P0172 and P0175: System Too Rich

Code P0172 indicates Bank 1 of the engine has a rich fuel mixture. P0175 indicates Bank 2 of the engine has a rich fuel mixture. Engine Bank 1 is the same side of the engine as Engine Cylinder 1. Engine Bank 2 is the opposite side of the engine. A firing order and cylinder location diagram may be needed to see which side of the engine is Bank 1 or Bank 2.

What Do P0172 and PO175 Mean?

P0172 is triggered by the engine bank 1 “upstream” oxygen sensor and P0175 is triggered by the engine bank 2 “upstream” oxygen sensor. The oxygen sensor alerts your system when the condition is too rich meaning there’s too much fuel int the exhaust stream or too little oxygen in the exhaust stream. The symptoms and causes will help you determine the exact issue and how to fix it.

Your bank 1 or bank 2 sensors will be referred to as “sensor 1” or “sensor 2”. Sensor 1 refers to “upstream” sensors, or the ones near the engine and before the catalytic converter. Sensor 2 is “downstream” or after the converter.

These upstream sensors verify the correct air/fuel ratio is entering the engine cylinders by measuring the oxygen levels in the exhaust gases leaving the cylinders. This is done by a continual but changing voltage signal to the computer. A continual high voltage reading indicates a rich fuel mixture. A continual low voltage reading indicates a lean fuel mixture.

There are various conditions that can cause the fuel mixture to change beyond the normal ratio range. The optimal ratio is 14.7:1 (air:fuel) When your powertrain/engine control computer receives a higher than normal voltage reading beyond the time allowed, indicating a rich air fuel ratio, trouble code P0172 or PO175 is set and the CEL is illuminated. Depending on the problem, it may set both codes if the issue is causing a rich condition on both engine banks.

It’s important to note here, the very nature of the oxygen sensor sending these codes, without any other oxygen sensor fail-codes tells us the sensor is working as it should. What makes this code tricky is if this code appears by itself, this is NOT a monitored component that has failed, or it would be getting a code as well. In short, the engine is running rich, but the computer cannot tell why if there are no other related trouble codes.

Check Your Check Engine Light

What are Common P0172 and P0175 Symptoms?

In many instances engine performance issues are not exhibited. However, some rich fuel situations can cause your vehicle to have these performance problems:

  • Strong smell of exhaust fumes
  • Black smoke emitting from the exhaust pipe
  • Increased fuel consumption
  • Reduced power
  • Rough idle
  • Engine oil dilution with fuel

Again, these symptoms are normally accompanied with a lit Check Engine Light.

Look for one of these common causes to determine how to solve the problem code.

The first thing to determine is what other codes are present? If either code (or both) arrives at the same time as, say, a mass air flow sensor code, it’s a logical step to assume the MAF sensor may be causing the rich condition. If either P0172 or P0175 is appearing by itself, or both codes are stored, you have a problem that requires more detective work, and you must look beyond sensors and items monitored by the engine’s computer.

Causes of P0172 and P0175

There are many minor or major repairs that could create this issue. Start with the easiest issues and work your way through to identify the cause of your problem code. Rich mixture conditions are often caused by insufficient air entering the engine. The first thing to inspect is the air filter and the entry air flow to ensure all is clear. An exceptional dirty air filter that is very restricted can cause both codes to set. Check the entire PCV system and crankcase for excessive fuel odor or dilution.

If nothing pops up here, there are other major issues that could be occurring. Any improper fueling, which could be caused by excessive fuel pressure (potentially a fuel pressure regulator / restricted fuel return line / fuel rail pressure sensor) It’s important to note that usually fuel issues such as these will often trigger a misfire code as well – either random or on particular cylinders. Also ensure the engine cooling system is within the correct temperature range. Extended driving with a low cooling system temperature will increase the fuel injector pulse time, meaning, more fuel. An additional trouble code may or not be set for this condition for older OBD-II vehicles.

How to Fix P0172 and P0175

If the cooling temperature is good, the air filter is clean, the engine oil is not diluted and there are no other trouble codes, you may try cleaning the MAF sensor. Regardless if this is the issue, a good maintenance cleaning is always good, especially during air filter replacement.

Do not put off having this problem fixed. A rich fuel mixture condition, depending on how severe, can cause damage to the engine, dilute the engine oil, increase carbon build up, foul spark plugs, overheat and damage the catalytic converter and fill the exhaust with black soot which coats the oxygen sensors.

If you need professional help, check out our list of Preferred Repair Shops in the area.

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