Code P0173: Definition and Meaning
P0173 Fuel Trim Malfunction (Bank 2) is a generic diagnostic trouble code (DTC) related to the powertrain, applicable to all vehicles equipped with the OBD-II standard. It signals a malfunction in the air/fuel mixture control system (stoichiometric ratio) managed by the vehicle’s computer (PCM – Powertrain Control Module).
Specifically, this code is triggered when the PCM detects that the “fuel trims” have reached their maximum limit for adding fuel. The module attempts to compensate for an actual or perceived condition of a too-rich mixture (excess fuel). When it can no longer correct the fault, it stores the P0173 code for the engine’s bank 2. A similar code, P0170, may appear to indicate the same malfunction on bank 1.
This code is particularly common in certain car manufacturers, especially Mercedes-Benz and Volkswagen (VAG Group).
Symptoms of Code P0173
When code P0173 is stored in the PCM’s memory, several symptoms can alert the driver:
- Illumination of the Malfunction Indicator Lamp (MIL) on the dashboard.
- Poor fuel economy (over-fueling).
- Hard starting and engine stalling.
- Hesitation, jerking, or misfiring at idle or during acceleration.
- Black smoke from the exhaust (indicating a too-rich mixture).
- Loss of engine power.
Possible Causes of the Malfunction
Several failures can be the origin of code P0173. Here are the most common causes, listed by probability:
- Faulty or dirty Mass Air Flow sensor (MAF sensor): This is the most common cause, especially on Mercedes-Benz, Volkswagen, Audi, and other European brands. A bad signal from the MAF sensor skews the mixture calculation.
- Air leaks (in the intake system): An unmetered air leak downstream of the mass air flow sensor (vacuum leak, cracked or disconnected air hose) disrupts the mixture.
- Faulty Oxygen sensor (lambda sensor): A worn or malfunctioning lambda sensor on bank 2 providing erroneous readings can mislead the PCM.
- Fuel pressure problem: A faulty fuel pressure regulator (internal or external leak) can cause excessively high pressure and a rich mixture.
- Specific turbo-related problems: A leak in the turbocharger pressure pipes or hoses.
- Oil intrusion into electrical connectors: An oil leak problem can contaminate the connectors of the lambda sensors or other sensors (camshaft, crankshaft), distorting their signals.
- Specific problems with Mercedes-Benz: Failure of the camshaft adjustment solenoids or, on some models, a cracked engine crankcase ventilation hose or faulty check valve (often subject to a recall).
Diagnosis and Solutions to Resolve Code P0173
The diagnostic procedure must be methodical to identify the root cause without unnecessarily replacing parts.
1. Checking the Mass Air Flow sensor (MAF)
Using a diagnostic tool, observe the live data from the MAF sensor (in g/s). Compare the read values with the manufacturer’s specifications. For example, for a Mercedes-Benz 1.8L engine:
- Stable idle: ~3.5 – 5.0 g/s
- 2500 RPM no load: ~9 – 12 g/s
- Wide Open Throttle (WOT): > 90 g/s
If the values are inconsistent, clean the sensor with a specialized product or replace it. Warning: Prefer OEM quality parts; low-end sensors are often the source of recurring problems.
2. Searching for Air Leaks
Visually and manually inspect the entire air intake circuit, from the air filter to the manifold. Check the tightness of the hoses, connections, air filter housing, and intake manifold. Use a smoke test to detect invisible micro-leaks.
3. Checking the Fuel System
Check the fuel pressure using a pressure gauge. Verify that the pressure is stable and conforms to specifications. Test the pressure regulator for a possible internal leak (via the vacuum hose).
4. Inspection of Lambda Sensors and Their Circuits
Using the diagnostic tool, analyze the signal and response of the lambda sensors (bank 1 and bank 2). Physically inspect their electrical connectors for any signs of corrosion, oil, or damage.
5. Specific Checks (especially for Mercedes-Benz)
On affected models, inspect the engine crankcase ventilation hose located under the intake manifold. Check its integrity and test the check valve. Also, check the condition and operation of the variable camshaft timing solenoids.
Conclusion and Call to Action
Code P0173 indicates a problem with the air-fuel mixture management, often related to a faulty MAF sensor or an air leak. Diagnosis requires a logical approach and the use of a diagnostic tool to analyze live data.
If you do not have the necessary equipment or expertise, it is highly recommended to entrust the diagnosis to a professional mechanic, particularly one familiar with European brands. Avoid replacing parts at random, as this can be costly and ineffective. Accurate diagnosis is the key to permanently resolving this code and restoring your vehicle’s optimal performance.