What does it mean?
OBD II fault code P0540 is defined as “Intake Air Heater "A" Circuit Malfunction”, and is set when the PCM (Powertrain Control Module) detects an abnormal input voltage from, or in the intake air heater circuit. Variations in the input circuit voltage that fall outside of a predetermined range will set a trouble code, and may trigger a warning light.
Note that not all applications will set a code and trigger a warning light on the first failure; some applications require multiple fault cycles to occur before setting a code, and/or triggering a warning light. Typical intake air heating systems consist of a heating element, a blower to move the heated air into the engine, and various electrical components that include wiring, connectors, a temperature sensor, and a relay.
The function of the intake air heater is to warm up the intake air to improve combustion at low temperatures. While most applications use a coiled element that is placed directly in the intake air’s flow path, others use a variety of designs that increase the elements’ surface area, thereby making the element more efficient. Regardless of the actual design of the element though, the purpose of the element is the same in all cases, which is to heat the intake air to improve combustion.
Note however that the intake air heating system only comes into operation when the ambient temperature falls below a level determined by the manufacturer. There are several advantages to heating the intake air. Reducing the volume of air entering the engine by heating it means that less fuel is required to maintain the optimal air/fuel ratio, since a hot air/fuel mixture combusts more completely than does a cold mixture. This results in fewer emissions, and improved fuel economy.
By way of contrast, applications that do not heat the intake air must add additional fuel to the incoming cold air to compensate for the greater density of cold air, which results in increased emissions and increased fuel consumption. The image below shows the typical construction and location of an intake-air heating element. Note however that actual designs vary between applications and manufacturers.
Note: “Circuit Malfunction” indicates that there is a malfunction in the control circuit, as opposed to a fault in a sensor or other component. With “Circuit Malfunction” codes, replacement of sensors and components in the affected circuit will almost never resolve the problem, since as the code suggests, the trouble is in the circuit .
This distinction between “circuit” and “sensor/component” is a great help to anyone trying to diagnose a circuit malfunction code, since it narrows the list of possible causes down considerably.