Ender 3 displaying wrong temperatures for hotend and bed
I got my printer fixed and am posting steps I took for people with similar problems in the future. 

Initial problem:

After I replaced my entire heating unit and reconnected the wires, my printer started to read about 135/185 °C for the nozzle and bed at room temperature. It gives the "Heating failed, printer halted, please reset" error when I tried to heat the hot end and bed. 

Troubleshooting steps:


Checked the resistance of the thermistors. Both the hot end and bed were about 100k. 
Reflashed Marlin with different thermistor settings. Only the static (dummy) ones worked (reading a static value of 25/100 °C). All the other settings were giving very high readings at room temperature. 
Replaced the mainboard. This fixed the bed reading but hot end was still not working. Checked the hot end thermistor again at this point and couldn't get any values, which suggests the thermistor wire was broken somewhere. 
Replaced the hot end thermistor and that fixed all the issues. 


So what I think happened was that I messed up the board when I was reconnecting the heating unit. Then at some point after I first measured the thermistor resistance, the hot end thermistor broke. Replaced the thermistor and that solved the issue.