31C400

What code 31C400 means

Code 31C400 indicates an air mass plausibility fault. The engine control unit (DME) compares the actual airflow measured by the mass air flow (MAF) sensor against a calculated model based on throttle position, crankshaft speed, and other parameters. If the measured value falls outside the acceptable range relative to the expected value, fault 31C400 is stored.

The most likely cause is a leak in the intake tract (unmetered air), especially around the charge air hose. A leak lets unmetered air into the intake system, which skews the MAF reading and throws off the air-fuel mixture. Less often, the problem is a faulty MAF sensor or its electrical circuits.

The intake and boost system includes the air filter, intake pipes, intercooler, charge air hoses, and the MAF sensor. All connections must be airtight for the airflow measurement to be accurate.

Symptoms

• Engine: rough running, especially at idle or under light load.
• Performance: power loss, hesitation under acceleration, sluggish response to the accelerator pedal.
• Warning lights: fuel mixture faults (for example, too lean or too rich), and the engine may drop into limp mode.
• Conditions: symptoms may get worse under load or when ambient temperature changes.

Diagnosis and repair

Step 1: Check the intake tract for leaks

• Visually inspect all air hoses, pipes, and connections for cracks, splits, and loose clamps.
• Use a smoke generator to find leaks (unmetered air).
• Pay close attention to the charge air hose and where it connects to the intercooler and throttle body.

Step 2: Check the mass air flow (MAF) sensor

• Read sensor data with diagnostic equipment (ISTA, INPA). Compare actual airflow against the calculated value across different engine operating modes.
• Check the sensor's electrical connectors and wiring for corrosion and damage.
• If needed, clean the sensor's sensing element with a dedicated cleaner (per the manufacturer's instructions).
• If the sensor readings clearly don't match the engine operating mode, replace the MAF sensor.

Step 3: Fix the fault

• Replace damaged air hoses or seals. When reassembling, make sure the clamps are tightened correctly.
• After the repair, clear the fault code and take a test drive to confirm it doesn't return.

Secondary problems

Running for a long time with fault 31C400 (unmetered air or incorrect MAF readings) throws off the air-fuel mixture. This can cause:

• A rich or lean mixture that puts extra load on the catalytic converter and oxygen sensors, speeding up their wear.
• Rough running, higher fuel consumption, and reduced power.
• Over the long term, catalytic converter damage and deposits on the valves and spark plugs.

Don't put off the repair, because ignoring the fault can lead to more serious and more expensive damage.