The A/C in my car doesn't work, before any repair, I try to understand which part in the A/C system is the problem.

Here the symptoms:

  • Everything looks fine: no error message, the interior ventilation blows, etc... but not cold

  • The pressure is ok: I've made a gas refill in specialized shop

  • No leaks (checked at the shop)

  • The big external fan of the A/C never start (the one at the front of the motor)

I think the compressor and the A/C clutch are working properly. At full stop, if I switch on the A/C the instantaneous fuel consumption increase from 0.5 L/h to 0.7 L/h. It decrease back to 0.5 L/h when the A/C is switched off. I assume it means the compressor is running, am I right?

Maybe it's a faulty sensor? It would explain why the big external fan of the A/C never start?

I've tried the Forscan OBD software to read car's data.

I have a P0531 default, which is about A/C pressure sensor, but seems to may be linked to other issues.

I've read some sensors data from the OBD, a lot looks normal. Here are all values from the HVAC module:

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The "PCM.PBDACCE" (A/C clutch engaged") value switch from disabled to enabled when I turn on the A/C.

The OBD can read also 3 A/C related pressures values:

  • "PCM.AC_PRES" (A/C pressure sensor) which is 0.2 kPa (A/C off) to 0.4 kPa (A/C on)

  • "PCM.CACRP" (corrected air conditioning refrigerant pressure) which is 158 kPa (A/C off) to 389 kPa (A/C on)

  • "PCM.ACRPSV" (A/C refrigerant pressure sensor A voltage) which is 0.68 V (A/C off) to 1 V (A/C on)

I also have various "out temperature" values "ABS.OUTTMP", "DDM.OUTTMP", "IPC.OUTTMP", "PCM.OUTTMP", "PCM.OUTTMP", "PDM.OUTTMP", "PSCM.OUTTMP" and "SASM.OUTTMP".

All have the same value (about 30°C which is correct) but "ABS.OUTTMP" and "PCM.OUTTMP" have -19°C value. Is it a faulty sensor? Do you know which one?

Any help to find the correct part to be changed? A temperature sensor? The pressure sensor? The compressor?

How can I test if the A/C pressure sensor is working (it's a 3-wire one)?

  • Welcome to Motor Vehicle Maintenance & Repair! Aug 5, 2022 at 18:54

1 Answer 1


What country are you in? Vehicle ac repairs vary as most attempt to discourage do-it-yourselfers from making repairs without advanced knowledge and certification in repairs to prevent releasing refrigerant into the atmosphere. R134a is restricted in some countries to certified repair specialists. R1234yf may also have restrictions to certified repair people. If this isn't a problem then the next question is dye detection as the easiest method to find a damaged system leaking refrigerant. An inexpensive uv blacklight can help search for the elusive leak no one wants to spend time finding because this is approximately 98% of most vehicle ac system problems. The three wire pressure transducer may use a 5v input source,varying output signals between 0.1 vdc to 4.9 vdc according to pressures. I think the middle range of voltages are used by the ecm to allow power to the compressor clutch; too low or too high signal voltage equates to loss of refrigerant (leak or catastrophic damage) or excessively high pressures that may burst a hose or trigger a high pressure relief valve on the compressor. Outside a specific range of signal voltages, the ecm interprets it as a disable command to remove power to the compressor clutch.

Do not presume the compressor is running or clutch engaged from OBD observation. Look at the compressor clutch plate with ac off then on; the clutch plate will remain still or rotate very slowly at engine idle rpm when ac is off. When running, the clutch plate will spin fast with a blur and someone cycling the ac switch inside the vehicle while you observe the compressor should show you whether the compressor is running or not. Hint; most if not all cooling system fan(s) run when ac is operating because the compressor generates pressures and high temperatures going into the condenser coils in front of the radiator. If the cooling fan isn't running, the cooling fan fuse is blown, fan failed, fan wiring disconnected or loss of refrigerant occurred triggering the pressure transducer to send a signal to the ecm. The ecm will disable power to the compressor, preventing it from running. Loss of refrigerant also means little to no lubricating oil returning to continually provide lubrication. Refrigerant moves lubricating oil throughout a system so loss of refrigerant also means zero lubrication. If allowed to run without refrigerant, the compressor will self destruct quickly therefore the reason for the pressure sensor.

  • Thanks for all these informations. I'm sure about the refrigerant, it was checked and refilled by a specialized shop. No leak was found and only a little refrigerant was added. I've tried several things : - replace pressure sensor : no change - shunt the compressor command relay : compressor is now running So the compressor is not broken. The control signal just never go to "ON" state. I'm reading some very low temperatures using and OBD connection. Like -17°C. I think a A/C temperature sensor is broken, but I don't find it.
    – Aurelien
    Aug 16, 2022 at 22:52
  • WIthout service manual info, this can be difficult to diagnose your 2014 Ford Transit 1.6L turbo diesel.
    – F Dryer
    Aug 18, 2022 at 1:15

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