Check engine light, p0420 error and o2 sensor voltages

Good Evening,

Some time ago I bought a used KIA ceed 2009. I’ve checked it with a mechanic who found it clean. The car was stationary for a long time and with little gasoline in the tank.

I changed his oils, filters (not gasoline) and put him a 15 liter 9 RON. Suddenly, the engine control lamp and error p0420 appeared. The engineer told me to put 100 RON petrol and move it a little bit because the stillness may have been precipitated. I put another 10 liters of 100 RON (plus the 95RON already in the tank) switched off CEL and moved it to Athens for a few days and suddenly the CEL lit up yesterday. I take the cellphone diagnostic (Torque pro) and measure the oxygen sensor voltages (front and back of the catalyst). I found that the voltages of the two sensors are the same, which I read is definitely a catalyst. The car has shown no signs of getting stuck, smelling or behaving in a strange way. Meanwhile, fuel consumption has increased (about 10.5 l / 100 km, 1400 cubic centimeters) and only when out of motion does it fall.

Exhaust control values are (I don’t know how hot the car was when it was measured)

Idle: CO 0.12%, HC: 64 ppm,
RPM: 0.15%, HC: 71 ppm, λ: 0.991

I’ve measured the O2 voltages. In the idle, rear voltage (downstream) is a flat line whereas the front voltage (upstream) oscillates slowly. After some time (2mins, still no moving car) the downstream and upstream oscillates the exact same way. The temperature is 90oC and the engine runs in closed loop . Catalytic converter temperature sensor shows about 400oC. (These are the readings from torque and OBD fusion). The p0420 is pending, I’ve erased the last one yesterday.

I plan to change a gas filter to see if it will come out again, otherwise, I suspect there is something wrong with the oxygen sensors (and based on what I’ve read). Do you think that immobility could cause such problems? Shall I move it further so that all deposits are cleared?
Could it be the bad coolant?