Error Code P0173 is described as Fuel Trim Malfunction Bank 2. This means the PCM (powertrain control module, also known as ECM or engine control module) has detected a fuel trim outside the specified range set by the vehicle’s manufacturer. It is usually detected when the fuel injection system doesn’t operate properly or if there’s an incorrect level of mixture ratio being compensated. This can be a negative or a positive fuel trim.

This trouble code is a generic code, meaning it is applicable to all vehicles equipped with OBD-II, especially vehicles made since 1996 up to present. Specifications on the definition, troubleshooting and repair may vary from one make/model to another.

Definition

The PCM works by receiving signals from the Oxygen (O2) sensors and adjusts the amount of fuel it allows the engine to receive based on the readings provided by the O2 sensors. If the signal from the O2 sensor means the engine needs less fuel, the PCM acts accordingly through a negative trim. If the O2 sensor says it needs more fuel, then the PCM will carry out a positive trim.

Error Code P0173 is set when the PCM has detected an anomaly on fuel (fuel trim) that is outside the range of the vehicle’s specifications. This can be a negative or positive trim. This code means there’s a malfunction in the computer’s control of the air to fuel ratio.

This code can also mean the fuel trim has reached its limit of adding fuel while it tries to compensate for actual or perceived rich condition. When the fuel trim reaches its limit, the PCM will set the error code, indicating a malfunction in fuel trims. It can also be the same malfunction but on Bank 1, which would be P0170.

Common Symptoms

Symptoms for Error Code P0173 may vary depending on the make of the vehicle. Also, other conditions present can also affect the symptoms. Obviously, it activates the Check Engine light. Some of the most common symptoms include:

  • Loss of power
  • Stumbling from the engine, hesitation on acceleration, start and stall
  • Increased fuel consumption
  • Black smoke coming out of the tailpipe
  • Misfire at idle or under load

Possible Causes

There are many conditions and factors that can cause the engine to run lean, such as a clogged fuel filter, a weak fuel pump, or faulty fuel pressure. The same thing applies for rich conditions, as factors like a faulty airflow sensor manifold, a faulty fuel pressure regulator, or an absolute pressure sensor.

Other potential causes of this error code include:-

  • Fuel saturated engine oil
  • Leak in turbo air charge hoses (if equipped)
  • Vacuum leak
  • Unmetered air leaks
  • Oil contamination in MAF connector or O2 sensor connectors
  • Leakage in ignition coils, Cam and Crank sensor and/or oil sensors which contributes to intrusion of oil in wiring harness
  • Bad O2 sensor
  • Defective MAF sensor
  • Defective fuel pressure regulator
  • Leaking camshaft adjuster solenoids

Some of these causes are prevalent in some makes of vehicles, such as Mercedes-Benz.

How to Check

You should know that the most common issues associated with these trouble codes is the MAF sensor or air mass meter. This is especially the case in many European cars, especially Mercedes-Benz and Volkswagen. This code rarely happens in Asian and American-made cars. For American cars, the most common fuel trim error codes are P0171, P0174, P0172 and P0175, which refer to fuel trim malfunctions. Meaning, there is very little information about P0173 and P0170.

As with other trouble codes, mechanics diagnose P0173 with the help of OBD-II scanner to ensure that no other codes exists as well. If there are other underlying trouble codes, then there’s a good chance that’s what’s causing the problem. If this is the case, then it is important to resolve those codes first. This may include repairing a rich or lean engine issue before moving forward.

After making the necessary repairs to solve underlying trouble codes, the mechanic will reset the Check Engine light and run the engine. If the code P0173 comes back, then there must be another problem. If there aren’t any existing issues, then the problem is mostly in the PCM.

How to Fix

Repairs for this error code are pretty straightforward;

  • For rich condition, you simply have to fix the problems that cause the engine to run rich.
  • For lean condition, you simply have to fix the problems that cause the engine to run lean.
  • You may also need to replace the PCM.

The severity of this problem can range from mild to problematic, depending on the underlying conditions and other factors that contribute to the error code. The code itself may not cause serious symptoms or no symptoms at all. In conjunction with other problems however, it may lead to various drivability issues. Also, if this code stays for longer period of time, it may cause serious damage to the PCM.