Hi all,
I have a 95 4wd 4.3 L Blazer.
I had new exhaust installed. The muffler splits out to 2 pipes so it looks like dual but really everything before the muffler is still stock. After the exhaust went in I am now seeing a P0138 (Downstream O2) being thrown out after hitting it with my OBD2 reader. I've replaced the O2 sensor just after the cat and it seems the error is coming right back. Is it possible that this new exhaust is causing the fault?

Any help is greatly appreciated.

Thanx,
Aqua

 

The new exhaust should not cause this code to set.

Check out the attachments below.

DTC Chart Test Description:
1. Determines if conditions necessary for setting DTC P0138 exist
2. Disconnecting HO2S 2 and jumpering CKTs 1670 and 1671 to ground should cause the Tech 1 to display HO2S 2 voltage below 0.2 volt (200mV). If the signal voltage is still high, the PCM (VCM-A) is faulty.

Diagnostic Aids:

• Rich exhaust - If DTC P0131, P0132, or P0134 are presetn, diagnose these DTCs prior to DTC P0138. An overly rich exhaust may load the catalyst causing high HO2S 2 signal voltages.
• False rich indication due to silicon contamination of HO2S 2. This will be indicated by DTC P0138 accompanied by a powdery white deposit on the sensor.
• Faulty HO2S 2 (catalyst monitor) - If HO2S 2 is internally shorted the HO2S 2 voltage displayed on a Tech 1 will be over 1 volt. Try disconnecting the HO2S 2 and jumpering CKT 1671 to engine ground; if the displayed voltage goes from over 1000mV to around 450mV, replace HO2S 2.
• Intermittent test - Observe HO2S 2 on Tech 1 while moving related connectors and wiring harness with key "ON". If the failure is induced, the HO2S 2 display will change. This may help to isolate the locatino of the malfunction.
• Damaged Harness - Inspect the PCM (VCM-A) harness for a short to voltage in CKT 1670.

Hope this helps!