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Mercedes Codes P0170 P0173

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Mercedes-Benz owners may notice the check engine light plus P0170 and P0173 codes.  In theory, these two codes mean that the fuel mixture is too rich or too lean. Most of the time these Mercedes-Benz fault codes are caused by a defective Mercedes Mass Air Flow Sensor (MAF) sensor. You can try cleaning the sensor but that doesn’t always work. It is best to replace the sensor.  If the codes remain it is possible you have a vacuum leak.

  • P0170 – Fuel Trim Malfunction Bank 1
  • P0173 – Fuel Trim Malfunction Bank 2
These codes can be misleading because if a mechanic goes by the book he may not suspect the problem to be the MAF sensor. This issue can affect any Mercedes-Benz model including C, E, S, ML, GL, CLK, GLK, SLK, GLE classes. Other codes that you may get are P0130, P0135. If you clear the P0170 and P0173 without fixing the underlying problem those two codes will return in a couple of days.

Symptoms

  • Check Engine Light On
  • P0170 and P0173 Codes
  • Car stuttering
  • CEL may come on for a few day, then turns off
  • Rough idle
  • Car shakes, running rough
  • Hesitation during acceleration

Under certain circumstances such as vacuum leaks, the car may run fine even if the CEL light is on and P0173/P0170 are present.

P0170 and P0173 Code Causes

  1. The most common cause of P0170 and P0173 on a Mercedes-Benz is a defective mass-airflow sensor. The MAF sensor gets dirty over time triggering the CEL and the two codes. If you are using an air filter such as K&N, you may be contributing to this problem because these filters allow larger particles to pass and also have oil that can coat the MAF sensor. Replacing the MAF sensor at the dealer can cost between $350-$500. Replacing the MAF sensor yourself will cost you under $100. Replacing the MAF sensor is easy and can be found here.
  2. Vacum leaks near the o-ring of the fuel injectors.
  3. Vacuum leak at the brake booster line.
  4. Cracked PCV hose or loose PCV valve.
  5. Fuel Supply Issue. If you have issues during a cold start, it is more likely a fuel related issue.

Misdiagnosis

mercedes p0170 p0173If you troubleshoot this issue based on the two codes, a mechanic can misdiagnose the problem. In fact, these two codes are often misdiagnosed. Common misdiagnosis includes bad Oxygen sensors or worn spark plugs. These items are unlikely to fix the P0170/P0173 codes. Always clean or replace the MAF sensor first.

How to Fix P0170 and P0173 on a Mercedes

  1. First, clean the Mass Air Flow (MAF) sensor.  You will need MAF Sensor Cleaner to spray on the sensor. Mechanics or the dealer are unlikely to perform the cleaning procedure so you will have to try this yourself. Cleaning may not always work.
    1. Remove the mass air flow sensor.
    2. Spray MAF sensor cleaner on the plate inside. Avoid touching the sensor with an object. Only spray sensor cleaner.
  2. If cleaning does not fix the problem, replace the MAF sensor. The sensor is very easy to replace. Make sure to get a set of secure torx bits. A Secure Torx bit set can be purchased online and is inexpensive. Ideally, you should install anOEM Mercedes Bosch MAF Sensor because aftermarket sensors don’t work or last very long. If P0170 and P0173 were caused by a bad MAF sensor and you replaced the sensor, the CEL should turn off in a couple of days. Even if you don’t clear the codes. If the fault codes are caused by a vacuum leak, the CEL will stay on or return.
  3. Check Fuel Filter
  4. Check Ful Pump, Fuel Pressure, Fuel Pump Relay
  5. Get an expert to look at your short trim and long trim fuel readings.

Replaced MAF sensor but P0170 P0173 are still on

If the CEL stays on after you replace the MAF there is a good chance there is a vacuum leak somewhere. You will need to check for vacuum leaks. In one case, a ML-Class we worked on had a vacuum leak by the brake booster. The vacuum helps to add pressure to the brake system. Vacuum on one side of the brake booster diaphragm pulls the brake cylinder when you press the brakes. When there is a crack on the diaphragm or the O-rings of the brake booster, you will hear air hissing sound near the brake pedal. Air is being pulled into the engine and is not being measured by the MAF sensor triggering the P0170 P0173 fault codes.

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Of course, the vacuum leak doesn’t have to be at the brake booster, it can be in any of the vacuum lines attached to the intake manifold.

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