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?

Many times P0 133 suggests either a vacuum leak OR excessive fuel on that bank (right).

Getting a P0 140 for the rear(right) sensor also suggests the same thing.

Possibly a leaking injector or fuel pressure regulator?

P0 140 is listed on my MB list of codes for that engine.

You did replace the RIGHT side sensors...bank 1?

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Thank you for quick and valuable hits.

Yes, I've changed sensors on the right/passenger side. Two days ago had the same tought and almost had a panic attack.

I followed your advice and investigated fuel pressure regulator. All 3 (air pump valve, MAP sensor, pressure regulator) vacuum lines were changed and vaccum at low idle is .6 mBar=20 inHg. Regulator is closed when no vacuum. Found this generic info for other engines at http://www.mercedestechstore.com/pdfs/508%20Systems%20II/508%20HO%20Part%2007%20Fuel%20supply%20(WJB)%2004-01-01.pdf . Managed to measure rail pressure: 3.0 Bar at idle and 3.6 Bar for 4000RPM. So looks like regulator is OK.

Could you point me to good info on he web on how to check which injector might be leaking and how to test and fix them? By leaking you mean dripping/partially closed orfices?

Thank you.

Compromised diaphragm in regulator? Sniff the vacuum tube - there should be no smell of gas.

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Fuel pressure at idle should be 3.4-3.5 Bar & with removed another .5Bar.

Fuel pressure regulator could be an issue. 000-078-18-89

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

M.B.Doc thank you and please clarify: your statement "with removed..." should be "with removed vacuum line...". So pressure should be 3.4-3.5 at idle and 3.5-4.0 at full throttle.
It means that I'm about .5 bar too low in both cases. But it also means that my pressure regulator is regulating correctly - it lowers the pressure by about 600mbar at idle. And I 've already checked that regulator is not leaking so why do you think it might be deffective?

It looks to me that for some reason I can't build enough pressure because: pump, injector leak, other leak in the system or...? But I'm not 100% sure why low pressure would affect only one bank. Unless it is a leaking injector which creates both pressure drop and sensors issues.
So my next step is to check injectors. Any other suggestions?

PS. Gmercoleza: no gas in the vacuum and regulator is moving and regulating. Thanks.

Problem solved.

I rechecked everything and found super small exhaust leak. It was a hairline crack in the first, upstream CAT on the weld where cone is attached to the cylinder. Crack was so small that at low idle leak was only detectably when CAT was still cold. It is hard to belive system is so sensitive. After welding and several drive cycle (system goes through some kind of adjustment for the first 3-5 drive cycles) the codes did not showed up anymore.

Found similar problem and solution on the other forum.

So if you have both codes first check for the exhaust leak before the oxygen sensor replacement.