AAT Rationality Test

The AAT rationality test determines if the AAT sensor is producing an erroneous temperature indication within the normal range of AAT sensor input.

The AAT sensor rationality test is run only once per power-up. The IAT sensor input is compared to the CHT sensor input (ECT sensor input on some applications) and the IAT sensor at key-on after a long (6 hour) soak. If the AAT sensor input, the CHT (ECT) sensor input and the IAT sensor input agree within a tolerance (+/- 30 deg F), no malfunction is indicated and the test is complete. If the AAT sensor input, the CHT (ECT) sensor input and IAT sensor input differ by more than the tolerance, the vehicle must be driven over 25 mph for 300 seconds to confirm the fault. This is intended to address noise factors like sun load that can cause the AAT sensor to indicate a higher temperature than the CHT (ECT) or IAT sensor after a long soak. Driving the vehicle attempts to bring the AAT sensor reading within the test tolerance. If the AAT sensor input remains outside of the tolerance after the vehicle drive conditions have been met, the test indicates a malfunction and the test is complete.

In addition to the start-up rationality check, an AAT "Out of Range" check is also performed. The test continuously, checks to see if AAT is greater than the "AAT Out of Range High threshold", approximately 150 deg F. In order to prevent setting false DTC during extreme ambient or vehicle soak conditions, the same count up/count down timer used for the IAT startup rationality test is used to validate the fault. If IAT is greater than 150 deg F and vehicle speed is greater than ~ 10 mph for 300 seconds then set a P0071.

Either the IAT startup rationality test or the IAT Out of Range High test can set a P0111 DTC. The logic is designed so that either fault can trigger a "two-in-a-row" P0111 MIL, however, both faults must be OK before the P0111 DTC is cleared.

Block heater detection results in a no-call.