If an automatic transmission malfunction is detected, check the ATF level first (see chapter Current service). If the level is insufficient, check the condition of the relevant oil seals. Carefully inspect all hoses related to the transmission fluid, as well as the coolant, and make sure that there are no signs of leaks. The only thing you can still do yourself is to check and, if necessary, replace the sensors related to the operation of the transmission. Before replacing any sensor, read fault codes from the memory of the control unit.
In case of a relatively serious breakdown, the box control system will switch to the safe operation mode, while entering the appropriate code (or codes) to the memory of the control unit. Codes can be read using special equipment (see chapter Engine electrical equipment).
If not, determine the nature of the problem (electrical or mechanical).
Do not attempt to remove the box, as the diagnostics are performed only on the box installed on the engine.