P0172 – Meaning, Causes, Symptoms, & Fixes

What Does Code P0172 Mean?

• P0172 definition: Bank 1 has too much fuel or not enough air
• Issue Severity: MODERATE– Extended driving with this code can cause internal engine damage.
• Repair Urgency:  Get this code fixed as soon as possible to avoid damage to spark plugs, pistons, and catalytic converters. 
• Diagnosis: It is important to complete the entire diagnostic process when diagnosing P0172. Many people will replace the air-fuel sensor or oxygen sensor as soon as they get a bad reading, but the root cause can also be a dirty or faulty mass airflow sensor or vacuum leak, thus causing the O2 or MAF sensor to read differently to compensate. Use the FIXD Sensor and app to diagnose and fix P0172 yourself, see the most likely fix and cost for your car, access our Mechanic Hotline, and much more!
• Common Reason: Code P0172 is often related to a vacuum leak.

Combustion engines run most efficiently when they maintain an air-fuel mixture ratio of 14.7 parts air to 1 part fuel. Too much air and/or too little fuel results in the engine running lean setting P0171 or P0174 trouble codes, while too much fuel and/or too little air results in the engine running rich resulting in a check engine light for the P0172 code. The rich condition can be caused by a vacuum leak, which introduces more air into the air-fuel mixture, or by a weak fuel system, which does not input enough fuel into the air-fuel mixture. To keep the engine running properly, the powertrain control module (PCM) tries to compensate for the rich condition by reducing the amount of fuel to the mixture in an effort to maintain the proper 14.7:1 ratio. When these adjustments become too large, the P0172 code is triggered.

P0172 Causes

• Faulty or dirty mass airflow sensor
• Vacuum leaks – PCV hoses, vacuum hoses, intake manifold gasket.
• Weak fuel pump
• Clogged or dirty fuel injectors
• Clogged fuel filter
• Exhaust leak
• Faulty oxygen sensor
• Faulty air-fuel ratio sensor

P0172 Symptoms

• Check Engine Light is on or flashing
• Lack of power from the engine
• Rough idle
• Engine hesitating or misfiring
• Strong fuel smell from the exhaust
• Soot buildup on spark plug diode
• Commonly associated with error codes: P0175

How Do I Fix Code P0172?

With a P0172 code, the first step is to get it properly diagnosed to figure out what is causing the engine to run rich. The FIXD Sensor and app can help you diagnose and fix P0172, saving you time and money. You can use FIXD to test your oxygen sensors at home and call our Mechanic Hotline for help. Using the live data function of the FIXD sensor and app allows you to read and analyze fuel trims to properly diagnose a P0172 code.

>Click here to try FIXD for just $19.99 for a limited time!

If your engine is running rich and you’re not comfortable diagnosing this issue at home, we recommend finding a RepairPal-certified shop to pinpoint the problem and give an accurate estimate for repairs. 

These shops can not only help you figure out what’s going wrong before you waste time and money on the wrong parts, but they also offer a minimum 12-month, 12,000-mile warranty and stand behind all their estimates with guaranteed fair pricing.

How Much Does It Cost to Fix Code P0172?

If you take your car to a shop for diagnosis, most shops will start with an hour of “diag time” (the time spent in labor diagnosing your specific issue). Depending on the shop’s labor rate, this typically costs somewhere between $75-$150. Many, if not most, shops will apply this diagnosis fee to any required repairs if you have them perform the repairs for you. From there, a shop will be able to give you an accurate estimate for repairs to fix your P0172 code.

Possible Repair Costs for P0172

When it comes to making repairs associated with the P0172 code, one or more of the below repairs may be needed to solve the underlying issue. For each possible repair, the estimated cost of repair includes the cost of the relevant parts and the cost of labor required to make the repair. 

• Vacuum leak $100-$200
• Clean MAF $100
• Replace MAF $300
• Fuel Pump $1300-$1700
• Fuel pressure regulator $200-$400
• Air fuel sensor or oxygen sensor $200-$300

DIY Steps to Diagnose Code P0172

If you’d like to try to fix code P0172 at home without throwing money at parts, you’ll want to follow the steps below for proper diagnosis. Keep in mind this is an intermediate-level diagnosis and repair and not recommended for beginners. Diagnosis requires more specialized equipment beyond what the FIXD Sensor can provide and it can be a time and labor-intensive process for inexperienced DIYers.

DIY difficulty level: Intermediate

This repair requires mechanical knowledge and is not recommended for beginners.

Tools/parts needed (our top picks from Amazon):

• FIXD
• Mass Air Flow Cleaner
• Fuel Pressure Testing Gauge
• Screwdriver
• Pliers

STEP 1: USE FIXD TO ENSURE NO OTHER ENGINE CODES ARE PRESENT.

Use FIXD to scan your vehicle to verify P0172 is the only code present. If other codes are present, they must be addressed first.

STEP 2: INSPECT VACUUM LINES.

Inspect all vacuum lines and hoses for leaks, and make sure they are properly connected. If a leak is present, you will hear a hissing sound, though it may be difficult to hear with the engine running. If you suspect a vacuum leak but are having trouble finding it, check out this detailed guide to finding vacuum leaks.

STEP 3: INSPECT MASS AIR FLOW (MAF) SENSOR.

Remove the mass airflow sensor and clean the sensor using mass air flow cleaner or contact cleaner.

> How to clean your MAF sensor

STEP 4: INSPECT EXHAUST SYSTEM.

Check the exhaust system components for leaks before the Air Fuel Ratio Sensors or Oxygen sensors; if you find any leaks have them repaired. Exhaust leaks can usually be spotted visually by a buildup of soot in the general vicinity of the leak.

STEP 5: CHECK FUEL PRESSURE.

Low fuel pressure can throw off the air-fuel mixture ratio, and this could be caused by anything from the fuel injectors and fuel pressure regulator to the fuel pump.

> How to check fuel pressure

• Check fuel injectors: Make sure the fuel injectors are functioning properly and activating. Random misfires can be a sign of faulty or clogged fuel injectors that need to be replaced. Also, check that the fuel injector wiring is not damaged and is connected properly.

> How to check fuel injectors by ear

> How to check fuel injectors with a digital multimeter

STEP 6: REPLACE AIR-FUEL AND/OR O2 SENSORS

At this point if the code still persists you may want to consider replacing your A/F sensor or O2 sensors.

STEP 7: INSPECT AND REPLACE SPARK PLUGS

After you repair the cause of your P0172 code, inspect the spark plugs. If the spark plugs appear to be covered in soot on the tip of the spark plugs (near the diode that screws into the engine), then you should replace all of the spark plugs.

Common P0172 diagnosis mistakes

It is important to complete the entire diagnostic process when diagnosing P0172. Many people will replace the air-fuel sensor or O2 sensor as soon as they get a bad reading, but the root cause is often a dirty or faulty mass airflow sensor or vacuum leak, thus causing the O2 or A/F sensor to read differently to compensate. Reading and analyzing fuel trims and the freeze frame data is the key to properly diagnosing P0172.

Still Need Help Fixing Code P0172?

If you’ve followed the steps above and are still experiencing a rich engine condition and code P0172, please contact the FIXD Mechanic Hotline if you’re a FIXD Premium subscriber or find a RepairPal certified shop near you to get the right repairs at a fair price. Don’t have a FIXD Sensor yet? Click here to get yours for just $19.99, including a free 14-day trial of FIXD Premium!

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Fixed It But The Check Engine Light Is Still On?

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