Accord
- Make
- Honda
- Segment
- Sedan
The Check Engine Light (CEL), otherwise known as the Malfunction Indicator Lamp, can be a rather intimidating sight while operating your vehicle. No matter what the fault or failure is, it is always a good idea to have the Diagnostic Trouble Code (DTC) retrieved. This will help you know where the problem area is and how much it could potentially cost to repair.
A common code that can plague many vehicle owners or even experienced automotive repair technicians is P0171, accompanied by the description "System Too Lean (Bank 1)." This code indicates that there is either too little fuel or too much oxygen in the air-to-fuel ratio, specifically relating to the first cylinder bank of the engine. Let's break it down and prepare you for a DIY repair or a trip to the auto shop.
| Code Number | Code Description | Related Codes |
|---|---|---|
| P0171 | System Too Lean (Bank 1) | P0172 - System Too Rich (Bank 1) P0174 - System Too Lean (Bank 2) P0175 - System Too Rich (Bank 2) P0148 - Fuel Delivery Error |
What Does The P0171 Error Code Mean?
The P0171 diagnostic trouble code indicates the vehicle's computer or Engine Control Module (ECM) has picked up too little fuel or too much air in the system. To do this, the computer constantly monitors specific sensors to know exactly how much air is entering the engine and how much fuel it will need to add for proper combustion. This scenario is also known as a "lean" condition.
As air enters the engine, it is initially monitored by a mass air flow sensor, and this sensor sends the data to the ECM. The ECM then reads the data from the throttle position sensor to adjust the volume of the fuel being delivered by the fuel injectors. Once combustion takes place, the ECM monitors the Air/Fuel Ratio sensor to calculate how much fuel and oxygen was burned. If the sensor shows too much oxygen content, the ECM will set the P0171 code and adjust the "fuel trim" values to increase the amount of fuel being delivered.
Along with the P0171 code, you may see the P0174 code, which specifically relates to the second bank of the engine. This code can appear depending on what the fault is or what type of engine your vehicle is equipped with. It is not uncommon to find P0300-P0308 codes when P0171 is triggered, which relate to misfiring cylinders. This is because an engine that is too lean can cause the cylinders to misfire.
What Causes The P0171 Code?
Both an air intake issue and/or a fuel delivery issue can cause the P0171 code. The most common cause for P0171 is typically a worn or cracked intake hose or gasket that allows unmetered air to enter the engine. However, several other causes may trigger P0171:
- Damaged or cracked intake system
- Dirty or malfunctioning fuel injector assembly
- Dirty or defective mass air flow meter sub-assembly*
- Engine coolant temperature sensor* (stuck high)
- Low fuel pressure (clogged fuel filter, worn pump, or restricted sending line)
- Gas leak from exhaust system* (leaking before the A/F sensor)
- Open or short in air-fuel ratio sensor circuit*
- Air-fuel ratio sensor*
- PCV valve and hose
- PCV hose connections
- Wire harness or connector*
- Faulty ECM*
*These issues will likely trigger additional DTCs
Symptoms Of The P0171 Error Code
The most common symptom of a P0171 code is a rough/erratic idle. The following are other symptoms your vehicle may exhibit:
- Rough or erratic idle
- Engine stalling
- Loss of engine power
- Illuminated engine light
- Delayed throttle response
- Reduced fuel economy
Can I Drive My Car With This Issue?
Although the ECM has made the corrections to maintain engine operation, this issue can worsen as time progresses. P0171 can become severe enough to cause power loss, engine stalling, or a damaged catalytic converter. If your car triggers this code, you should have the issue diagnosed as soon as possible.
How To Fix The P0171 Error Code On Your Vehicle
The exact fix to the P0171 code varies because quite a few issues can make this code appear. Dirty or worn injectors, worn fuel pumps, defective fuel pressure regulators, a faulty or dirty Mass Airflow Sensor, lazy or slow responding oxygen/AF sensors, and even a defective ECU can all lead to this code being set. To diagnose this code, we'll need more information.
First, we have to establish whether or not the P0171 code is active. You can have this done by most parts stores or by a quick scan from a local shop. If the only stored code is P0171, we can begin our diagnosis.
One simple check we can perform is investigating worn or loose hoses around the intake manifold. It's not uncommon to see these air hoses become hard and dry from the heat of the engine. If you carefully listen to the engine while it is running, you may hear a "hissing" sound from an area that is drawing unmetered air into the engine.
