I have a 1997 C230 with 118,000 miles on it. The CEL is on with:
P0133 O2 Sensor Circuit Slow Response (Bank 1 Sensor 1)
and pending:
P0136 O2 Sensor Circuit Malfunction (Bank 1 Sensor 2)

I replaced the front O2 sensor with a universal Bosch, and the code remained. So I bought an OE Bosch replacement, installed it, and the code came back again. Since I've gotten the code with 3 different O2 sensors, I know the problem is something else. I don't know if the P0136 problem is related or if it just went undetected because it was pending. I tried cleaning the throttle body and running some Sea Foam through the system, and cleaning the MAF but to no avail.

Do you have any ideas as to what would cause a slow response on an O2 sensor when the sensor itself is fine? Thanks in advance for any help.