Automatic Transmission Goes into Failsafe Mode: Fault Causes and Troubleshooting Methods

Commonly, automatic transmissions go into failsafe mode in case of severe system disruptions to protect the powertrain from damage and to inform the car owner about the necessity to visit the automobile repair shop as soon as possible. In this article we will consider the main peculiarities of the automatic transmission failsafe mode, and delve into the possible causes of this issue.

The Electronic control unit (ECU) keeps track of the car components operation statuses and the automatic transmission in particular. When the car has any problems related to its automatic transmission, the ECU notifies the driver about transmission issues by means of special indicators (Check AT, HOLD, Check Engine, O/D OFF, “Transmission Failsafe Program”) displayed on the dashboard. Failsafe mode may be referred to by various names such as:

• Failsafe Mode
• Fault Mode
• Limp Mode
• Default Mode

They all mean the same thing, and that thing is that the ECU in the car has detected that something is wrong and has decided that the car gearbox is at risk as a result of whatever has gone wrong.

BMW6 transmission goes into failsafe mode

The most vivid demonstration of a transmission in failsafe mode (in addition to the dashboard warning) is the transmission being limited to operation in one gear, unable to shift gears either automatically or manually. The particular gear it will engage varies from transmission to transmission, but usually it turns out to be 2-nd or 3-rd gear.

Depending on the cause of the transmission failsafe mode activation, the driver may restart the engine and drive normally for a time, but there is a high probability that the cause of the failsafe mode activation hasn’t been eliminated, and the driver has to consult the transmission repair specialist to ensure that the car won’t go into limp mode again. Now we will consider the main cause that usually leads to activation of the automatic transmission failsafe mode.

ATF level issues

There is a certain range of ATF volume required for normal operation of the automatic transmission. The transmission failsafe mode may be triggered by the following issues with ATF:

- ATF overflow, when the ATF level is above the recommended level;

Learn how to check ATF level without a dipstick

An excess of ATF in the automatic transmission leads to its foaming. Wearing surfaces receive less lubrication and tears appear in points of contact. Eventually, it leads to increased wear of transmission parts. Excessive ATF may be taken away via the breather hole. This issue may be fixed simply by drain of excessive ATF. Afterwards, it is required to delete the error code stored in the memory of the ECU.

- ATF incomplete filling, occurs when the ATF level falls below the norm.

There are several reasons of potential ATF shortage in automatic transmissions:

• During maintenance operations specialists have added insufficient amount of ATF. This problem can be easily detected with the dipstick. To fix the problem, it is necessary to add ATF to its normal level (it is extremely important to use the same type of ATF). Otherwise, a chemical reaction may occur between the additives; it may lead to significant deterioration of ATF properties;
• Mechanical damage of the automatic transmission body. In this case, the plan of actions depends on the damage degree. If you continue only to add ATF, then the transmission will regularly go into failsafe mode. To fix this problem properly, it is required to detect the source of the ATF leakage.

ATF leakage

• Failure of the gasket located between the engine and the gearbox. Usually, transmission repair specialists fix this problem by means of the gasket replacement. It is quite a tricky procedure, as it may require a complete teardown of the powertrain with associated units. Only in a number of cars, it is possible to tear down the automatic transmission separately. Therefore, this procedure should be performed at professional workshops, which are equipped with necessary machinery
• Problems with the seal. If the seal loses its functional qualities, drivers can solve this problem independently, by replacing the worn/damaged component.

Oil starvation often leads to increased wear of the contact surfaces. Wear debris contaminates the ATF and leads to numerous damages. The automatic transmission overheats and imposes an additional load on the engine. A long-lasting ride with a low ATF level may result in an expensive repair of the automatic transmission.

Hydraulic system malfunctions or physical damage of the automatic transmission

During the process of self-diagnostics, the ECU can detect a malfunction in the mechanical part of the automatic transmission and, as a result, go into failsafe mode. Wear of clutches is one of the most common reasons of the failsafe mode activation. Electronic diagnostics does not always work properly and rarely specifies the exact trouble spot. Therefore, it is necessary to perform the automatic transmission disassembly and visually inspect each part.

Friction clutches

Broken teeth of gears or pieces of friction clutches increase the wear of accompanying elements of the automatic transmission. Long-continued driving in the car with engaged failsafe mode may lead to costly overhaul, so transmission specialists do not recommend using the car before proper transmission examination.

Failure of electronic components

Issues with the following electronic components of the automatic transmission also may trigger Failsafe mode:

- sensors;

- connecting tails;

- Contact sockets;

- Transmission control module (TCM).

1997 Jaguar XK8 Transmission Control Module

The sensor contacts can oxidize or come off, resulting in failsafe mode actuation. The wiring used in automatic transmissions has a small section and is prone to wear and breaking off. As a result, the ECU will not receive any data. When dealing with problems related to the transmission electronics, it is necessary to read out the error code with the help of a special scanner. The self-diagnostics mechanism of the automatic transmission is not always capable of detecting a faulty sensor, therefore it is recommended to visually inspect the state of all available circuit elements.

Wearing of valve body channels

In case of ill-timed ATF change chances are high that the valve body channels will be clogged or damaged. The automatic transmission has to work with wedged plungers and ATF flows through the channels under increased pressure. Small debris contained in ATF damages the aluminum channels. All these factors lead to activation of failsafe mode.

Damaged valve body channels

It is required to regularly monitor the ATF state in order to avoid unexpected problems related to ATF contamination. It has to be noted that modern automatic gearboxes became highly sensitive to improper maintenance. Therefore, it is recommended to use only high-quality oil and monitor the transmission condition. It is better to take preventive measures than deal with negative consequences later.

Regardless of the reasons why the automatic transmission goes into failsafe mode, it is necessary to visit the service station or, if possible, solve the problem independently. When the driver hears suspicious transmission growl, then it’s better to put the hazards on and call a tow truck, despite the fact that the car control system allows to keep moving. In this case, continued operation becomes risky for the whole powertrain, as it may lead to increased wear of transmission components. The essential point to remember is that the automatic transmission won’t go into failsafe mode for no reason, therefore it’s better to take such problem manifestations very seriously.