One of the most common trouble codes with the Toyota Mirai is P0174. It indicates that your Toyota Mirai is running lean on the bank two side of the engine.
P0174 and P0171 (fuel trim lean bank one) are the most frequently seen air/fuel mixture-related codes. If they appear together, it reduces the list of potential causes (more on that in the last section of the article.
The most common causes of P0174 are a vacuum leak or a dirty MAF sensor.
|Definition||P0174: Fuel Trim System Lean (Bank 2).|
|Symptoms||Check engine light, decreased fuel economy, misfire, hard to find idle.|
|Common Causes||Dirty MAF sensor, vacuum leak, O2 sensor.|
|Repair Cost (Parts)||It varies, but the most common repairs are usually under $200.|
|Repair Difficulty||The common P0174 repairs are usually simple.|
Table of Contents
P0174 Definition: Fuel Trim System – Lean (Bank 2)
P0174 is a generic code, so it has the same meaning for the Toyota Mirai as any other vehicle.m Here’s all three parts of the definition, divided by its three parts:
Fuel Trim System
The ideal air/fuel ratio is 14.7:1 (14.7 parts air to one part fuel). Your Mirai’s PCM will try and stay as close to this number as possible (it can range anywhere from 12.0:1 for power and 16.0:1 for economy).
Here are the main components of the fuel trim system.
- O2 sensors– Oxygen sensors meter the exhaust and report the oxygen content to the powertrain control module.
- MAF or MAP sensor– Depending on your Mirai’s model year, it can have a mass airflow sensor or manifold absolute pressure sensor (or both). Their job is to meter the air as it enters the engine.
- Powertrain control module– Takes info from these sensors and uses it to adjust the air/fuel ratio.
There’s more to how your Mirai gets air and fuel than what’s listed above, but these are the big three components that meter and adjust the fuel trim.
P0174 is thrown when your carotruck’s B2S1 O2 sensor detects the air-fuel mixture is “lean” (too much air or not enough fuel).
Your Mirai’s PCM will add more fuel to the mixture to bring the air-fuel mixture where it needs to be for optimal combustion. It also throws the code P0174 to let you know it has done this. Most of the time, there won’t be any noticeable drivability issues.
Bank two is the side of your Toyota Mirai’s engine with the second cylinder. For most engines, that’s the cylinder head a little further from the engine accessories.
If your car has P0171 + P0174, it indicates the entire engine is running lean, so you don’t really need to concern yourself over finding bank 2.
Toyota Mirai P0174: Symptoms
There aren’t usually any drivability issues associated with P0174. Typically, the first sign that anything is wrong is the check engine light.
- Mileage- may or may not suffer depending on how much the air-fuel mixture has changed.
- Misfire– If the air-fuel mixture has gotten too far from the 14.7:1 sweet spot, this can cause your Toyota Mirai to misfire.
- Idle Issues– With the air/fuel mixture altered, the vehicle can idle erratically as the computer compensates to keep the engine running.
Causes of P0174 in the Toyota Mirai
The P0174 code is thrown when something has caused too much air to get into your Mirai’s combustion chamber or the fuel pressure at the rail is below where it needs to be.
Your Mirai’s MAF and/or MAP sensors meter the air as it enters the engine. If there is an air leak after the MAF sensor, it’s not being measured as it enters the intake, which increases the air in the air-to-fuel ratio.
One of the most common causes for the P0174 code to show is a vacuum leak. Listen and look around your Toyota Mirai’s engine bay to see if you can find the vacuum leak.
Look at any hoses or sensors that connect to the intake manifold. Verify that they aren’t cracked and that they are tightly secured. Listen for unusual noises.
The P0174 code is thrown when the bank two sensor one O2 sensor reports that your Mirai’s engine is running lean.
If your Mirai has P0174 without P0171 present, you can swap the bank one and two O2 sensors and see if the code “jumps” to the other bank. Here’s what you do.
- Clear the diagnostic trouble codes with your scanner.
- Swap the bank one sensor one oxygen sensor with its bank two counterpart.
- Start your Mirai and wait for the check engine light to come back on.
- Once you pull the codes, if it changes to P0171, that’s enough evidence that a faulty O2 sensor was causing P0174 in your car.
Mass Air Flow (MAF) Sensor
The mass air flow sensor calculates the volume of air entering the engine. If it is malfunctioning, your car’s PCM won’t know how much air is entering the engine. When this happens, it can cause your Toyota Mirai’s engine to run rich or lean. If it were running rich, you’d get P0175.
Cleaning the MAF or MAP Sensor
Your Mirai’s MAF sensor is covered with tiny electrodes that can detect how much air is passing through it. Unfortunately, it can’t read the air properly if it gets dirty.
Local parts stores sell dedicated MAF sensor cleaners. If you use anything else to clean it, you risk damaging its sensitive electronics.
P0174 indicates that your Toyota Mirai is either getting too much air or not enough fuel. A clogged or damaged fuel injector will cause the air-fuel mixture to lean out in one specific cylinder. You may see a cylinder misfire code or fuel injector-specific code with P0174.
It’s unlikely that you’ll see P0171 if this is the case. You may also get P0300 or P030X (X being the missing cylinder, which should be an even number).
Fuel Pressure Regulator or Fuel Pump
With P0174, there’s possibly not enough gas entering the combustion chambers. This can happen if the fuel pressure regulator isn’t allowing enough fuel to get to the fuel rail.
A faulty fuel pump behaves exactly the same way. The only way to know why the fuel pressure is too low and causing P0174 in your Toyota Mirai is to trace it back from the fuel rail and see where the pressure starts weakening.
An exhaust leak will change the amount of oxygen the B2S1 O2 sensor picks up. When this happens, it’ll report a false lean condition. You’ll likely hear the exhaust leak coming from under the front of your Mirai.
While it’s not the most common reason to throw the P0174 code, a clogged air filter can cause it. It only takes a second to check it.
Possible P0174 Fixes
Here are the steps you can take to diagnose P0174 in the Toyota Mirai.
- Check for other trouble codes. They will likely point you to what part of your car is malfunctioning.
- Listen for an exhaust leak or vacuum leak within the engine. The exhaust leak should be close to the B2S1 O2 sensor.
- Inspect all of the hoses that connect to the intake manifold, as well as the intake hose in front of the throttle body.
- Check to see if the air filter is dirty.
- Test to see if the intake manifold gasket has a leak.
- Test the fuel pressure. Some vehicles will tell you how hard the fuel pump is working on a scanner.
- Verify fuel trim on a quality scanner.
Is P0174 a breakdown risk?
It is rare that a vehicle breaks down by itself when P0174 is the only code. Still, you don’t want to ignore it. Doing so may eventually damage your Mirai’s catalytic converter.
My Mirai has P0174 and P0171 together; what does that mean?
If your Toyota Mirai has P0174 and P0171 together, it rules out a few problems and provides anecdotal evidence to help narrow the problem.
- Your Mirai’s O2 sensors are likely working fine since they are both reporting the car is running lean.
- One bad fuel injector will not make a lean code appear on BOTH sides of the engine, so you can rule those out.
- It’s unlikely there’s an exhaust leak.
- It leaves the MAF (or MAP) sensor, vacuum leak, air filter, or fuel pressure on the table as possible causes since they would equally affect both sides of your Mirai’s engine.
Is P0174 difficult to repair?
P0174 in the Toyota Mirai is not difficult to repair. On the other hand, it is difficult to diagnose. Looking up any other codes that appeared with it can save time.