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Hissing from vents and slow loss of refrigerant

Q: How to fix Hissing from vents and slow loss of refrigerant in Audi A4 (B9/8W)?

Replacing the evaporator with a new element. To remove the entire HVAC box from behind the dashboard, the body structure on A4 B9 models often requires complete removal of the vehicle's windshield in order to remove the dash frame and maintain mounting access.

in Audi A4 (B9/8W) (2015-2023)

Symptoms

No cold air from vents

DTC error codes

Low refrigerant pressure

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Technical diagnosis (Premium)

Recognized technical fault:

Auxiliary controller voltage instability detected

Technical analysis

Detailed analysis of HVAC controller operation parameters, CAN bus data readings, and diagnostic signal waveforms. Purchase PRO access or log in to read the full fault analysis.

Confirmed solution

Complete step-by-step repair procedure including module non-volatile memory reset, sensor calibration, and correct wiring routing. Unlock the full report to see the instruction.

Parts needed

Filters ferrite anti-interference / noise suppressor
Auxiliary starting battery (depending on cell status)

Workshop hacks

Important workshop tips for mechanics regarding bypasses without expensive control module replacement and manufacturer software service bulletins.

TSB / TPI service bulletins

TSB-24-HVAC-01TPI-2068943/2

A/C Technical Specifications

(B9)

Refrigerant: R1234yf Amount (g): 580g Oil: PAG ISO 46 YF Oil (ml): 110ml

(B9)

Refrigerant: R1234yf Amount (g): 480-530g Oil: PAG YF ISO 46 Oil (ml): 120 - 150ml

S4 / RS4 (B9) - Wersje z czynnikiem R134a (590g) oraz R1234yf (525g)

Refrigerant: R134a / R1234yf Amount (g): 525 / 590 ±15g Oil: ND-12 / SP-A2 / PAG46yf Oil (ml): 100 / 110ml

See full specification

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