IAT Rationality Test

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

The IAT 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) at key-on after a long (6 hour) soak. If the IAT sensor input and the CHT (ECT) sensor input agree within a tolerance (+/- 30 deg F), no malfunction is indicated and the test is complete. If the IAT sensor input and the CHT (ECT) sensor input differ by more than the tolerance, the vehicle must be driven over 35 mph for 5 minutes to confirm the fault. This is intended to address noise factors like sun load that can cause the IAT sensor to indicate a much higher temperature than the CHT (ECT) sensor after a long soak. Driving the vehicle attempts to bring the IAT sensor reading within the test tolerance. If the IAT 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 IAT "Out of Range" check is also performed. The test continuously, checks to see if IAT is greater than the "IAT 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 ~ 40 mph for 250 seconds then set a P0111.

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.

The IAT rationality test employs alternate statistical MIL illumination. This protocol allows up to 6 trips before MIL illumination based on the magnitude of the measured error. The greater the error the fewer number of trips before a DTC will be indicated. In the case of the IAT rationality test the measured error is the difference between the IAT input and the CHT (ECT) input.

The error space is divided into bands. Each band represents a range of error. There are two bands for each of; 5 trips to pending DTC, 4 trips to pending DTC, 3 trips to pending DTC, 2 trips to pending DTC and 1 trip to pending DTC. There are two bands for each because there is one band for positive error and one band for negative error of the same magnitude range.

Counters are maintained that keep track of how many trips a malfunction has occurred within each band. When a sufficient number of trips with a malfunction has been achieved in any band, a P0111 DTC will be set.

If an IAT error, trip to trip, remains just above the IAT-out-of-range error threshold, it will take 6 trips to illuminate the MIL. If the IAT-out-of-range error, trip to trip, is much larger (80 deg F), the MIL will illuminate in the standard 2 trips.

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