2006 Toyota RAV4 DTC P0037 Keeps returning

I have an 06 Rav4 with a 2AZFE engine and 62K kms. On the Odo. Recently we had some work done on the headlights and the day after that we had rough idling on a cold start until the engine just died.

First scan with an ODB-2 reader said MAF sensor and VSV purge. So we had it sent to the dealership and they scanned again and saw DTCP0037 and P0031. This was after they cleaned the throttle body and assured us that the MAF-S and also the VSV purge were working properly.

Dealership assured us that replacing the 02 sensor and air/fuel ratio sensor would solve the problem.

2 days later we got the parts and replaced the old ones. Everything was fine for about 5 minutes, no CEL, no DTCs and engine was idling at 600 rpm. Smooth and stable.

After the 5 minutes the CEL came on again.

Scanned again and P0037 again.

Removed the negative terminal overnight to see if it would re-set the ECU.

Next morning, cold engine start, revs at 2000 rpm for 2-3 minutes before dropping to 1000 rpm.

Even after reaching operating tempt. Rpm stays at 1000 and sometimes it goes up to 1500 when stopping in traffic and putting gear in Park.

Had it scanned again and same DTC P0037, cleared the DTC and engine returned to 600 rpm at idle but

CEL came back on after 5 minutes.

Mechanics are saying it could be the ECU/ ECM.