Trouble code solution P0173: decoding, causes, resetting, meaning

Contents

Technical description and explained code P0173
Symptoms of vehicle malfunctions
Factors that can cause this error code
How to fix or reset OBD-2 code P0173
Diagnose and repair of problems
On which vehicles does this problem occur most frequently
Video

Fault code P0173 is called “Fuel Trim Malfunction (Bank 2)” but in different programs it may be called differently. This fault designation applies to all vehicles equipped with OBD-II.

Technical description and explained code P0173

This Diagnostic Trouble Code (DTC) is a generic powertrain code. Error P0173 is considered a generic code because it applies to all makes and models of vehicles. Although the specific repair steps may vary slightly from model to model.

This code, although common, is found on some makes of cars more often than others. In the text below, the emphasis will be on Mercedes-Benz as well as Volkswagen. They are the ones most prone to the P0173 error along with ignition misfire codes or other fuel correction codes.

Typically, a P0173 OBD2 code indicates that the ECU has detected a failure to meet the specifications specified by the manufacturer in the fuel mixture. The malfunction may be caused by positive or negative fuel settings. Otherwise it tells us that there is a malfunction in the ECU fuel/air ratio control.

The ECU control module receives information from the oxygen sensors (O₂), which are used to adjust the amount of fuel delivered to the engine. This action performed by the ECU is called fuel compensation.

If the oxygen sensor signal (O₂) indicates that less fuel is needed, the ECU perceives this as a negative fuel setting. And if the oxygen sensors indicate that more fuel needs to be added, it indicates a positive fuel setting.

In short, code P0173 means that there has been a failure in the computer’s control of the air/fuel ratio. There may also be a similar error P0170, referring to the same fault, but for (Bank 1).

Symptoms of vehicle malfunctions

The main signal that an error P0173 has occurred is the Malfunction Indicator Lamp (MIL) is also known as the CheckEngine Light.

It can also be warning signs such as:

Check engine” control lamp on the control panel will light up.
The engine stops or starts badly.
Increased fuel consumption.
Black smoke coming out of the exhaust pipe.
Ignition jerking/missing at idle or under load.

Factors that can cause this error code

The error code P0173 can mean that one or more of the following problems have occurred:

Vacuum problem.
Unregulated air leakage.
There may be oil in the fuel.
Leak in turbo hoses (if vehicle is equipped).
Possibly a faulty O2 sensor (on Mercedes, may need to be adapted with an M-Benz compatible scan tool).
Mass air flow sensor (MAF) or oxygen (O2) sensor connector may be contaminated with oil.
Also check the ignition coils, cam and crank sensors, and oil sensor for leaks contributing to oil in the wiring harness.
Defective MAF (Mass Air Flow) sensor (especially on Mercedez-Benz and other European vehicles). There are many aftermarket MAF sensor problems.
Faulty fuel pressure regulator.
Problem with the camshaft solenoid regulator (Mercedes-Benz).

For some Mercedes models, there is a service recall for the crankcase vent hose located under the intake manifold. It should be checked for leaks/cracks as well as the operation of the check valve in the hose. The check valve should only flow in one direction, if it does not, fault P0173 may occur.

How to fix or reset OBD-2 code P0173

Some suggested steps for troubleshooting and fix the error code P0173:

Check the mass air flow sensor (MAF). Make sure the electrical connections are in good condition. Replace the sensor if necessary.
If you cannot see the oil sensor in the connector, check the fuel pressure regulator for leaks.
If you have access to a scan tool, observe the MAF (mass air flow) sensor reading.
Compare with the manufacturer’s specifications and if they don’t match those specifications, replace the mass airflow sensor.
Make sure all vacuum hoses are intact and not cracked.
Check the oxygen sensor connectors (O₂). Make sure that there is no oil in them. Eliminate the cause of oil leakage and replace the oil contaminated sensor O₂.

Diagnose and repair of problems

It should be noted right away that the most common problem associated with this code is the MAF mass air flow sensor or air flow meter. This is especially true for Mercedes and Volkswagen, as well as other European cars.

For American and Asian vehicles, codes P0171, P0172, P0174, P0175 are most commonly found in relation to fuel supply correction faults.

Simply put, if you have error P0173, your ECU has noticed that the fuel balancing has reached its correction limit. Basically, it’s adding fuel to try to compensate for the poor condition, real or perceived.

If you have this code and access to a scanner, observe the MAF sensor reading in grams/sec. The readings will be different for different cars. Let’s push back on what would be normal for a Mercedes (1.8L), since they have the main problem.

Expect to see 3.5-5 g/s at idle (ideally). At 2500 rpm with no load, it should be 9 to 12 g/s. In road tests, at WOT (fully open throttle) it should be 90 g/s or much higher.

If it is not in the specs, replace it. If there is no oil penetration into the connector when you check the MAF, check the fuel pressure and make sure there are no leaks in or out of the regulator.

Check all vacuum hoses and make sure none are cracked, disconnected or missing. Make sure there are no vacuum leaks on the intake manifold gaskets or ruptures in the air supply hose.

If the engine is turbocharged, make sure the hoses are in good condition and have no leaks. Leaky turbocharger hoses can cause a rich mixture condition.

Check the condition of the crankcase vent hose under the intake manifold and the operation of the check valve in the hose. If there are no problems with fuel pressure, MAF hoses or vacuum hoses, check O₂ sensor plugs for oil penetration.