Both upstream signal wires are maxed out

My pcm for my gmt900 Silverado isn't showing the 450mv bias voltage. It is fixed high at 1.275v on both upstream sensors. I can force both signal wires low with a test light, I have unplugged both upstream sensors and I have jumped the signal wires and signal grounds at the O2 sensor connectors and verified continuity at the pcm conmector. As a last resort I cut both signal wires just after the pcm and it's still fixed high on my scan tool. Most of the time it shows open loop fault but when it does go into closed or I forced it in closed loop both short term and long term fuel trims peg out negative. When I unplug both upstream sensors truck runs fine. I have codes P0172 and P0175. I have checked power to the pcm and added an extra ground from pcm to battery and now I'm pulling my hair out. Help please. I also have pending codes P0132, P0134,P0152 and P0154. My fuel pressure is 57.9 psi and I have checked to see if the injectors are leaking by pressuring up the system and clamping on the fuel line going to the rail and the pressure holds for 15 minutes.

Hey, Nali! I moved your thread to the Repair section.

What year/make/model/engine is your GMT900, specifically? That's important because some years ran a 450mV signal bias, and some use a 1900mV bias. Your specific Y/M/M/E will decide which one your truck uses.

Have the upstream oxygen sensors been replaced recently? If so, what brand of sensor?

It's a 2012 Chevy Silverado 2500hd with a 6.0L flex fuel.

Nali wrote:

It's a 2012 Chevy Silverado 2500hd with a 6.0L flex fuel.

Gotcha. I snipped this from the P0132 'Diagnostic Aids':

This ECM uses a bias voltage on the high and low signal circuits of the HO2S. The voltage measured between the high and low signal HO2S terminals is approximately 1,900 mV. The voltage measured between the low signal HO2S terminal and ground varies based on the temperature of the HO2S and will typically be between 0 and 1,000 mV, under normal operating conditions. The value of the high signal circuit when measured to ground will be approximately equal to the voltage measured between ground and the low signal circuit plus 1,900 mV.

If you're using Global OBD data to observe the oxygen sensor signal voltages, then seeing 1.275V makes sense, since 1.275V is the highest that PID is programmed to read. The actual signal voltage is probably closer to the 1,900mV described by service info.

Have the upstream oxygen sensors been replaced recently?

I have replaced both sensors and they are Bosch.

Okay. Thanks. Maybe I can stop beating my head up against the wall now. Yes. I changed the O2 sensors about six months ago and everything was fine until now. Before I messaged you I got the truck to operating temperature and both sensors were oscillating with a dvm connected to the signal wires. So since the bias voltage is correct what should the voltage be on the signal wire with a dvm attached when I force the signal wire high? So maybe my pcm doesn't like the bosch sensors????

Yes. I changed the O2 sensors about six months ago and everything was fine until now.

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So maybe my pcm doesn't like the bosch sensors????

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I have replaced both sensors and they are Bosch.

I can almost guarantee that your ECM doesn't like Bosch sensors. :lol: Been here a couple times. There's something about this bias circuit that just doesn't play well with Bosch sensors. Very similar to the way aftermarket sensors don't play nice with Chrsyler/Dodge/Jeep/Ram NGC controllers.

Regardless of where you end up with this diagnosis, I'd change those sensors out with NTK or OE. They're available from RockAuto if you'd rather not visit your local dealer.

Before I messaged you I got the truck to operating temperature and both sensors were oscillating with a dvm connected to the signal wires.

Do you recall what the fuel trims were when you observed the oxygen sensor signal switching?

So since the bias voltage is correct what should the voltage be on the signal wire with a dvm attached when I force the signal wire high?

If you're forcing the signal high with a resistive short to power (like your body), then the signal will likely remain at 1.275V on your scanner, as the PID won't read any higher.

That 'Diagnostic Aids' snip I posted before pretty well outlines what voltages to expect at the oxygen sensor connector with a DVOM.

The OEM sensors did the trick. Thank you. She's running like a top again.

Nali wrote:

The OEM sensors did the trick. Thank you. She's running like a top again.

Thanks for the follow up, great to hear!