C240 OBD codes 136 & 156
 

I got CEL on my 2001 C240, the car has 124k on the clock. It read 2 codes P0136 and P0156. The CEL went off in the meantime. Should I replace both rear (after cat) O2 sensors? I have emission sticker expiring in 3 months, so was wondering if the codes will clear too, since the CEL did.

Check engine light
 

Follow up to my earlier posting, now I have 4 codes 130, 136, 150, 156. CEL is not on and there are no pending codes. Is this just a momentary malfunction of the O2 circuit or something more serious?
Thanks for your advice.