A recent AC issue in a 2015 Volkswagen Golf led to an in-depth investigation using VCDS, highlighting the importance of correct sensor readings for proper climate control function. The problem stemmed from inconsistent readings from the G805 pressure sensor, responsible for monitoring refrigerant pressure in the AC system. This article details the diagnostic process undertaken to isolate the faulty component.
Initial VCDS Readings and Comparisons
Initial VCDS scans revealed discrepancies between the expected and actual pressure readings. Comparing data from a functioning 2017 Golf TSI provided a baseline:
- 2017 Golf TSI (Functioning):
- Key On, Engine Off (KOEO), AC Off: Manifold Absolute Pressure (MVB) = 8.6 bar
- KOEO, AC On: MVB = 14.2 bar
These values contrasted sharply with the problematic 2015 Golf. Further testing on the 2015 model yielded confusing results, suggesting a faulty G805 sensor.
Testing the G805 Pressure Sensor
Several tests were performed to isolate the G805 sensor as the potential culprit:
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KOEO Test: With the engine off, the MVB reading was 2.2 bar, while manual pressure gauge readings indicated approximately 6.8 bar (calculated from 100 psi / 14.7 psi/bar). Engaging the compressor didn’t alter the MVB reading, despite significant changes in high and low-side pressures.
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Disconnected Sensor Test: Testing both the original and a replacement G805 sensor while disconnected from the refrigerant line showed readings of 0.8 bar and 1.0 bar respectively. Considering the vehicle’s elevation of 2800 feet, the 0.8 bar reading from the original sensor was deemed plausible.
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Bypass Harness Test: Bypassing the wiring harness between the sensor and the HVAC module didn’t change the MVB reading, ruling out wiring issues.
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Disconnected Signal Wire Test: Disconnecting the G805 sensor’s signal wire resulted in a 51 bar reading from the Engine Control Module (ECM), confirming the ECM’s responsiveness to the sensor’s presence or absence.
Considering the HVAC Module and Reset Procedures
While the evidence strongly pointed towards a faulty G805 sensor, the possibility of a malfunctioning J301 Manual HVAC module was also considered. Documentation suggested performing a “Replace Control Module” function and adaptation procedures using VCDS when installing a new HVAC module.
Furthermore, VCDS offered two reset options:
- IDE01475 – Resetting all adaptations: This option remained untested at the time of writing.
- IDE02325 – Reset to factory settings: This option resulted in an “ERROR: Request out of range” message.
Conclusion: Likely G805 Sensor Failure
The extensive testing strongly suggested that the aftermarket G805 sensors were not functioning correctly. The inability of the sensors to respond accurately to pressure changes likely caused the AC compressor to shut down intermittently, particularly at idle when high-side pressure could build up excessively.
The next step involved procuring a genuine VW G805 sensor to confirm the diagnosis. This case underscores the importance of using OEM parts, especially for critical components like pressure sensors, and the power of VCDS as a diagnostic tool for identifying complex automotive issues.
