Depending on the model of your MPU, if the green varta battery is discharged the mpu will not boot. What leds are lit on your mpu as you power on. Also the green varta battery (Rechargable barrel type battery) is nothing but a headache as it leaks as it ages, it is best to remove it. If you have a newer 1270 enhanced MPU (the backplane connectors are the same size on these) there is a couple of components you have to remove in addition to the battery. If you have a 500 series mpu (backplane connectors are different sizes), you just remove the battery. Once that battery leaks it causes all kinds of damages where you start getting door open errors.