Hello, please help.
Got codes P0133 (O2 bank1 sensor1 slow response) and P0140 (bank1 sensor2 no activity?). Replaced both sensors with OEM Bosch and cleared codes. Got the same errors. Cleaned MAF just in case, checked electrical connections, checked for exhaust leaks, and checked for vacuum/intake leaks. Same errors are comming back.

Searched web: code P0140 is not listed in Mercedes startechinfo.com, it is probably a generic OBD. Found info somewhere that 140 follows P0133 so I should concentrate on this one. Also, don't think it is fuel trim because it relates to one bank only. FYI: use Actron reader and E420 1997 has 130k miles. Works fine but I want to clear CEL.

I measured voltage at the sensors with voltmeter. All sensors read .44V with KeyOnEngineOff and cold engine running. With hot engine running: Bank 1 sensors: S1 .1-.8V flactuates fast all the time. S2 flactuates slow and is more speed dependent: .2V @low, .7V @hi RPM. Sensors on Bank2 have the same reading as on Bank1 except S2 is not speed dependent. Keep in mind that I don't know what the ECM is reading but should be the same since no other codes for the sensors. Again: both sensors on Bank1 are new.


Additional info: when criss cross wires on left and right sensors got many codes related to malfunctioned sensors and missfire (but no 133 or 144) - which means that both sensors are read by the ECM.

Question 1: what to do next? I found in this forum that Ethan had problem with sensors even after replacing them but he did not posted solution from the Mercedes shop. Ethan, please respond.

Question 2: why the code P0140 is not listed on the official Mercedes StarTech for my model - is it valid error?

Any suggestions?