DIY DIAGNOSTICS
As with any DIY repair, extra effort spent on air-conditioning diagnosis can save you from costly mistakes. Before suspecting the pressurised circuit, interrogate the electrical system via the EOBD socket. Obviously, more advanced equipment is needed than equipment that communicates solely with the engine management ECU. Should the compressor refuse to engage, which is characterised by a ‘click’ sound emanating from the clutch (if fitted), you can check the pressure readings diagnostically, as well as checking that the low-refrigerant switch hasn't activated. This shuts down the air-conditioning if the pressure drops too low, protecting against oil starvation.
Research your vehicle’s specification, because other parts may prevent the air-conditioning from working, such as faulty cabin sensors. Most cars switch on their engine cooling fans when the air-conditioning is activated, so a problem with the fan control unit (or relays) may be to blame. Climate control systems may shut down the air-conditioning if either an ‘Eco’ button is depressed or the outside air temperature is too low (usually below 5°C).
Therefore, check the live data readings taken from the air-conditioning ECU, and suspect relatively inexpensive sensors first rather than snap-diagnosing costly parts, such as the compressor.