What does it mean?
Aside from controlling the engine, the engine control module (ECM) is in constant communication with other modules, such as those can control the anti-lock braking system, automatic transmission, and climate control system. With regards to climate control, the ECM needs to know when to activate the air-conditioning compressor, which command is received from the climate control module (CCM).
Similarly, the CCM needs to know coolant temperature, ambient air temperatures, and air-conditioning compressor function, which signals it gets from the ECM. On vehicles equipped with automatic climate control, one simply selects the desired cabin temperature, or temperatures, depending on how many zones the system is equipped to handle. The CCM simply handles the rest, including application of heat or air conditioning, blend door positions, vent door positions, fan speed, and others.
To illustrate, if the driver selects 72 °F in automatic, the CCM takes into account outside and inside temperatures and coolant temperature when calculating how much heating or cooling to apply. If it’s 45 °F outside and 50 °F inside, the CCM will apply a heat from the heater core, until the inside temperature reaches the desired 72 °F.
On the other hand, if it’s 85 °F outside, and 100 °F inside, because you just got in your car on a hot summer day, the CCM will command maximum air conditioning and fan speed to cool off the cabin. To make these calculations, the CCM counts on a number of signals, such as outside ambient air temperature, one or more inside air temperatures sensors, air-conditioning clutch operation, engine coolant temperature, and others.
If there is a problem with one of the signals, the CCM sets a diagnostic trouble code (DTC) in memory. Depending on system design, a CCM DTC may need to be read directly from the CCM or it may command a DTC to be stored in the ECM. In case the CCM or ECM detects a problem with the outside ambient air temperature sensor circuit, it will set DTC P0070 Ambient Air Temperature (AAT) Sensor Circuit.
Additionally, the AAT signal may be used to display the exterior temperature, perhaps in the instrument cluster or in the climate control head unit itself. Note: Circuit High Input codes are mostly caused by defective alternators that develop excessive voltages, although there may be other causes as well, such as short circuits that “leak” current into a particular system from other, unrelated systems that work on higher voltages.
Note that in the case of short circuits that leak power into a control circuit, there is likely to be other, seemingly unrelated trouble codes present along with the code(s) being investigated. Diagnosing a “high input” code will always involve a thorough testing of the charging system as a first step, followed by measures to isolate the system from all other possible sources of power during resistance, continuity, and reference voltage tests.