Engine code p0136 03 ranger
 

Hello, just got a Ranger for my kid and has the check engine light on. Initially the code was p0137, I replaced downstream oxygen sensor with a Bosch and now every 3 days after resetting check engine, I'm getting code p0136. From what I've researched, may be exhaust leak, intake leak or electrical signal. Fuse is good. Any suggestions anyone? Thanks, Carlos