles. Check out our offer by selecting using our car selection tool and searching for the parts you need. It’s that easy!