Trouble code P0172 (VW 16556) Fuel Trim Bank 1 Rich.
Tests/Procedures: 1. Check and verify that the fuel pressure is 36 PSI at idle, with vacuum to the fuel pressure regulator (44 PSI with no vacuum). Be sure that there is no fuel leakage into the vacuum line and that the vacuum line is intact and not leaking.
2. Remove and replace the vacuum line to the fuel pressure regulator a number of times while observing fuel pressure. Make sure it changes between 36 and 44 PSI (approximate values) and does not stick high or low. Sticking indicates a faulty regulator.
3. Check the N80 canister control valve, located on the passenger side of the engine compartment near the coolant reservoir, for leaking or sticking. The valve should not leak when de-energized.
4. Using a full function factory compatible scan tool, observe data in the engine address word 01, function 08 read data blocks, display group 033 field 2 actual O2 sensor voltage. Verify that voltage fluctuates rapidly by at least 0.3 volts. Fixed voltages can indicate wiring harness or internal faults of the front O2 sensor.
5. Check and verify that the engine oil is not fuel-fouled. Change it as necessary.
6. Check and verify that the intake screens ahead of the air filter are clean and unobstructed. Tech Tips: Most common failures: faulty vacuum lines to the fuel pressure regulator, sticking fuel pressure regulators or a sticking N80 canister control valve. Start diagnosis by checking these items first.
Trouble code P1296 (VW/Audi code 17704).
Tests/Procedures: 1. Check and verify that the Coolant Temperature Sensor (CTS) data stream accurately indicates actual engine temperature (access indicated engine temperature at the Engine Address word 01, Function 08 Read Measuring Blocks and Display Group 004, Display Field 3).
2. Measure actual engine temperature with an external thermometer and compare it to the CTS data stream indicated temperature. Both should be nearly the same.
3. Check the CTS connections and wiring for damage or corrosion. Verify that the CTS resistance is 250 to 350 ohms at full engine operating temperature (200 degrees F).
4. Check for a possible stuck thermostat (open or closed) or a faulty water pump impeller (check if the radiator temperature matches the temperature in the engine). Tech Tips: Trouble code P1296 (VW code 17704) indicates that the engine has not reached correct operating temperature within a given time frame. Water pump, thermostat, CTS wiring or a faulty CTS could potentially set this code.