Mobile air conditioning – Punch Newspapers

Automotive air conditioning is a self-contained system which cools the cabin of a vehicle by changing the phase of refrigerant from gas to liquid and back again by applying and releasing pressure on it. Each part of the system is connected by pipe tubing or hoses which form a closed loop through which the refrigerant flows.  Since this is a continuous circle, there’s no specific start or end point; everything occurs simultaneously. Let’s start with the compressor; often considered to be the “heart” of the system


Like a heart, the compressor is a pump which pushes the refrigerant through the pipe lines or hoses. As it pumps the refrigerant it also compresses it from a low pressure gas to a high pressure gas. This high pressure also heats the refrigerant.  Because of its high pressure contents, this is called the “high side” of the system.


The next procedure occurs in the condenser.  The hot, high-pressure refrigerant flows through the high-pressure tube towards the condenser. The condenser is a series of coils, similar to a radiator, which sits in front of the car’s actual radiator.  As air passes over the condenser it removes heat from the refrigerant. The refrigerant condenses into a liquid as it cools (which is why it’s called a “condenser”).

Thermal expansion valve or orifice tube

The now-liquid refrigerant flows through high-pressure tubing back towards the dash, where it passes through a small valve. The valve restricts the flow of refrigerant allowing only a small amount through at a time, and also relieves the pressure created by the compressor. When the pressure is suddenly reduced, the refrigerant becomes cold.  Depending on the system, this valve is either a thermal expansion valve or an orifice tube.  Since the pressure has been reduced, the refrigerant returns to a low-pressure gaseous state.


The cold, low-pressure refrigerant then goes into the evaporator:   the only part of the air conditioning system that is actually in the passenger compartment. Since the refrigerant is cold now, an electric blower fan pushes the cold air through the vents and into the passenger cabin.

Since refrigerant has a low boiling point, the hot air causes it to quickly evaporate back into a gas (which is why this part’s called the “evaporator”). As it evaporates, it absorbs heat from the air passing through.

Accumulator or receiver/drier

Next, the refrigerant enters into either a receiver drier or an accumulator. Although different, both devices function to filter the refrigerant and absorb any moisture from the system.

After completing the loop, the refrigerant cycles through to the compressor through the low-pressure tube restarting the entire cycle, and repeating continuously.


The ability of refrigerant gas to change its physical properties as it is compressed and cooled is what makes the air conditioning system produce cool air

This series continue.


P1256: Engine coolant temperature sensor open/short to positive


The engine control module monitors the engine coolant temperature sensor. The ECM sets the OBDII code when the engine coolant temperature sensor is not to factory specifications.

Possible causes

Faulty engine coolant temperature sensor

Engine coolant temperature sensor harness is open or shorted

Engine coolant temperature sensor circuit poor electrical connection

P1257: VTEC system malfunction


The VTEC system, in conjunction with the engine control module, activates the VTEC solenoid valve to control oil pressure to the VTEC mechanism that is installed in the valve system to switch valve timing between low and high. If the VTEC solenoid valve has an open or short circuit, the P1257 code is set.

Possible causes

Engine oil level low

Low engine oil pressure

Blocked oil passage

Faulty VCT oil control valve

P1258: Engine coolant overtemperature protection mode active


In order to reduce the engine temperature, the powertrain control module has the ability to disable a number of fuel injectors during an engine over-temperature condition. The PCM considers the engine over-temperature whenever the engine coolant temperature reaches a predetermined temperature. This DTC sets in order to show that the PCM detected an over-temperature condition and that the system engaged the protection mode.

Possible causes

Low engine coolant

Faulty thermostat

Faulty engine coolant temperature sensor

Engine coolant temperature sensor harness is open or shorted

Engine coolant temperature sensor circuit poor electrical connection

P1259: VTEC system malfunction


The variable valve timing and lift electronic control system activate the VTEC solenoid valve by the command from the powertrain control module, and it charges/discharges the hydraulic circuit of the VTEC mechanism that is installed in the valve system to switch valve timing between low and high. The PCM monitors oil pressure conditions in the hydraulic circuit of the VTEC mechanism according to the VTEC oil pressure switch downstream of the VTEC solenoid valve. If there is a disparity between oil pressure condition in the hydraulic circuit that is determined by the PCM command, and the oil pressure condition that is determined by the status of the VTEC oil pressure switch, the system is considered faulty and the P1259 code is stored.

When is the code detected?

The P1259 is set when there is a problem in the VTEC oil Pressure switch circuit or VTEC solenoid valve circuit

Tech notes

Make sure the oil level is good and it is not dirty. If the oil is dirty replace it with the correct oil weight. Dirty oil can clog the VTEC assembly so you might have to take it apart and clean it (and replace the seals).

Possible causes

Low engine oil level or oil pressure

Faulty variable valve timing and lift electronic control (VTEC) solenoid

VTEC Solenoid harness is open or shorted

VTEC circuit poor electrical connection

Faulty VTEC oil pressure switch

P1260: Theft detected vehicle immobilised


The P1260 code can be set if the passive anti-theft system (PATS) has determined a theft condition existed and the engine is disabled or an engine start was attempted using a non-PATS key. This diagnostic trouble code is a good indicator to check the PATS for DTCs. This DTC can also be set when a new instrument cluster, instrument panel cluster or powertrain control module is installed without correctly programming either module even if the vehicle is not equipped with PATS.

Possible causes

Incorrectly programmed powertrain control module Incorrectly programmed instrument cluster or instrument panel cluster

Previous theft condition

Anti-theft system concern.


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