System Description

WARNING: This page is about a different car, the 2010 Lexus IS 250. However, it is still accessible from the selected car via links, so may be relevant.

The ECM uses signals from the output shaft speed sensor and input speed sensor to detect the actual gear position (1st, 2nd, 3rd, 4th, 5th or 6th gear).

Then the ECM compares the actual gear with the shift schedule in the ECM memory to detect mechanical problems of the shift solenoid valves, valve body or automatic transmission (clutch, brake or gear, etc.).

DTC No.	DTC Detection Condition	Trouble Area
P0751	S1 stuck ON malfunction^*1 : The ECM determines there is a malfunction when the following conditions are both met. (2-trip detection logic) When the ECM directs the gearshift to switch to 1st gear, the actual gear is shifted to 2nd. When the ECM directs the gearshift to switch to 5th gear, the actual gear is also shifted to 5th.	Shift solenoid valve S1 remains open Valve body is blocked Automatic transmission (clutch, brake or gear, etc.)
↑	S1 stuck OFF malfunction^*2 : The ECM determines there is a malfunction when the following conditions are both met.(2-trip detection logic) When the ECM directs the gearshift to switch to 6th gear, the actual gear is shifted to 5th. When the ECM directs the gearshift to switch to 5th gear, the actual gear is also shifted to 5th.	Shift solenoid valve S1 remains closed Shift solenoid valve SLT remains open or closed Valve body is blocked No. 2 brake malfunction (Driving is difficult.) Automatic transmission (clutch, brake or gear, etc.)

HINT:

Gear positions in the event of a solenoid valve mechanical problem:

ECM command gearshift	1st	2nd	3rd	4th	5th	6th
*1: Actual gear position under S1 stuck ON malfunction	2nd	2nd	↑	↑	↑	↑
*2: Actual gear position under S1 stuck OFF malfunction	1st	1st	↑	↑	↑	N*

N*: Neutral

Gear position during fail-safe operation:

If any malfunction is detected, the ECM changes into the fail-safe mode to shift into the gear positions as shown in the table below.

Gear position under normal conditions	1st	2nd	3rd	4th	5th	6th
*1: Actual gear position under fail safe mode when S1 stuck ON malfunction	2nd	2nd	↑	↑	↑	↑
*2: Actual gear position under fail safe mode when S1 stuck OFF malfunction	1st^*3	1st^*3	↑	3rd	3rd	3rd

*3: Under engine braking downshifting to 1st or 2nd gear is prohibited.