99 ML 320 with 235,000 miles. Seems to run fine. Started it up to come home last night and CEL was on. Went ot Autozone and they pulled codes for me. P0136 and P0161. Autozone guy said Bank 2 refers to "after the cats." (The cats were both replaced 3.5 years ago at 166,000 miles. Would they have replaced "post cat" O2 sensors at that time?)

Did a search on this forum and got nothing on P0136 and very little on P0161.

Google search yielded:

P0136: Essentially the same as P0137, a P0136 code refers to the second oxygen sensor on Bank 1. The O2 oxygen sensor produces a voltage between .1 mV and .9 mV. The ECM monitors O2 sensor voltage and determines if exhaust is lean or rich. O2 sensor voltage is high when exhaust is rich and low when exhaust is lean. The ECM monitors this voltage and increases or decreases fuel injector pulsewidth according to engine fuel/air ratio. If the ECM detects low HO2Sensor voltage for an extended period, it will set P0136 Conditions required to set: HO2 sensor voltage is low for longer than 2 minutes (minutes depend on model of vehicle. Could be up to 4 minutes)
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Possible Solutions - Replace the second oxygen sensor on bank 1
Repair open or short in signal circuit
Clean corrosion from connector
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P0161: It basically means the PCM detected either an excessive current draw, or an open or short in the circuit powering the oxygen sensor's heater circuit. This is in reference to the Heated Oxygen Sensor (2) located rear of catalytic converter (Bank 2).
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Possible Solutions - Replace HO2 Sensor
Repair open or short in power circuit
Repair open or short in ground circuit
Repair damaged connector due to tracking


Anyone care to suggest where I should start to resolve this issue.

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