Audi Allroad Aircon problem

[et0]

Hi all, I got a new old car recently and (of course) the aircon doesn't work.

Plugging in the laptop shows an error that the HP sensor (located on the side of the condenser) "signal too low".
There is an LP sensor as well.

So you switch on the AC and after about 2 seconds the "econ" light comes on and the fault code reappears.
The condenser fan doesn't run and I don't hear the compressor kick in.

My question is: does this indicate insufficient refrigerant, or simply that the compressor didn't start so the high side pressure didn't increase, so the ECU turned it off again?

There aren't many places around that claim to do automotive AC other than putting more gas in so I just want to try and diagnose it first.

Thanks for any pointers

[hyperion]

Put some gauges on and physically check the pressures that are present. This should confirm if there is any gas in the system. Obviously you would need to observe the normal safety precautions when attaching gauges to a system.
Normally the HP sensor has several functions, one of which is to prevent the compressor from running if the standing pressure is below a pre-set minimum value. Another is to sense a higher than pre-set value to bring on the condenser fan and thirdly to act as a high pressure safety cut out in the event of excessive pressures. All of these various values are relayed back to the ECU for interpretation.
The HP sensor would not normally be used to sense a lack of increase in pressure if the compressor failed to start. The reason the compressor failed to start is due to no control signal being sent from the ECU.

[et0]

Thanks for that, I'll get gauges on it.

[charlie patt]

you have to split this into two ways before clutchless ie before 2004 and after 2004 ie clutchless like above says fit gauges but on both cases the ec is flashing because its bringing up a self diagnoistic code on the ac system both have two common faults because you dont here a click it is probable after 2004 these have two common faults firstly the trianary pressure switch witch locks out due to switch fault or condensor fans faulty which is a common issue or fault two comps break up fills orifice with debris and pressure has no where to go fit your gauges and compare with scanner and hope its that if its a clutchless comp its a days work if you need a trianary switch they are epromed so matched to reg of car