p0136 - Rear o2 Sensor - 99 C230K Troubleshooting
 

I do NOT have a CEL light but recently plugged in my very basic OBD II scanner (no live data graphing or anything) and recieved a P0136 code. This the o2 Sensor Circuit Malfunction (Bank 1, Sensor 2) error which indicates the o2 oxygen sensor after the catalytic converter. There are no other codes.

When I try to clear the code with my scanner, the code comes back immediately without even starting the engine.

My 99 c230 Kompressor with the M111.975 engine has just under 40k miles. The gas mileage has been low for quite some time but the car runs smoothly and doesn’t misfire. I recently replaced the MAF with a new Bosch unit from a reputable parts place but the code was there before the MAF replacement and after (it is a long story but I have had a suspicion my MAF was unreliable for a long time). I am very confident that all of the intake connections from the intercooler back to the throttle body (including the MAF) are not leaking.

Any help troubleshooting this would be appreciated. Is this one of those issues where I need to go get it diagnosed with a high tech system like the Mercedes SDS diagnostics versus just replacing o2 sensor or is it pretty certain with this code that the problem is going to be sensor itself?

Is it possible on the w202 C Class to disconnect the rubber exhaust hangers from the cat back and be able to get enough clearance to swap out the rear o2 sensor or do I have to disassemble the whole exhaust system?

Is an o2 socket wrench necessary or will an open wrench work just fine? If a socket is necessary, is the full socket (like a lawn mower spark plug wrench) best or is the smaller crow’s foot type better?

Any advice would be appreciated.

Thanks!

The rear O2 sensor monitors cat converter function. It does not affect fuel management.
A code P0136 may mean that one or more of the following has happened:

• Faulty O2 sensor leak in exhaust close to O2 sensor
• Short to voltage on O2 signal circuit
• Open in circuit resistance caused by corrosion in connector

I can't tell you too much but I know a little... First of all some OBDII scanners can report live O2 data (my ScanGauge can), so you could watch it and see if it looks OK. That might give you some idea about the wiring. The ScanGauge is really cool but costs about 200.00, but it is cool... I did replace my cat and I remember the exhaust system is in two pieces. If you need to you could try lowering it just by disconnecting the hangers, but you would want to loosten the manifold bolts first. Sounds like Chas knows more than me. Good luck.