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Matt, Mechanical Engineer
Category: Mercedes
Satisfied Customers: 22750
Experience:  BEng hons Mech engineering, in auto industry 22 years
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C230k 51 plate running very rich, fault code 2031 showing and

Customer Question

C230k 51 plate running very rich, fault code 2031 showing and no output from lambda/o2 sensor
Submitted: 4 years ago.
Category: Mercedes
Expert:  Matt replied 4 years ago.

I'd try replacing the oxygen sensor relay which I believe is in the underbonnet fusebox, this powers up the heaters in teh lambda sensors and as they pull a reasonable amount if current they are prone to failure
Customer: replied 4 years ago.

Hello Matt.

please can you confirm relay and location. Had already checked fuse 53 next to relay8 also fuse number48 is ok. Is there a exhaust Gas temperature sensor that is only other fault code that came up once.





Expert:  Matt replied 4 years ago.
Hello Alec

Mercedes tend not to follow the rules for generic OBD readers and unless a dealer level tool (STAR diagnostic ) is used then you can get all sorts of odd codes

the reason for saying this is that a petrol C class has no exhaust temperature sensor and the issue is really one of a failing cataylic convertor

as for the oxygen sensor relay I was mistaken its just on a fuse not a relay

so check no 48, 53 &54 in the fusebox thats up on the drivers side A pillar / by the drivers right knee under the dash
Customer: replied 4 years ago.

Checked fuse 54 and checked fuses 48 and 53 again, all are ok. Assume you were referring to engine bay fuse box on drivers side bulk head.

Thanks Alec


Expert:  Matt replied 4 years ago.
Hello Alec

that should be the right fusebox is the fuse numbers match up

I'd next confirm if the lambdas are getting hot / are the heaters actually working ?

if you turn the ignition on for about 30 seconds and don't start the engine then briefly / carefully touch the body of the sensors they should be hot ( 100 degrees C +)
Customer: replied 4 years ago.

Hi Matt.

Sorry for delay, had to wait for it to stop raining before jacking up again. All wiring seems to be connected.

The lambda/ oxygen sensors do not heat up, when I turned on ignition and checked as you suggested.

Thanks Alec

Expert:  Matt replied 4 years ago.
OK that confirms that they're not getting power

in that case check the wiring to each sensor is in good condtion with no damaged insulation or hard sections of wire ( indicating a short)

if thats OK then its possible that the fault is internal to the ECU as this does control the switching on / off of the heaters

both of these firms offer a cheap test / repair service