Technical Description of OBD-II Trouble Code
Malfunction in Fuel Trim (Bank 2)
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code that is applicable to vehicles equipped with OBD-II. While it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
This code is more commonly found in certain makes of automobiles compared to others. In this article, I have included specific information for Mercedes-Benz vehicles since it appears that M-B (and VW) are more prone to experiencing this P0173 code along with misfire codes or other fuel trim codes. P0173 indicates a malfunction in the computer’s control of the air:fuel ratio.
It also indicates that the fuel trims have reached their limit in adding fuel while attempting to compensate for an actual or perceived rich condition. When the fuel trims reach their rich correction limit, the PCM (Powertrain Control Module) sets a P0173 code, indicating a problem or malfunction in the fuel trims. It may also have a P0170 code referring to the same malfunction but on bank one.
Indications
Indications of a P0173 DTC may include:
Causes
Potential causes include a vacuum leak, unmetered air leaks, fuel saturated engine oil, leak in turbo air charge hoses (if equipped), and possibly a bad O2 sensor (If Mercedes, it may require adaptation with an M-Benz compatible scan tool.) Other causes may include oil contamination in the MAF connector or O2 sensor connectors, faulty ignition coils, cam and crank sensors, and oil sensor leakage contributing to oil intrusion in the wiring harness. Additionally, a defective MAF (Mass Air Flow) sensor, especially on Mercedes-Benz and other European autos, or a defective fuel pressure regulator, or leaking camshaft adjuster solenoids (Mercedes-Benz) can be causes.
NOTE: For some Mercedes-Benz models, there is a service recall for a crankcase vent hose located under the intake manifold. It should be checked for leaks/cracking and also for the operation of the check valve in the hose. The check valve should only flow in one direction.
Possible Solutions
It should be noted that the most common problem associated with this code is the MAF sensor or air mass meter. This is especially true for Mercedes-Benz, Volkswagen, and other European cars. At the time of writing, this code is not commonly seen with American-made cars and minimally with Asian cars, and the reason for this is unknown. It appears that the PCM logic used by some European car manufacturers to set a P0173 (or P0173) fault code is simply not used by American vehicle manufacturers. It is more common to see P0171, 0174, 0172, 0175 codes set in relation to fuel trim malfunctions on American cars. There is very little information available on the setting conditions for a P0173 or P0173, but the available information seems to be redundant with the setting conditions for P0171, P0174, P0172, and P0175. The similarities between these codes may be the reason why we do not often see the P0173 code on domestic vehicles. It is simply unnecessary. So, in simple terms, if you have a P0173 code, your PCM has detected that the fuel trims have reached their rich correction limit. Essentially, it is adding fuel to compensate for a lean condition, whether real or perceived.
If you have this code and have access to a scan tool, observe the MAF sensor reading in grams/sec. The reading will vary for different automobiles, so refer to the appropriate specifications. For a Mercedes (1.8L), which experiences this issue most frequently, expect to see a reading of 3.5-5 g/s at idle (ideally). At 2500 RPMs with no load, the reading should be between 9 and 12 g/s. During a road test at wide open throttle (WOT), the reading should be 90 g/s or higher. If the reading is not within specifications, replace the MAF sensor. Be cautious of purchasing MAFs from eBay, as they often do not work according to OE specifications. If the MAF checks out and there is no oil intrusion at the connector, check the fuel pressure and ensure that there are no leaks at the regulator internally or externally. Inspect all vacuum hoses and confirm that none are cracked, disconnected, or missing. Also, ensure that there are no vacuum leaks at the intake manifold gaskets or tears in the air supply hose. If the engine is turbocharged, make sure the hoses are in good condition and do not have any leaks, as leaking turbo pressure hoses could cause a rich condition. Inspect the condition of the crankcase vent hose under the intake manifold and check the operation of the check valve in the hose. (Refer to the “Causes” section) If there do not appear to be any problems with the fuel pressure, MAF, or vacuum hoses, then inspect the O2 sensor connectors for oil intrusion. A faulty O2 sensor could cause a P0170 or P0173 code. Repair the cause of the oil leak and replace the oil-fouled O2 sensor.
