My '03 suburban threw a P0135 code recently. Being lazy, I purchased a new O2 sensor without troubleshooting the problem to confirm. After installing the new sensor and not fixing the issue, I started to troubleshoot, here's what I found and where I'm at now....need some help.

• Scanning with HPT, I noticed that B1S1 O2 doesn't warm up (increase voltage) during startup like its brother B2S1. B2S1 increases voltage from ~0.450mV to 0.9 mV during startup. B1S1 stays at 0.450 mV during this time.
• I verified 12VDC power and ground from the battery.
• I'm getting 0.450mV from the PCM, but only when I connect to battery or chassis ground. This leads me to believe that the ground wire back to the PCM is open
• I tried connecting the PCM ground with the battery ground (at the O2 wire harness) but this doesn't improve things.

Aside from tracing the ground wire from the harness back to the PCM, any suggestions on how to fix this? BTW, no other codes are present, so I'm not thinking there is a common ground (at the PCM) issue, or I'd see other codes/malfunctions.

Thoughts?

 

The sensor voltage is carried to the PCM on 2 wires (tan, ppl). You should be checking for the signal across those 2 wires. If the signal is changing at the connector but not on the scanner, then yes you may have a wiring / connector issue. Did you check both fuses for the O2 sensor heaters? The pink wire provides 12V to the sensor heater, and the PCM provides the ON/OFF grounding of the heater circuit.

Can you explain what you tested.

 

Hmmm, didn't realize there were 2 fuses to the O2 sensor. Are they under the hood or driver's door. What are they labeled?

It's raining right now, but here's what I recall seeing/checking.

Pink to chassis ground= 12 VDC
Pink to "C" on plug= 12 VDC
Purple (B) to ground= Stepped increase from 0.450 mV to ~0.9mV
Tan (A) to ground= Open (no voltage)

I believe I also checked A to B (thinking the low O2 signal [tan] was a ground path back to the PCM) and got no reading.

 

Under the hood:
Fuse O2A: Sensors-B1S1, B2S1
Fuse O2B: Sensors-B1S2, B2S2

 

I suspect if a fuse were blown, I'd be getting 2 codes rather than 1, but I will check anyhow.

BTW, when I hooked the truck up to my ODB scanner, I could see the Passenger side O2 increasing voltage from 0.450 at startup to 0.900 and then begin to oscillate about 2 minutes later. The D-side stayed flat during this time. I switched O2s and the problem stayed with the D-side.

 

Good reading, the heater circuit is getting it's power, assuming the heater is OK. I'd test the resistance of the unplugged heater (C to D).
Compare the A to B readings with one of the working sensors.

 

Good test! If the D side had 12V power across C-D, then I'd be looking for wiring issues, exhaust pipe burned wires, critter chewed wires, corroded connectors.