Climate Control Diagnostics and Repair


The Audi Quattro 20V Home

Problems and Diagnostics

Climate Control Diagnostics & Repair

Heater Not Working
Heater Blower Motor Replacement
Diagnosis and Fault Codes
A/C Not Cold
A/C Compressor Clutch
Outside Temperature Sensors


Heater Not Working
There are many possible causes of heating system malfunctions. These are but a few of the more common ones:

  • Stuck thermostat.
  • There is a vacuum actuated heater (hot water) cutoff valve in the input heater hose, located in the engine compartment at the end of the engine block very close to the firewall. Looks like a plastic cone with a visible small actuator attached to a valve in the heater hose. It's function is to cut the hot water flow into the passenger compartment (heater core) when the air conditioning is working and calling for cold air (i.e. climate control in "auto" position and temp setting below the cabin ambient temp). Set climate control to "econ", temp to 85, and check to see that the actuator is extended away from the cone (a little finger help is acceptable). Test by hand feel to see if the hot water is flowing past the valve (after engine is hot, of course). If it does you probably had a sticking valve. If it doesn't, make sure the climate control is in "econ" mode, you may then disconnect vacuum line from the valve actuator (the cone), and plug line with a golf tee. If you can move the actuator away from the "cone" to the open position and the valve is not stuck, you will have heat in the passenger compartment, and maybe a sick climate control programmer. All above assumes coolant levels are ok.
  • (Dan Simoes) The foam that covers the flaps inside the heater box tend to deteriorate, especially in humid climates.

Heater Blower Motor Replacement
(Ray Calvo, Bill Powers)
Some fans rattle or groan on low speed. This is caused by moisture and lack of bearing lube. A product from WURTH called HHS2000 sprayed into the bearings can alleviate this problem for a long time...and it's much cheaper than a new fan that you don't really need yet.

Take out the glovebox -- look for the screws around the perimeter behind the glovebox door. You'll then see the fan mounting on the air plenum. Unscrew these screws and disconnect the power leads. Now, with a lot of wiggling, prying, and forcing, you should get the old fan out (there is just a little lack of room between the fan mounting and the local brickabrack). For a little clearance, remove the dash screws on the passenger side next to the A-pillar. These are under the little round caps. This will allow you to move the dash out slightly, making it much easier to pull the fan out.

Take a page from Audi Dudi's book "if it won't come out, remove something else". The trick is to remove the fan from the housing which holds it. Two screws hold the fan to a plastic housing. Remove these, pull the plastic housing off, leaving the fan inside the plenum. With a minor amount of wiggling, the housing will now come out. Then you are left with the motor and blower. They will also come out. This housing normally comes off with the fan, and taking it off first gives you just enough clearance. Reverse the procedure to install your new fan.

Diagnosis and Fault Codes from the Climate Control Head
22 Diagnostic 'channels' are available on the A/C control head.
(For diagnostic values greater than 199, the first digit is represented as a horizontal and vertical line or "arrow")

Pulling up the display:

  1. Ignition ON
  2. Push & hold the OUTSIDE TEMP button
  3. Push & hold the OFF button
  4. Release OUTSIDE TEMP button then the OFF button

Now you'll see a little vertical line followed by 01. (Looks like '01) This is your 'channel' display. You can adjust the channel by using the warmer cooler buttons on the control head. Once the display shows the proper channel number, press the OUTSIDE TEMP button to show that channel's value. End the session by switching to any A/C mode or switch the ignition off.