If there are no obvious signs of damaged or leaking air hoses, we'll need to dig deeper using a scan tool. We have to establish whether or not the engine is currently running lean. This level of diagnostics would typically be performed by an experienced automotive repair technician. However, if you are feeling capable or trying to save some money you can attempt this yourself.
Identifying A Lean Engine
You'll need a diagnostic scan tool for this; there are many of these on the market and all these options can lead you to feeling indecisive. Fortunately, the level of information we need is considered "generic," so even a basic $20 option should work.
Begin by connecting the scan tool to the Data Link Connector (DLC) or OBD-II port, often located near the driver's footwell. In some cases, the port will be in a different spot. Start the engine and select "view live data" on the scan tool menu.
The data we need to view is referred to as "Fuel Trim," a numeric value generated from the ECM that indicates exactly how much fuel adjustment - or "trimming" of fuel - is needed to ensure proper engine operation. The value is expressed as a percentage. There are both Short- and Long-term Fuel Trim values to be aware of. Short-term fuel trim is a value that expresses the immediate/current adjustment taking place at idle or in closed-loop. Long-term fuel trim is a value that expresses the change indicated over a longer period of time including changes in throttle position and loop status.
A positive percentage indicates the ECU is adding more fuel to a mixture that's too lean, while a negative percentage indicates a reduction in fuel flowing into the injectors, as data suggests the mixture is too rich.
More specifically, if the scanner shows "Short-term Fuel Trim Bank 1 - 13%," it means the computer is adding that much more fuel to the engine. If the value is shown as a negative number, such as -13%, the engine is running "rich," and the ECM is reducing the amount of fuel being delivered by the percentage reflected.
There are up to four data PIDs (Personally Identifiable Data) for fuel trims:
- Short-term Fuel Trim Bank 1
- Short-term Fuel Trim Bank 2*
- Long-term Fuel Trim Bank 1
- Long-term Fuel Trim Bank 2*
*Only used on certain engine types.
The expected values to see on a correctly running engine range from -9% to +9%. Once the value passes -10% or +10%, the check engine light is set. However, since the vehicle information below has set a lean code, we are expecting to see a value of +10% to +25% either on the short-term or long-term PID.
Now that we know what data to view, and what to expect, we are going to monitor the fuel trim while revving the engine (slightly) approximately to 3,000 RPM.
| Engine RPM | 1,000 |
|---|---|
| Short-term Fuel Trim (Bank1) | 12% |
| Long-term Fuel Trim (Bank 1) | 22% |
| Engine RPM | 3,000 |
|---|---|
| Short-term Fuel Trim (Bank1) | 0% |
| Long-term Fuel Trim (Bank1) | 17% |
If we see the short-term fuel trim data change to 0% at 3,000 rpm, the engine is mostly suffering from a vacuum leak. Inspect all air intake hoses thoroughly. If no issues can be found, the intake plenum or manifold gasket may be the culprit.
If the short-term fuel trim rises to a greater percentage, the engine is suffering a fuel delivery issue and the most likely cause is worn/dirty injectors or low fuel pressure. You can try using special fuel additives to help clean the injectors over time (these results vary) or you can see a repair facility to perform a fuel injection cleaning service. You'll need a fuel pressure gauge set to further inspect fuel pressure.
How Much Does It Cost To Fix?
This entirely depends on the cause of the lean condition and engine configuration. Diagnostic time and air intake hose replacement would typically cost around $150. Intake manifold gaskets are fairly labor intensive; for the average four-cylinder engine, such as a Toyota RAV4, it could cost up to $300. If your vehicle was a full-size pickup, like the Ford F-150, expect that number to rise to $500.
Fuel injector cleaning services can go for between $100 and $200. Using a Honda Accord as a reference, a worn in-tank fuel pump can cost over $600 easily. Meanwhile, if the high-pressure fuel pump requires replacement, costs can exceed $1,200. According to RepairPal, the average cost for a fuel pump replacement is roughly between $1,000 and $1,300.
Driving the vehicle with an active P0171 code will most likely not result in major engine damage. However, we would highly recommend paying close attention to the symptoms your vehicle displays. If your vehicle has a noticeable lack of power, fix the issue immediately, as this can become a safety issue. If your vehicle feels mostly normal or only has a rough idle, you may have some time while you attempt to perform the repair yourself or save up some extra cash to cover the services of an experienced workshop.
