Seat Heater Mat Faults - Diagnostic Guidance

Seat Heater Sensor Faults - Each seat has two heating elements: one in the base (seat cushion), and one in the back (squab). There is only one sensor, which is located in the base. This sensor controls both heating elements. If there is a fault with the sensor signal, neither element will operate. If the signal fault is traced to the sensor itself (in the base/seat cushion mat), there is no need to replace the back/squab heater mat.
Diagnosis Of Seat Heater Elements Using Resistance Measurements - The back/squab heater mat consists of a heating element, the base/seat cushion heater mat consists of a heating element with a sensor. Most faults can be confirmed by simple electrical resistance measurements, using the values listed below. It is possible that the sensor could develop a fault causing it to exhibit either higher resistance (resulting in the seat heaters operating at too high a temperature), or lower resistance (resulting in the seat heaters operating at too low a temperature or not at all), but these kinds of faults are uncommon. The maximum resistance of the seat elements are only about 1 ohm each under normal operation, so it is difficult to take an exact reading with a standard multimeter. However, the most likely cause of failure is breaks in a number of the heater element wires, which causes both a significant loss of heating and a corresponding significant increase in resistance. In this situation, both (seat cushion AND squab) elements produce less heat, but contrary to expectation, it is the undamaged element which will show the greatest loss of heating. Given this, suspected seat heater element failures should always be confirmed by electrical resistance tests before replacement. Do not replace components based only on temperature performance.
Simultaneous Failure of Both Seat Heater Mats Is Unlikely - The base/seat cushion and back/squab seat heater elements connect to the HVAC Control Module, but the control module only acts as a splice box and the elements are electrically wired in series (each drawing proportion of battery voltage). The only way for one element to operate while the other is faulty is if one is the elements has a short-circuit fault. Under normal circumstances, where short-circuit faults are present these will be detected by the HVAC Control Module which will switch off both elements. Similarly, if the sensor (in the base/seat cushion element) is faulty, the HVAC Control Module will not apply voltage to the elements and, if one element is open-circuit, then no voltage can be supplied to the other element. Given this, while it may often be the case that both elements cease to operate, it is extremely unlikely that both heater mats are faulty. Consequently, if a fault is confirmed with one of the seat heater mats, do not also replace the other.
Seat Heater Sensor Open Circuit Faults - Previous investigations have found the mostly likely failure mode to be associated with the seat heater sensor (in the base/seat cushion element) developing an open-circuit fault. This can be measured by disconnecting the 4-way plug from the HVAC Control Module, and (Test 1) testing the circuit into the harness; if an open-circuit fault is detected, then test the circuit at the connector under the seat (Test 2). If the open-circuit fault is present both at Test 1 and Test 2, then suspect the seat heater sensor and replace the base/seat cushion element. If the open circuit fault is detected at Test 1 but NOT at Test 2, then the harness is most likely faulty. It may not be necessary to check for DTCs (which log in the HVAC Control Module), but if this is done: B103A-01 = Right Front Seat Heater Sensor fault; B1038-01 = Left Front Seat Heater Sensor fault.