Customer Concern: P1128 (VW Code 17536) Long Term Fuel Trim (LTFT) Lean.
Tests/Procedures: To diagnose a code P1128, proceed as follows:
1. Check and verify that there are no vacuum leaks or unmetered air leaks.
2. Check and verify that the air filter, all inlet screens and throttle body are clear and clean. Never use an aerosol cleaner to clean a throttle body, damage to the throttle body electronics may result. Use a rag dampened with cleaner to wipe the throttle clean.
3. Check and verify that fuel pressure is 36 PSI at idle, with vacuum to the Fuel Pressure (FP) regulator.
4. Check Mass Airflow Sensor (MAF) grams/second (G/S) readings as follows:
A. Idle (about 830 RPM) 3.5 G/S or more (4 to 4.5 G/S preferred).
B. 2500 RPM no load, 9 to 15 G/S (11 to 12 G/S preferred).
C. Wide Open Throttle (WOT) road load (drive vehicle at 45 MPH in 3rd gear and perform a WOT acceleration, observe maximum indicated MAF G/S readings) 90 or more G/S should be present (prefer above 100 G/S).
D. A MAF that exhibits consistently lower than normal G/S readings can cause a P1128 code to set.
Tech Tips: P1128 or VW proprietary code 17536 indicates that the LTFT numbers show a lean condition. P1128 will set when LTFT numbers approach +25.
Volkswagen has issued a warranty extension due to a high number of MAF failures. MAF's are now warranted for 7 years/70,000 miles on 1999 through 2001 VW vehicles.