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 speed sensor SP2 and input speed sensor NT 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.).

Fig 1: Identifying Shift Solenoid Valves And Valve Body

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DTC No.	DTC Detecting Condition	Trouble Area
P0729	6th gearshift malfunction: The ECM determines there is a malfunction when both of the following conditions are met. (2-trip detection logic) When the ECM directs the gearshift to switch to 5th gear, the actual gear is also shifted to 5th. When the ECM directs the gearshift to switch to 6th gear, the actual gear is shifted to 4th.	Valve body is blocked up or stuck (sequence valve) Shift solenoid valve SLT remains open or closed 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
Actual gear position under malfunction	↑	↑	↑	↑	↑	4th

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
Actual gear position under fail-safe mode	↑	↑	↑	3rd	3rd	3rd