Hello and Assalamu Alaikum to our readers! Today, we have a very interesting case study on a Toyota Passo. The owner brought the car to our workshop explaining that two days after a routine tune-up, a warning light suddenly appeared, and the car went into Limp Mode—it would not accelerate past 40 km/h.
The owner mentioned he had already taken it to another workshop where they tried to clear the trouble codes using a scanner, but the problem remained. Watch our complete diagnostic video below to see the fault.
1. Initial Scan and Code Analysis
The first step was to connect our diagnostic scanner to the Toyota Passo. Several "U" codes were stored, which indicate a loss of communication between electronic control modules.
Lost Communication with TCM (Transmission Control Module)
The TCM is responsible for controlling the automatic transmission. When the engine control unit (ECU) cannot communicate with the TCM, the vehicle enters a protective "Limp Mode" to prevent damage. We tried clearing the codes, but they returned immediately, confirming a permanent (hard) fault.
2. Checking the Basics – Fuses and Wiring
Before replacing any expensive components, we always check the basic systems first. This included inspecting:
- Transmission-related fuses.
- Battery terminals and voltage.
- Grounding points (Earthing).
- Wiring harness connectors.
Everything was found to be intact and in factory condition, with no external power or communication breaks.
3. Final Diagnosis – Internal TCM Failure
With all external checks completed, we removed the Transmission Control Module (TCM) for internal inspection. Externally, the unit looked clean. However, once we opened the casing, the fault was immediately visible.
The main processor IC on the TCM circuit board had physically blown / burnt out.
This internal short circuit destroyed the processor, cutting off all communication with the ECU. This is why the scanner could not "talk" to the transmission, forcing the car to stay in 40 km/h limit mode.
4. The Solution – Replacing the TCM
Because the processor itself was damaged, the only effective solution was to replace the faulty TCM unit. We sourced a compatible unit, installed it, and carried out the necessary calibration.
This case highlights how an internal electronic failure can cause major drivability issues even when the wiring and mechanical parts are perfect. Thank you for reading, and Allah Hafiz.
🔧 Need more expert fixes? Explore The Auto Pulse homepage for more diagnostic guides.
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