Hey, Why is My Transmission Warning Light on?

This image shows a wiring harness dirty by the transmission connector's leaks. This type of issue affects the transmission control module.

A customer came into the shop complaining that their car was performing weaker and wouldn't pass 25 miles per hour. In addition, they said that before the check engine and transmission lights popped on the dash, the car was driving well.

First, the technician did a test drive, verifying that the check engine and transmission warning lights appeared on the dash. They also noted that the vehicle was in limp mode, limiting the transmission to the 2nd gear and the rev limiter at 3000 rpm.

Moreover, a vehicle health inspection didn't lead to any new findings. So, the customer agreed to perform additional testing, revealing failures in the transmission control module and system. As it turns out, a code activated the car's safety limp mode, so the technician cleared all codes.

The additional tests didn't show a faulty connection or electrical components. So, the mechanic finally diagnosed the issue as a failed transmission control module (TCM).

This image shows debris and particles contaminating the car's transmission system; these are worrying signs and can progressively damage the vehicle.

A customer brought in their car because it wasn't shifting gears smoothly. They said that when the car shifted gears while driving, it would slow down, the engine revs would go up, and then it would finally shift gears after a few seconds.

The technician did a test drive, noticing the car's erratic gear-shifting behavior and engine revving. In addition, they also saw both the check engine and transmission warning lights on the dashboard.

During a vehicle health inspection, the mechanic noted that the transmission fluid was dark and produced a burnt smell.

The technician used a tool to check the car's computer and found that one of the sensors on the transmission was not working correctly; the output speed sensor. So they took the car for a test drive with two technicians, one driving and the other monitoring the transmission with the scanner.

They saw some worrying signs, so they removed the pan on the transmission and found a lot of debris and worn parts inside.

This image shows a scanning tool's monitor, which is an additional test instrument. The duty cycle and slip speed RPM values indicate transmission issues.

A customer brought in their car because it was making a strange shudder when going at a steady speed, but it would stop when they accelerated more. Apart from that, the car was driving normally.

During a test drive, the technician found the car displayed both the transmission and check engine lights. In addition, they noticed that the car would shudder when its torque converter should be locking up.

The torque converter is a fluid coupling transmitting torque from the engine to a rotating driven load. It transfers power from the engine to the wheels, allowing the car to change gears smoothly, and protecting the transmission from damage; a "bridge" between your engine and the transmission.

The technician did a vehicle health inspection that showed no relevant leads to the customer's complaint. So, the auto shop got authorization to perform a more in-depth testing procedure.

The technician used a tool to check the car's computer and found the converter clutch system malfunctioning. So they took the car for a test drive with two technicians, one driving and the other monitoring the transmission with the scanner.

The results indicated that the car's computer told the torque converter to lock up. When the convertor was supposed to be locked, the RPMs did not change. This failure was also causing a drop in fuel mileage, likely due to a faulty torque converter.

This image shows a valve body. The red arrow points toward a stuck solenoid; rust and wear signs surround the solenoid. The computer sends a signal to the solenoid to move, but the valve is not changing position. This indicates that the internal transmission requires repair.

A customer came in with their car and said that it feels like it's in two gears at once when it shifts gears. Plus, the gear-shifting was unusually hard.

The technician did a test drive, noticing both the check engine and transmission lights on the car's dashboard. Also, they verified that the car's transmission was hard shifting between first, second, third, and fourth gear.

They also performed a vehicle health inspection, revealing that the transmission fluid was very dark. But, since there were still underlying matters, the customer agreed to additional tests.

The technician used a tool to check the car's computer and found that both the shift solenoid "A" and "C" had "performance or stuck off" issues. So they took the car for a test drive with two technicians, one driving and the other monitoring the transmission with the scanner.

The results indicated a delay in the shift solenoids' transmission. So, the technician took out the transmission pan, discovering a lot of dark fluid. Finally, they flushed the transmission with a flushing solution, replaced the valve body, cleared codes, and filled the transmission. At last, the symptoms were gone.

This image shows a locked shift interlock. For the transmission to operate properly, the interlock should unlock when the engine is running and the brake pedal depressed. The red arrow points toward the faulty part.

A customer came into the shop complaining that their vehicle's transmission wouldn't get out of the "park" gear. Consequently, they had to ask for a tow.

During a test drive, the mechanic used a screwdriver to access the shift interlock system's bypass switch, to get the car into the "drive" gear. They already suspected the shift interlock system since the car wouldn't shift from the "park" mode.

But, after getting the car running, there were no driving issues. The customer agreed to additional testing.

After disassembling the shifter assembly, the technician measured to see if power was getting through the shift interlock; they found out that it was. Then, after manually depressing the shift interlock, the transmission moved out of the "park" mode.

Still, since this part comprises many other transmission components, the entire shifter assembly needs replacement.