P0173 Fuel Trim Malfunction (Bank 2)
OBD-II Trouble Code Technical Description
Fuel Trim Malfunction (Bank 2)
What does that mean?
This diagnostic trouble code (DTC) is a generic powertrain code, which means that it applies to OBD-II equipped vehicles. Although generic, the specific repair steps may vary depending on make/model.
This code is one that surfaces with certain makes of automobiles more than others. In writing this article I've added Mercedes-Benz-specific info since it seems that M-B (and VW) are most prone to having this P0173 surface along with misfire codes or other fuel trim codes. P0173 means there was a malfunction in the computer's control of the air:fuel ratio.
It also indicates that the fuel trims reached their limit of adding fuel while trying to compensate for a actual or perceived rich condition. When the fuel trims reach their rich correction limit, the PCM (Powertrain Control Module) sets a P0173, indicating a problem or malfunction in the fuel trims. It may also have a P0170 referring to the same malfunction but on bank one.
Symptoms
Symptoms of a P0173 DTC may include:
MIL (Malfunction Indicator Lamp) Illumination Start and stall Poor fuel economy Black smoke at tailpipe Hesitation/misfire at idle or under loadCauses
Potential causes include a vacuum leak, unmetered air leaks Fuel saturated engine oil Leak in turbo air charge hoses (if equipped) Possibly bad O2 sensor (If Mercedes, may require adaptation with M-Benz compatible scan tool.) Oil contamination in MAF connector or O2 sensor connectors. Also check ignition coils, cam and crank sensors, and oil sensor for leakage contributing to oil intrusion in wiring harness. Defective MAF (Mass Air Flow) sensor (especially on Mercedez-Benz and other European autos. There are a lot of problems with aftermarket MAF sensors.) Defective fuel pressure regulator Leaking camshaft adjuster solenoids (Mercedes-Benz).
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 operation of check valve in the hose. The check valve should flow only one way.
Possible Solutions
It should be stated right off the bat that the most common problem associated with this code is the MAF sensor or air mass meter. This is especially the case with Mercedes-Benz and Volkswagen and other European cars. At time of writing, you don't normally see this code with American-made cars, and minimally with Asian, and, to be honest, I have no idea why. It appears to me that the PCM (powertrain control module) logic used by some European car manufacturers to set a P0173 (or P0173) fault code simply isn't used by American vehicle manufacturers. It is more common to see P0171, 0174, 0172, 0175 codes set with regard to fuel trim malfunctions on American cars. There is very little information on the setting conditions for a P0173, or P0173, but what information that is available almost seems to be a redundancy of the P0171,4,2 & 5 setting conditions. I'm sure there's a reason for it, but I can't get anyone to tell me what that is. The similarities between them may be why we don't see this code surface on domestic vehicles very often. It's simply unnecessary. So, simply put, if you have a P0173, your PCM noticed that the fuel trims reached their rich correction limit. Basically it's adding fuel to try to compensate for a lean condition, real or perceived.
If you have this code and access to a scan tool, observe the MAF sensor reading in grams/sec. The reading will be different for different automobiles, so get a good spec. I'm going to stick with what would be normal for a Mercedes (1.8L), since they have the bulk of the trouble. Expect to see at idle 3.5-5 g/s (ideally). At 2500 RPMs with no load it should be between 9 and 12 g/s. On road test, at WO
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