System Description

WARNING: This page is about a different car, the 2015 Lexus IS 250 and 2014 Lexus IS 250. However, it is still accessible from the selected car via links, so may be relevant.

The ECM uses signals from the transmission revolution sensor (SP2) and transmission revolution 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.).

DTC No.	Detection Item	DTC Detection Condition	Trouble Area	MIL	Memory
P0729	Gear 6 Incorrect Ratio	6th gear malfunction: The ECM determines there is a malfunction when both of the following conditions are met. (2-trip detection logic) (a) When the ECM directs the gearshift to change to 5th gear, the actual gear is also shifted to 5th. (b) When the ECM directs the gearshift to change 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.)	Comes on	DTC stored

HINT:

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

ECM commanded gear	1st	2nd	3rd	4th	5th	6th
Actual gear position under malfunction	1st	2nd	3rd	4th	5th	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 under fail-safe mode	1st	2nd	3rd	3rd	3rd	3rd