Heating,Ventilation, Air Conditioning > Troubleshooting > B1238 Ambient Temperature Sensor Open (High)

Component Location

General Description

The ambient temperature sensor located at the center stay of the condensor detects ambient air temperature. It is joined to AQS sensor as one boby. It is a negative type thermistor whose resistance is inversely proportional to temperature. Its output is used for sensor fail-safe, temperature regulation door lock, blower motor level control, mix mode control and in-car humidity control

DTC Description

The ECM sets DTC B1238 if there is an open circuit in ambient temp sensor signal harness or the measured voltage value of the sensor is more than the threshold value(4.9V)

DTC Detecting Condition

Item

Detecting Condition

Possible Cause

DTC Strategy

•	Resistance check

•	Open Circuit in harness

•	Faulty ambient temp.sensor

•	Faulty A/C Control Unit

Threshold value

•

>271.1kΩ

Detecting time

•

0.3sec

FAIL SAFE

•	Control with the value of 20°C

Signal Waveform And Data

※ Resistance value of ambient temp.sensor as a function of temperature.

Temperature(°C)	Resistance(kΩ)	Temperature(°C)	Resistance(kΩ)
-20	271.4	50	11
0	95.1	80	3.8
25	30

Schematic Diagram

Monitor DTC Status

1.	Connect scantool to Data Link Connector(DLC).

2.	Warm up the engine to normal operating temperature.

3.	Monitor DTC(Diagnostics Trouble Code) on the scantool.

Is the same DTC displayed again?

	▶ Go to " Inspection/Repair " procedure .
	▶ Fault is intermittent caused by poor contact in the sensor's and/or A/C-ECM connector or was repaired and A/C-ECM memory was not cleared. Thoroughly check connectors for looseness, poorconnection, bending, corrosion, contamination, deterioration, or damage. Repair or replace as necessary and go to "Verification of vehicle Repair" procedure.

W/harness Inspection

1.	Many malfunctions in the electrical system are caused by poor harness and terminals. Faults can also be caused by interference from other electrical systems, and mechanical or chemical damage.

2.	Thoroughly check connectors for looseness, poor connection, bending, corrosion, contamination, deterioration,or damage.

Has a problem been found?

	▶ Repair as necessary and go to "Verification of Vehicle Repair" procedure
	▶ Go to "Signal circuit Open Inspection" procedure.

Signal Circuit Open Inspection

1.	Ignition "OFF"

2.	A/C-ECU, ambient sensor : Disconnect.

Measure resistance between Signal(+) terminal of ambient sensor harness connector and Signal(+) terminal of A/C-ECU harness connector.

Specification : 1Ω below

Is "resistance" display near the specified value?

	▶ Go to "Sensor ground circuit Open Inspection " procedure.
	▶ Check for open in harness. ▶ Repair as necessary and then go to "Verification of Vehicle Repair" procedure.

Signal Ground Circuit Open Inspection

1.	Ignition "OFF"

2.	A/C-ECU, ambient sensor : Disconnect.

Measure resistance between Sensor ground(-) terminal of ambient sensor harness connector and Sensor ground(-) terminal of A/C-ECU harness connector.

Specification : 1Ω below

Is "resistance" display near the specified value?

	▶ Go to " Component inspection " procedure .
	▶ Check for open in harness. ▶ Repair as necessary and then go to "Verification of Vehicle Repair" procedure.

Component Inspection

Check ambient sensor

1.	Ignition "OFF"

2.	A/C-ECU, ambient sensor : Connect.

Measure resistance between Signal(+) terminal of ambient sensor harness connector and Sensor ground terminal of ambient sensor harness connector .(Component side)

Specification : Refer the specifications in fig.1

Temperature(°C)	Resistance(kΩ)	Temperature(°C)	Resistance(kΩ)
-20	271.4	50	11
0	95.1	80	3.8
25	30

※ Resistance value of ambient temp.sensor as a function of temperature.

Is "resistance" display near the specified value?

	▶ Go to "Check A/C-ECU" procedure.
	▶ Substitute with a known-good ambient sensor and check for proper operation. If the problem is corrected, replace ambient sensor and then go to "Verification of Vehicle Repair" procedure.

Check A/C-ECU

1.	Ignition "OFF"

2.	A/C-ECU : Connect, ambient sensor : Disconnect.

3.	Ignition "ON"(ENGINE "OFF").

Measure voltage between Signal(+) terminal of ambient sensor harness connector and and chassis ground .(Component side)

Specification : approx. 5V

Is "voltage" display near the specified value?

	▶ Check connectors for looseness, poor connection, bending, corrosion, contamination, deterioration, or damage. Repair or replace as necessary and then go to "Verification of Vehicle Repair" procedure.
	▶ Substitute with a known-good A/C-ECU and check for proper operation. If the problem is corrected, replace A/C-ECU and then go to "Verification of Vehicle Repair" procedure.

Verification Of Vehicle Repair

After a repair, it is essential to verifying that the fault has been corrected.

1.	Connect scan tool and select "Diagnostic Trouble Codes(DTCs)" mode.

2.	Using scantool, Clear DTC.

3.	Operate the vehicle within DTC Enable conditions in General information.

Are any DTCs present?

	▶ Go to the applicable troubleshooting procedure.
	▶ System is performing to specification at this time.