P0172 – Meaning, Causes, Symptoms, & Fixes
Code P0172 Definition
System Too Rich (Bank 1)
What Does P0172 Mean?
Combustion engines run most efficiently when they maintain an air-fuel mixture ratio of 14.7 parts air to 1 part fuel. When the upstream oxygen sensor detects there are less than 14.7 parts air to 1 part fuel in the air-fuel mixture, a rich condition exists. To keep the engine running properly, the powertrain control module (PCM) tries to compensate for the rich condition by injecting less fuel to the mixture in an effort to maintain the proper 14.7:1 air-fuel ratio. When these adjustments become too large, code P0172 is triggered.
- Check Engine Light is on
- Lack of power from the engine
- Rough idle
- Engine hesitating
- Engine misfiring
- Dirty or faulty Mass Air Flow (MAF) sensor
- Faulty oxygen sensor
- Leaky fuel injectors allowing too much fuel into the combustion cylinders
- Worn spark plugs
- Stuck fuel pressure regulator
- Faulty coolant temperature sensor
- Faulty coolant thermostat
Code P0172 Severity – Moderate
It is okay to drive a vehicle with P0172 for a short period of time, but driving with this code for an extended period of time can overheat the engine and cause internal engine damage.
Code P0172 Common 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 Air Flow (MAF) 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.
- Tools Needed to Diagnose:
Difficulty of Diagnosis and Repair – (1-4)
- Use FIXD to scan your vehicle to verify P0172 is the only code present. If other codes are present, they must be addressed first.
- Inspect your air intake box and air intake snorkel for any obstructions that would prevent sufficient air flow in the engine. Inspect your air filter to ensure it is not dirty and it is seated properly.
- Remove the Mass Air Flow (MAF) Sensor and clean the sensor using mass air flow cleaner.
- Reinstall the Mass Air Flow (MAF) Sensor and clear the check engine light using FIXD. If the check engine light comes back on with code P0172 continue the diagnostic process.
- If check engine light code P0172 persists after you have inspected the air intake system and cleaned the Mass Air Flow (MAF) Sensor, perform a fuel pressure test. If any components in the fuel system are failing, replace them as necessary. Pay special attention to the fuel pressure regulator and the fuel injectors. If the fuel pressure regulator is stuck, it can cause a rich condition due to the high pressure. If the fuel injectors are faulty, they could leak fuel into the cylinders rather than delivering the precise amount needed for the air-fuel ratio.
- Check that your coolant temperature sensor and coolant thermostat are functioning properly (see steps 3 and 4 of P0128). If either of these are not functioning properly, the vehicle will stay in “open loop” operation and continue to deliver a fixed rich mixture.
- If the check engine light persists after this diagnostic process, it is most likely time to change the oxygen sensor. You can test the oxygens sensor to verify this is the fix. Here is a great video that clearly explains this process!