What does it mean?
OBD II fault code P0890 is a generic code that is defined as “Transmission control module (TCM) power relay sense circuit low”, and is set when the PCM (Powertrain Control Module) or in some cases, the TCM Transmission Control Module) detects an abnormally low voltage from the relay that supplies power to the shift solenoids.
All automatic transmissions use a relay to supply power to the shift solenoids, However, before the relay is activated, the PCM/TCM checks that there is no voltage at the shift solenoids. In this condition, the transmission is held in a fail-safe or “limp mode”, and gear shifts cannot be initiated. When this condition is verified, the PCM/TCM activates the relay, and power can be supplied to the shift solenoids.
When all required conditions are met, the PCM/TCM supplies power to the relay and the output from the relay is then fed back to the control unit via a dedicated sensing circuit.
In a fully functional system this feedback voltage should be equal to (or very close to) battery voltage, but should the control unit detect a feedback voltage that is lower than expected, or is lower than battery voltage by a predetermined value for a predetermined period of time it will set code P0890, and may illuminate a warning light.
The image below shows the typical appearance and location of a transmission control module (TCM) power relay, such as might be found on most modern applications.