how to fix P0171 and P0174 codes on Tesla Outback 4th Generation (2010-2014)

Subaru Outback 4th Generation (2010-2014)

Error Codes

2-4 hours

Time

medium

Difficulty

high

Severity

Error Code: P0171, P0174

3 min read

600 words

⚠️ Safety Warning

This issue requires immediate attention. If you are not experienced with automotive repairs, consult a professional mechanic. Working on critical systems can be dangerous.

Problem Statement

Learn how to diagnose and fix P0171 and P0174 codes in a 2010-2014 Subaru Outback. Step-by-step guide with diagnostic procedures and repair instructions.

Error Code

P0171, P0174

Affected Component

Fuel System

Estimated Time

2-4 hours

⚠️ Safety Warnings

Important safety information - read before proceeding

•Disconnect the battery before working on electrical systems.
•Allow the engine to cool before inspecting or working on the fuel system.

Symptoms

•Check engine light illuminated
•Rough idle
•Decreased fuel economy
•Engine stalling during acceleration
•Increased emissions

Diagnostic Steps

1
Scan for error codes using an OBD-II scanner to confirm P0171 and P0174.
2
Inspect the air intake system for vacuum leaks, including hoses and connections.
3
Check the mass air flow (MAF) sensor for contamination or failure.
4
Verify fuel pressure using a fuel pressure gauge to ensure it meets specifications.
5
Inspect the fuel injectors for proper operation and leaks.
6
Examine the engine coolant temperature (ECT) sensor for accurate readings.

Solution

💡 Expert Tip

For complex repairs like this, consider taking photos at each step. This helps if you need to reverse a step or consult with a mechanic. Also, keep all removed parts organized - you may need to reference them during reassembly.

Verification

After completing the repair, verify that the issue has been resolved:

✓Test the affected component to ensure it functions correctly
✓Check for any error codes or warning lights
✓Monitor the vehicle for a few days to ensure the issue does not recur

Prevention Tips

To prevent this issue from occurring again:

•Follow the manufacturer's recommended maintenance schedule
•Address warning signs early before they develop into major issues
•Use quality parts and fluids recommended for your vehicle
•Keep detailed records of all repairs and maintenance

Tools & Equipment Required

•OBD-II scanner
•Smoke machine or carb cleaner
•Multimeter
•Fuel pressure gauge
•Basic hand tools (sockets, wrenches, etc.)

Parts Required

•Vacuum hoses (if damaged)
•MAF sensor (if faulty)
•Fuel filter (if clogged)
•Fuel pump (if necessary)
•ECT sensor (if faulty)

Related Faults

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Diagnostic Steps

1
Scan for error codes using an OBD-II scanner to confirm P0171 and P0174.
2
Inspect the air intake system for vacuum leaks, including hoses and connections.
3
Check the mass air flow (MAF) sensor for contamination or failure.
4
Verify fuel pressure using a fuel pressure gauge to ensure it meets specifications.
5
Inspect the fuel injectors for proper operation and leaks.
6
Examine the engine coolant temperature (ECT) sensor for accurate readings.

Solution

1. Preparation

Gather tools and parts required for the repair.
Ensure the vehicle is parked on a flat surface and the parking brake is engaged.
Disconnect the battery to prevent electrical shorts.

2. Inspect and Repair Vacuum Leaks

Sub-steps:
1. Visually inspect all vacuum hoses connected to the intake manifold for cracks or disconnections.
2. Use a smoke machine or carb cleaner to identify leaks while the engine runs.
3. Replace any damaged hoses or fittings.

3. Clean or Replace MAF Sensor

Sub-steps:
1. Locate the MAF sensor in the air intake duct.
2. Disconnect the electrical connector and remove the sensor from the duct.
3. Clean the sensor using MAF cleaner; do not touch the sensor element.
4. If cleaning does not resolve the issue, replace the MAF sensor with a new one.

4. Check and Adjust Fuel Pressure

Sub-steps:
1. Connect a fuel pressure gauge to the fuel rail.
2. Turn the ignition key to the "ON" position without starting the engine to check the pressure.
3. Compare the reading to the manufacturer's specifications (usually 50-60 psi for Subaru).
4. Replace the fuel pump or filter if the pressure is below specifications.

5. Inspect Fuel Injectors

Sub-steps:
1. Remove the fuel rail to access the injectors.
2. Inspect injectors for clogs or leaks; clean them if necessary using a professional injector cleaner.
3. Reinstall the fuel rail and ensure all connections are secure.

6. Check ECT Sensor

Sub-steps:
1. Locate the ECT sensor on the engine.
2. Disconnect the electrical connector and measure the resistance with a multimeter.
3. Compare the resistance readings against the manufacturer's specifications for temperature.
4. Replace the ECT sensor if it is faulty.