Code P0172 - System Too Rich: Symptoms, causes, common problems, diagnostic

By Vlad Samarin June 25, 2023
The OBDII trouble code P0172 - System Too Rich (Bank 1) means that the engine air-fuel mixture is too rich, or there is too much fuel and too little air. The code P0175 - System Too Rich (Bank 2) is the same code only for Bank 2 in an engine with two cylinder banks (e;g;, V6 or V8)


When an engine runs rich it may produce black smoke from the exhaust and you may notice increased fuel consumption. In some cases, the engine may run rough at idle. The engine may also be difficult to start or may sputter when started.


Common causes include:
- Faulty Air/Fuel ratio sensor.
- Failed or dirty fuel injectors.
- Engine oil level is too high.
- Presence of fuel in engine oil due to short driving intervals.
- Faulty Mass Air Flow sensor (MAF).
- Faulty PCV valve, PCV diaphragm, and other parts of PCV system.
- Stuck open purge valve/solenoid.
- Excessive fuel pressure (faulty fuel pressure regulator).
- Faulty high-pressure fuel pump (e.g., leaking internally into the crankcase).
- Faulty engine coolant temperature sensor (ECT).
- Incorrect aftermarket intake system installed.
- Contaminated fuel.
- Faulty PCM.

How the code P0172 is diagnosed:

The first step is to check for service bulletins and do the basic check under the hood. For example, the code P0172 could be caused by the engine oil level being too high, so the oil level must be checked. If there is a strong smell of fuel from the engine oil, it might be diluted by fuel, for example, due to the vehicle being driven for only short distances wihout being fully warmed up. If engine oil looks diluted with fuel, clearing the code, resetting learned values and doing an oil change may help. The air filter and the air induction system must also be checked.

If there are no obvious problems and no service bulletins, your mechanic will have to check basic engine data with a scan tool and follow the diagnostic procedure for this code from the service manual for your vehicle. It may also be helpful to the check the freeze frame, which is a snapshot of the engine parameters at the time the code P0172 was set. Read more about the freeze frame. The PCV system and the purge valve must be tested. The injector balance test may help pinpoint a faulty fuel injector.
If you have your own scan tool, check Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT) readings at idle and at higher rpms (read more about fuel trims below). Normally, both LTFT and STFT should be close to zero. If LTFT shows a larger negative number (higher than 10-12), the air/fuel mixture is currently rich.

Common problems causing the code P0172 in different cars:

We checked some of the service bulletins, here is what we found:
The GM service bulletin 18-NA-246 for certain 2013-2019 models with a 2.0L LTG engine mentions that the code P0172, as well as P06DE and/or P0521 can be caused 'by driving the vehicle in short intervals, allowing excessive fuel to collect in the crankcase.' If the diagnosis is confirmed, the bulletin advises to check and if needed to replace the PCV valve with an updated part and to reprogram the Engine Control Module.
GM issued another service bulletin 22-NA-056 for some 2020-2022 models with the 2.0L Turbo (RPO LTG) engine advising that 'If P0172 is caused by excessive fuel in the crankcase, the long term fuel trim should be a large negative number.' If the diagnosis is confirmed, the bulletin advises to perform an oil change, clear the code and test drive the vehicle to confirm that the code has not returned.

The Mazda service alert SA-026/18 refers to a similar problem with some of the 2014+ direct-injected Mazda vehicles. It explains that if the vehicle is driven on short trips most of the time without letting the engine to fully warm up the engine oil can become diluted with fuel and this can cause the code P0172. The service alert advises to change the oil and filter more often if the vehicle is driven mostly on short trips.

Basic car knowledge: how the air/fuel ratio is adjusted

Fuel trim Control diagram - OBD II system Fuel Trim Control diagram
In modern cars, the air/fuel ratio sensor (front oxygen sensor) measures the amount of oxygen in the exhaust gases and provides feedback to the engine computer (PCM). The sensor can detect if the air-fuel ratio is lean (too much air and too little fuel) or rich (too much fuel and too little air).

The PCM adjusts the fuel supply accordingly to keep the air/fuel ratio at the optimum level, which is 14.7/1 (14.7 parts of air to 1 part of fuel).

This process is continuous: when the air/fuel ratio sensor detects lean mixture, the PCM adds more fuel. When there is too little oxygen in the exhaust (air/fuel mixture is rich), the PCM reduces the amount of fuel. In technical language this adjustment is called fuel trim.

What is Fuel Trim?

Short term fuel trim (STFT) Once the front oxygen sensor is warmed up after a cold start, the engine computer (PCM) starts cycling the air/fuel ratio between a little lean and a little rich. This cycling happens around once or twice per second. This air/fuel ratio adjustment is called short term fuel trim (STFT). A typical scan tool displays the STFT as a percentage with "-" or "+" in the Live Data mode. For example, if your car has a rich condition, you would see the STFT going up to a negative 10-15%.

Long-term fuel trim (LTFT) There is also a long-term fuel trim (LTFT), which is a long-term compensation of the base air/fuel ratio. For example, if over time, the engine gradually runs richer, the engine computer (PCM) compensates for this condition by adjusting the long-term fuel trim (LTFT) to reduce fuel. The long-term fuel trim is also displayed as a percentage with a "-" or "+" sign in a scan tool. For example, a long-term fuel trim (LTFT) at -5% means that the PCM has subtracted fuel supply, although 5% is considered within normal range.

How the code P0172 is set

The P0172 code indicates that the powertrain control module (PCM) has detected a rich air-fuel mixture in the engine's bank 1. The rich mixture means that there is more fuel than needed and the air/fuel ratio is too rich. To compensate for excessive fuel, the engine computer (PCM) adjusts the air/fuel ratio (long-term fuel trim) to negative values within possible range. However, if the long-term fuel trim falls outside the admissible range (typically higher than a negative 10-14 %), the PCM sets the fault P0172 and illuminates the MIL (Check Engine light).

What does the term "Bank 1" mean?

v6 engine Bank1 and Bank2 in a V6 engine.
The terms "Bank 1" and "Bank 2" refer to a separate group of engine cylinders. Most in-line 4-cylinder engines have only one bank: Bank 1. In V6 and V8 engines, as well as in some 4-cylinder engines, the engine is "split" into two groups of cylinders; the Bank 1 is the one that includes the cylinder number one. To know which is Bank 1 and Bank 2 in your car, you can look it up in the repair manual or google it, mentioning the year, make, model and engine size of your car.