Slew of DTC Codes
 

Hi All, three long years and I'm back. I've had the SES light on for a few months and decided I should check it. I was expecting one, maybe two codes, but instead was greeted by this:

Generic:
P0137 Oxygen Sensor Circuit Low Voltage B1S2 (Stored)
P0403 Exhaust Gas Recirculation Circuit Malfunction (Stored)
P0404 Exhaust Gas Recirculation Circuit Range/Performance (Stored)
P0405 EGR Sensor A Circuit Low (Stored)
P1404 CHEVROLET - Exhaust Gas Recirculation Closed Position Performance (Stored)
P0137 Oxygen Sensor Circuit Low Voltage B1S2 (Pending)
P0403 Exhaust Gas Recirculation Circuit Malfunction (Pending)
P0405 EGR Sensor A Circuit Low (Pending)

Chevrolet:
P0403 Exhaust Gas Recirculation Circuit Malfunction (MIL)
P0405 EGR Sensor A Circuit Low (MIL)
P1404 CHEVROLET - Exhaust Gas Recirculation Closed Position Performance (MIL)
P0137 Oxygen Sensor Circuit Low Voltage B1S2 (Current)
P0403 Exhaust Gas Recirculation Circuit Malfunction (Current)
P0405 EGR Sensor A Circuit Low (Current)
P1189 CHEVROLET - Engine Oil Pressure Switch Fault (Current)
P0137 Oxygen Sensor Circuit Low Voltage B1S2 (FSC DTC)
P0403 Exhaust Gas Recirculation Circuit Malfunction (FSC DTC)
P0404 Exhaust Gas Recirculation Circuit Range/Performance (FSC DTC)
P0405 EGR Sensor A Circuit Low (FSC DTC)
P0620 Generator Control Circuit Malfunction (FSC DTC)
P1189 CHEVROLET - Engine Oil Pressure Switch Fault (FSC DTC)
P1404 CHEVROLET - Exhaust Gas Recirculation Closed Position Performance (FSC DTC)
P0137 Oxygen Sensor Circuit Low Voltage B1S2 (History)
P0403 Exhaust Gas Recirculation Circuit Malfunction (History)
P0404 Exhaust Gas Recirculation Circuit Range/Performance (History)
P0405 EGR Sensor A Circuit Low (History)
P0620 Generator Control Circuit Malfunction (History)
P1189 CHEVROLET - Engine Oil Pressure Switch Fault (History)
P1404 CHEVROLET - Exhaust Gas Recirculation Closed Position Performance (History)

So, it's looking like I have issues with the following:

• EGR system
• Downstream O2 Sensor (or catalytic convertor??)
• Generator Control Circuit (or PCM??)
• Oil Pressure Switch

In the last year the EGR valve, battery and alternator have been replaced. I was having an intermittent P0404 when weather is cooler/damp which is why I had the EGR valve replaced. Mechanic said it had carbon buildup and replaced...never solved the intermittent P0404 issue and it has obviously worsened. The battery was replaced because it was the original (yes, 17 years old!) and the alternator was replaced with an Autozone remanufactured because the original had a bearing go out.

About 4 years ago my PCM burned out and was replaced with a remanufactured unit (I was told new OEM was unavailable) and I've always been skeptical of its quality. I am also unsure if it has been flashed with the latest OEM tune.

If this was your car, where would you start with repairs?? Do you think any of the issues are tied to one another?

Thanks,
Richard