P0171 Code Causes And Fixes

Overview of how to inspect, diagnose, and repair the causes of a P0171 fault code.

What does the P0171 code mean?

The P0171 code is set when there is a lean condition on the first bank of the engine. Too much air or insufficient fuel in the fuel-air mixture is called a “lean” condition. Fault code P0171 usually occurs when a vacuum leak is present or the fuel system is running weak.

What is running lean?

Running lean is when the engine gets too much air or insufficient fuel. If there is too much air, the Engine Control Module (ECM) may try to compensate by injecting more fuel.

Possible Causes

The most common problems that trigger the P0171 fault code are:

Bad MAF (mass airflow sensor)
Bad injectors
Bad O2 sensors
Vacuum leak
A malfunctioning or bad PCM
Clogged fuel filter
Weak fuel pump
Bad fuel pressure regulator

Symptoms

You notice common symptoms when the check engine light comes on due to the P0171 code.

Engine stall
Engine derate
Check engine light
Engine hesitation
Poor starting
Possible catalytic converter damage

Possible Repair

Repair or replace intake/vacuum hoses
Replacement of lousy fuel system components (filters, injectors, pumps)
Replacement of inadequate intake or exhaust gaskets
Repair or replacement of O2 sensors
Air filter replacement
PCM repair or replacement
Mass airflow sensor replacement

Diagnosis

Disconnected or leaking vacuum/air hoses:

The supply tube from the airbox to the throttle body (or the snorkel) can become loose. Check all fasteners and check the tube for any punctures or looseness.
Listen for whistling or rushing air sounds while your engine is running. Follow the noise and inspect for holes, dry-rotted hoses, open ports, or loose hoses.

Faulty/dirty/plugged mass airflow sensor or air filter

Debris blocking the mass airflow sensor will prevent the sensor from sending the correct readings to the PCM.
The air filter is probably one of the most overlooked components. Check your filter for debris and damage. Verify that no residue got into the airbox snorkel if the filter appears damaged.

Damaged or disconnected engine intake components

Check your plenum gasket for dry rot or other indications that the gasket wouldn’t be sealing.

(to do this, removal of the air filter box and plumbing is often required, as well as the engine cover. Once the plenum is exposed, bolts fasten the plenum to the cylinder head. Remove these and remove the plenum. Inspect the gasket for damage.)

While the plenum is off, inspect for any damage to the cylinder head.

If there are no obvious signs of damage or possible leak points, assemble everything and tighten all bolts and fasteners to OEM specifications.

Once everything is back together, you can use a carburetor cleaner to check for possible leaks.

(with the engine running, spray carb cleaner in the suspected leak area. If a leak is present, the engine will either run smoother or rougher than before you sprayed the carb cleaner)

Check the area where you got the most response from again for any of the possible causes mentioned earlier. (if you still can’t find the source of your problem, taking your car to a certified shop will help you. Technicians have computers that can graph engine performance and help them find the system that isn’t performing correctly)

Fuel system checks

If fuel pressure is too low, your engine won’t get the right fuel. Begin by checking fuel filters, fuel injectors, and fuel pumps for blockages.
Borrow a fuel pressure gauge from your local auto parts store and perform a fuel pressure test.

Exhaust system checks

Check for leaking exhaust flex pipes and manifold gaskets.
Check for adequately functioning O2 sensors. (sensors sending incorrect signals to the PCM can trigger a P0171 code)

Troubleshooting with YOUCANIC Full System Scanner

One good example of a multi-system scanner is the YOUCANIC Full Systems Scanner. This scanner can read and clear fault codes from all the systems in your Mercedes-Benz. It can also perform bi-directional tests, maintenance and repair resets, etc.