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Faulty MAF Sensor Can Cause Repeat 'Check Engine' LightPosted 1/16/2004By Jim Newkirk
Vehicle: 2000 Volkswagen Jetta 2.0L AEG Engine. Test and Fix 1) Verify that there are no vacuum leaks or any other faults that might cause a lean indication (low fuel pressure, exhaust leaks, faulty injectors, faulty secondary air system, etc.) 2) Use a scan tool to measure air flow at idle speed. The typical readings for a properly functioning MAF sensor will be 3.5 or more grams/sec at 800 rpm. 3) Replace the MAF sensor if the grams/sec reading is 2.0 or below at 800 rpm. It is sometimes possible to clean the MAF, but only use a cleaner that does not leave any residue on the MAF. Diagnosis: One of the most common codes we see on OBD-II Volkswagens is P1128, a long-term, fuel trim lean code. Any unmetered air introduced into the engine can cause P1128 to set. Tips: Carefully check for vacuum leaks. Include all areas that might introduce unmetered air to the engine. Don't forget exhaust leaks before the O2 sensors. I have found that using propane while monitoring the oxygen sensor signal is a good way to locate small vacuum leaks when a smoke tester is not available. 1) Check and verify that fuel pressure is in specification. 2) If vacuum leak and fuel pressure checks are normal, check the air flow signal from the MAF sensor using a scan tool. The 2000 Jetta 2.0L AEG motor will typically idle at approximately 800 rpm. At 800 rpm the MAF should indicate grams/sec at 3.5 or above. 3) Snap accelerate the engine. The MAF will typically show a snap reading in excess of 60 grams/sec. 4) If the MAF grams/sec reading at idle is 2.5 to 3.0 or if the snap acceleration grams/sec is below 60, inspect the sensor for dirt or contamination. This is a good time to check the entire intake system for dirt or contamination. Make sure it is ALL clean. Use a cleaner that does not leave a residue (an electrical contact cleaner is a good example). 5) Once the sensor and intake are clean, recheck the grams/sec air flow. If it is now above 3.5 at idle and 60 on snap accel, the problem should be solved. 6) But if the MAF grams/sec at 800 rpm is below 2.5, or the snap accel grams/sec is below 50, the MAF may be out of calibration. The problem is that the MAF is incorrectly measuring the amount of air entering the engine. The vehicle power control module (PCM) calculates the required fuel based on the measured air flow. Since more air is entering the engine than the MAF is accounting for, the result is an overly lean condition and a code P1128. Once again, try cleaning the MAF and rechecking the grams/sec air flow numbers. If they fall in line, the problem should be solved. However, if grams/sec are low after cleaning, it's time to replace a faulty MAF.
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