System Error
00 = no system error
01 -- 16 = See error table
Measurement Value of inside temp sensor G86 on roof
Depends on temp. (table, Section D8-310)
Measurement Value of inside temp sensor G56 on instrument panel.
Depends on temp  (table, Section D8-310)
Measurement Value of outside temp sensor G57 (plenum)
Depends on temp  (table, Section D8-240)
Measurement Value of outside temp sensor G17 (cowl)
Depends on temp  (table, Section D8-240)
Measurement value of coolant temp sensor G62. 
Depends on temp  (table, Section D8-320)  Note: Display value for vehicles without coolant temp sensor  255 = open circuit
Graphic display of output control info for A/C programmer.
See  Section D8-40
Actual value of feedback potentiometer on temp regulating flap servo motor, v68
Dep on prog:  Numerical value for the position of the servo motor is shown.
Spec value of feedback potentiometer on temp regulator flap servo motor
Dep on prog:  ONLY CHECK when value in CH 8 is between 30 and 200. 
Specified voltage on fresh air blower
Specified in volts
Vehicle Electrical System Voltage
Depends on instantaneous operating condition greater than 9.5 volts.
Total of Electrical system voltage interruptions - values between 5 and 9.5 volts.
Depends on running performance since display was last erased. 
Program Number
No determination... no use in troubleshooting
Switch position on high pressure switch F118 (red housing)
0-5 (high pressure switch closed)
Specified voltage on fresh air blower V2 in volts
Depends on program
Pulse Counter
No significance
Graphic display of compressor shut-off conditions
Depends on program, see sec. D8-50
Graphical display of electrical outputs
Depends on program, see sec. D8-60
Number of times the compressor shut off via high pressure switch F118 (red housing)
Depends on running performance since display was last preset.
Number of times the compressor shut off via high pressure switch F118 (red housing) since last ignition switch cycle or reset from OFF button
Note: After 8th shut off during a driving period, the compressor does not switch on again.
Program number
Not significant
Speed signal
Note: at high vehicle speeds fresh air blower speed is limited in "fresh air" mode
00 when vehicle stopped
01 or higher dependant on vehicle speed.

A/C Not Cold
Symptoms: no more cold.
Lots of possibilities here, but these are some common ones:

A/C Compressor Clutch
(Ian Duff)
A replacement clutch for the Nippondenso compressor is Audi P/N 034 260 811, but you may be able to find it cheaper through an A/C shop as Warner Electric P/N 819-0316 (also used for: Ford FS-6, Chrysler C-171, A-590, Nippondenso 6E171, 10P15, 6P148, 6C17, Tecumseh HR980). The parts fiche says you also need "attachment parts" (Audi P/N 034 298 012), but these are probably included with clutch, itself, so don't get this unless you know you need it.

(Igor Kessel)
I'm not sure the following P/N applies to our compressor, but we do have a Nippondenso:
AC Clutch Bearing (Nippondenso): This double row ball bearing is made by Nachi, P/N NAC40BG05S1DS. cost is $21.50.

Eastern Bearings Inc.
7096 So. Willow St.
Manchester, NH 03103
603-668-3300 phone
603-669-5714 fax

the man to talk to or email is Tom Breault, Operations Manager

Outside Temperature Sensors
(Fred L. DeRoos, Jim Fraser)
These cars use a pair of negative temperature coefficient thermistors to sense the outside temperature. One of them is located in the plenum where the climate control air intake is located (under the hood on the passenger side) and the other is attached to grille next to the driver's side headlight. Both thermistors are identical and are 1000 ohms at 25 deg C. They have an R value of 9.10 and can be purchased from Digikey Corp. in Thief River Falls, MN for approximately $2 each. The part number is KC016N-ND.  This is actually a Keystone Thermometrics part. The K.T. Eng. P/N is RL2004-582-97-D1. The AC head unit reads both sensors, then displays the one that senses the lower of the two temperatures. This is done to prevent the display of an artificially high temperature due to engine or sun heating. The temp may still be a little high, but at least it would be the lower of the two. The problem is when one of these sensors goes bad, it typically increases in resistance. That means that your temp display will show low. If the climate control thinks the outside temp is really low, you'll always get heat from the vents at all settings, except "LO".

The display will operate with only one sensor, so you can do one of two things. Best choice is to check the resistance of each sensor at approximately 25 C and see which one is reading high, then replace it. Second approach is to unplug each sensor separately and see which one makes the temp read correctly (assuming you haven't been parked in the bright sun or just run the car and the engine is hot).

To replace the thermistor, you will need to remove the bad assembly, then remove the plastic tip from the holder. The original thermistor is soldered to the two wires and fits into a plastic tube. The new one is just slightly larger, so you will need to insert a screwdriver into the plastic tube and enlarge it approximately 0.02 -- 0.04 inches. Audi would be happy to sell you a replacement, however, they list for approximately $100.

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