DTC 121

DTC 121 Chart:

A/C Compressor Control Circuit:

*** Test Revised Per TSB 55-65-04 ***

CIRCUIT DESCRIPTION
The A/C high side temperature sensor uses a thermistor to control the signal voltage to the Powertrain Control Module (PCM). The PCM applies a voltage on CKT 732 to the sensor. When the sensor is cold, its resistance is high; therefore, the PCM will see a high signal voltage. As the sensor warms, its resistance becomes less and the signal voltage is pulled low through the sensor ground, CKT 452.
DTC 121 will set if the signal voltage indicates less than -12°C (10°F) (open circuit) or over 210°C (410°F) (shorted circuit). These conditions can be observed in Tech 1 data display as an "A/C Hi Side Temp" reading.

DTC 121 WILL SET WHEN
- Outside air temperature sensor failure not indicated by the PCM.
- Outside air temperature between 4°C (40°F) and 99°C (210°F).
- A/C high side temperature sensor less than -12°C (10°F) or greater than 210°C (410°F).

ACTION TAKEN (PCM WILL DEFAULT TO)
- With a current DTC 121 set, the PCM will turn the high speed cooling fans "ON."
- With a current DTC 121 set, the PCM will illuminate the Malfunction Indicator Lamp (MIL) (Service Engine Soon).

DTC 121 WILL CLEAR WHEN
The fault no longer exists.

DTC CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.

1. Determines if conditions to set DTC 121 are present.
2. With A/C high side temperature sensor harness terminals "A" and "B" jumpered, the Tech 1 should display 215°C (419°F).
3. With the A/C high side temperature sensor disconnected, the Tech 1 should display "A/C High Side Temp" of -13°C (9°F).
4. When CKT 732 is jumpered to ground, the Tech 1 should display "A/C High Side Temp" greater than 215°C (419°F).
5. When backprobing PCM terminal "A 1" to ground the Tech 1 should display "A/C High Side Temp" greater than 215°C (419°F).