Ambient Air Temperature

For many years, Ambient Air Temperature was not able to be directly measure by a sensor input. Ambient Air Temperature was inferred from the Intake Air Temperature (IAT) sensor. The IAT sensor is normally located as part of the MAF sensor. Under cold start conditions (long soak, ECT < 140 deg F, no block heater), there is minimal underhood heating and the IAT sensor is a good estimate of ambient. The inferred ambient temperature is the IAT sensor value. Under normal warmed up conditions, the IAT sensor reading is normally higher than actual ambient due to the underhood heating from engine, exhaust, A/C, etc. The value of inferred ambient is updated at higher vehicle speeds under conditions where IAT reflects actual ambient. When the IAT reading is higher than the current inferred value, the ramp rate is slow due to lower confidence that the elevated temperature is the result of increasing ambient temperature. When the IAT reading is lower than the current inferred value, the ramp rate is fast since underhood effects are less likely.

Beginning in the 2013 MY some vehicles will have an Ambient Air Temperature (AAT) sensor wired to the PCM. The PCM simply reads the sensor and sends the sensor value out to the rest of the vehicles (instrument cluster, HVAC system, etc) over the CAN network. The AAT sensor is typically located as part of the driver's door mirror.

Beginning in the 2015 MY, some vehicles will begin using the AAT sensor as a measure of ambient temperature for some OBD monitors rather than inferring it from IAT as described above. This change was prompted by the replacement of the MAF sensor by a MAP sensor on some port fuel injected engines. The IAT sensor is part of the MAP sensor. It is close to the intake manifold and reflects manifold charge temperature rather then the temperature of the air passing through the MAF sensor. The higher temperature environment makes it less able to accurately infer ambient air temperature.

If utilized by the software, the ambient temperature from the AAT sensor will be used by the evaporative system monitor, the PCV system monitor and the thermostat monitor rather than the inferred ambient from the IAT sensor.

AMBIENT AIR TEMPERATURE SENSOR CHECK OPERATION

DTCs	P0072 - AAT Sensor Circuit Low P0073 - AAT Sensor Circuit High
Monitor Execution	Continuous
Monitor Sequence	None
Sensors OK	Not Applicable
Monitoring Duration	5 seconds to register a malfunction

TYPICAL AAT SENSOR CHECK MALFUNCTION THRESHOLDS

Voltage < 0.51 volts or voltage > 4.93 volts

ECT, IAT, EOT TEMPERATURE SENSOR TRANSFER FUNCTION

Volts	A/D counts in PCM	Temperature, degrees F
4.89	1001	-40
4.86	994	-31
4.81	983	-22
4.74	970	-13
4.66	954	-4
4.56	934	5
4.45	910	14
4.30	880	23
4.14	846	32
3.95	807	41
3.73	764	50
3.50	717	59
3.26	666	68
3.00	614	77
2.74	561	86
2.48	508	95
2.23	456	104
1.99	407	113
1.77	361	122
1.56	319	131
1.37	280	140
1.20	246	149
1.05	215	158
0.92	188	167
0.80	165	176
0.70	144	185
0.61	126	194
0.54	110	203
0.47	96	212
0.41	85	221
0.36	74	230
0.32	65	239
0.28	57	248
0.25	51	257
0.22	45	266
0.19	40	275
0.17	35	284
0.15	31	293
0.14	28	302

The Cylinder Head Temp Sensor uses a switchable input circuit to create two transfer functions for cold and hot range temperatures

CHT TEMPERATURE SENSOR TRANSFER FUNCTION, COLD END

Volts	A/D counts in PCM	Temperature, degrees F
4.899	1002	-40
4.861	995	-31
4.812	985	-22
4.75	972	-14
4.671	956	-4
4.572	936	4
4.452	911	14
4.309	882	22
4.14	847	32
3.95	808	40
3.737	765	48
3.508	717	58
3.26	666	68
3.00	614	77
2.738	560	87
2.478	507	96
2.226	455	105
1.985	406	114
1.759	360	122
1.551	317	132
1.362	279	141
1.193	244	149
1.043	213	159
0.91	186	168
0.794	162	176
0.693	142	186
0.604	124	194
0.528	108	203
0.462	95	204

CHT TEMPERATURE SENSOR TRANSFER FUNCTION, HOT END

Volts	A/D counts in PCM	Temperature, degrees F
4.235	866	168
4.119	843	168
3.993	817	176
3.858	789	185
3.714	760	194
3.563	729	203
3.408	697	212
3.244	664	221
3.076	629	230
2.908	595	239
2.740	561	248
2.575	527	257
2.411	493	266
2.252	461	275
2.099	430	284
1.953	400	294
1.813	371	303
1.680	344	312
1.556	318	320
1.439	294	329
1.329	272	338
1.228	251	347
1.133	232	356
1.046	214	366
0.965	197	375
0.891	182	383
0.822	168	392
0.760	155	401
0.701	144	408
0.648	133	415
0.599	123	422
0.555	113	428
0.513	105	433
0.476	97	438
0.441	90	442
0.409	84	447
0.380	78	450
0.353	72	454
0.328	67	457
0.306	63	460
0.285	58	463
0.265	54	465
0.248	51	468
0.231	47	470
0.216	44	472
0.202	41	474
0.190	39	475
0.178	36	477
0.167	34	478
0.156	32	480