Can this be fixed by me?

A malfunctioning control panel or membrane can cause erratic behavior in your microwave oven. If the control panel is not registering the start command correctly, it may interrupt the cooking cycle. Try cleaning your membrane and control panel with a soft and damp cloth to remove the dust.
If cleaning doesn't resolve this issue, try resetting the microwave oven by unplugging it from the power source for a few minutes, and then plugging it back in.
The microwave may not generate the required heat to cook the food properly if the magnetron or high voltage capacitor is defective. This could lead to the timer counting down without any heating action.

To cook with microwave only, press "Microwave/Grill/Combi", once, the screen will display "P100". Press "Microwave/Grill/Combi" to select the power level. Then press "Start/+30Sec./Confirm" to confirm and turn the dial to set cooking time. Then press "Start/+30Sec./Confirm" to start cooking on microwave only.

To switch to microwave/ grill combination cooking, choose C1 or C2 when selecting the power level.

See page 14 and 15 in the user manual.

TCP is "True Cook Plus" in Kenmore speak.
Here is a cut/paste of a section of the manual.

*CUSTOMIZING TrueCookPlus' You may find that you prefer TrueCookPlus® to cook your food more or less. TrueCookPlus " can adjust to your tastes - to cook your food hotter or cooler - automatically: Example Clear 1. Press the Clear button. (§H§ 1 2. Press the TrueCookPlus® button twice. 3. If you want less cooking time when you enter a TrueCookPlus code, press 1. This will decrease the time by 6%. 345 4. If you want more cooking time when you enter a TrueCookPlus® code: Press 3 for 3% more cooking time. Press 4 for 6% more cooking time. Press 5 for 9% more cooking time. 2 5. Press 2 to reset TrueCookPlus* to factory setting default time. 6. Press the Start button.
I didn't try to clean up the copy
Hope this makes sense and helps.

There are door switches and one of them is evidently out of adjustment or the little actuator arm on it is.

The first thing I would suggest is a hard reset. Unplug the unit for a minute or so. Then plug it back in, set the clock and try again.

If that doesn't work, leave it unplugged for about 5 minutes.

You must dismantle the circuit board and check if some grease vapours between touch buttons and the the circuit film. Clean with some wet soft cloth.

HELLO.

Solution to your microwave problem is given as below.

* 1 Set your Daewoo microwave on a countertop near an electrical outlet and plug it in. Open the door and place the circular roller guide in the center of the oven cavity, then place the glass cooking tray over it.

* 2 Set your food on a microwave-safe dish and then put the dish on the cooking tray and close the door. Make sure that it latches properly.

* 3 Turn the upper knob to select a power level. Power levels include "Defrost," "Med-High," or "High." If your Daewoo has a touch pad, touch the "Microwave" button once to set the highest power level. Touch it consecutive times to lower the power level.

* 4 Turn the lower knob to set the cooking time. For cooking times less than two minutes, turn the knob up past the two-minute mark, then back to the correct time. For touch pad models, enter in the cooking time with the numerical keys and then press "Start."

* 5 Open the door at any time to stop the cooking and check the temperature of your food. Shut the door to automatically continue cooking, or turn the timer knob to the right to add more time. For touch pads, press the "Stop/Clear" button, then key in a new cooking time and press "Start."

* 6 Open the door when the timer reaches the "Off" position and take out your food.

Thanks.

The owner's manual is here. On page 10 we find:

COOKING IN ONE STAGE
1. Press the POWER button; the Microwave and High
Cook indicators will light and “PL 10” will light in the
display.
2. Press the POWER button different times to set the
desired power; the display will show the percentage
you chose. If selecting power level 70%-10%, the
Low Cook indicator will light instead of the High
Cook indicator.
3. Press number keys to set the desired cooking time.
OPERATION(CONTINUED)
4. Press the START/+30SEC. button to start cooking;
the Microwave and one of the Cook indicators will
blink and the display will countdown the remaining
cooking time in minutes and seconds. It will beep
five times when cooking is completed.

We're happy to help you with free advice and we'd appreciate your thoughtful rating of our answer.

Pressing "timer/clock" button 4 times turns on and off demo mode.

Quite often this is due to a bad door switch. Opening the door while cooking (without first hitting the STOP pad) is the primary cause of door switch failure. If you or a friend decide to work on it, we have *critical* safety, disassembly, and troubleshooting info (as well as a text file which helps in diagnosing and repairing door switch problems) at our site, which is linked at our listing here on FixYa: http://tinyurl.com/yzjozk