Causes & Meaning of Volkswagen Check Engine Light On

In this guide, we address a yellow glowing or flashing engine control light in different VW vehicles. We will discuss the symptoms, potential causes, and also the costs of possible solutions.

Causes of the Yellow Engine Control Light – Which VW Models are Affected?

The cause of a yellow glowing or flashing engine control light on VW models is very likely related to the exhaust system (catalytic converter or diesel particulate filter). However, a variety of other errors could also be responsible.

Other possible causes for the engine light appearing on VW:

• Errors in the mass air flow sensor or the lambda probe
• Defective EGR valve
• Misfire of individual cylinders (uneven engine running)
• Broken thermostat (coolant temperature not normal)
• Clogged fuel filter
• General errors in the exhaust system (including catalytic converter or DPF)
• Newer VW diesel vehicles: Problems with the AdBlue system

The error message can basically occur in all newer VW models (Polo, Passat, Sharan, Golf, Transporter, Touran, Tiguan, Touareg, Up!, T-Cross, T-Roc & Co.).

Symptoms: How is the Error Noticeable in the Car?

The error is recognized by the constant lighting or flashing of the engine control lamp. At the beginning, the indicator light usually only appears at irregular intervals and nothing is noticeable in daily driving.

Later, this becomes a constant light and the effects can then be noticeable in the irregular running of the engine and a significant loss of performance, increased fuel consumption, the failure of one or more cylinders up to the engine switching to its emergency program.

To avoid irreparable damage to the catalytic converter of petrol vehicles (for diesel vehicles to the diesel particulate filter (DPF)), the cause of the error message should be determined and remedied immediately after its first occurrence.

Symptoms of possible causes of a glowing VW engine control lamp:

• EGR Valve: Poor gas acceptance, jerky engine, smoke
• Broken thermostat (coolant temperature low, heating cold)
• Mass air flow sensor / lambda probe: Uneven idle, vehicle does not pull, smoke
• Fuel filter: Car starts poorly, less power
• AdBlue: further error messages / no reliable range display anymore

Problem Solving / Problem Handling – How Can the Error Be Fixed?

As always, when problems occur with warning or indicator lights in the instrument cluster (IC), the first step should be to read out the vehicle's control unit for any stored error messages (error codes).

Only then can a precise error analysis begin. If the code P0420 appears here after reading out, then the cause of the problem is definitely the catalytic converter. If this is not the case, then some other components come into question.

The error message "Catalytic converter" stored in the diagnostic device usually has its actual origin in other faulty vehicle components. In the above case, damage or pre-damage to the injection system or the ignition system would be possible for petrol engines.

For diesel engines, on the other hand, an existing problem with the diesel particulate filter, the exhaust gas temperature sensor, or the differential pressure sensor would be suspected.

The vehicle catalytic converter is a wear part. Accordingly, the durability or functionality is limited depending on the km mileage or the general use of the vehicle in everyday life.

Normally, a catalytic converter should last at least 100,000 km. There are numerous cases where these have worked without problems for up to 150,000 km, in some individual cases even more than 200,000 km.

The exhaust gas catalytic converter is not to be confused with the diesel particulate filter. Vehicles with a petrol engine have a catalytic converter and vehicles with a diesel engine have both a diesel particulate filter and an additional pre-catalytic converter as well as an underbody catalytic converter.