04 sierra 1500 4.8l 4x4 evap vent circuit

I had the truck dropped off by another shop that couldn't get the catalyst & evap I/M monitors to go through after many attempts with no fault codes stored. They said they originally had vent code & tank pressure sensor codes & replaced both parts ( I'm assuming they just threw the parts at it). So I tested evap system & found the vent valve not working with scan tool activation. I unplugged connector & tested power ( good ) & control & found the control wire to be grounded at all times with koeo. So I found control wire at pcm & cut wire by pcm & same result testing on pcm side. Anyone run into this before ? Possibly bad pcm or am I overlooking something? I've seen control wire grounded n replaced it from pcm to valve before but not this.

Do you have the e light on?
To run the monitors on some vehicles you need to do special driving conditions at different drive cycles to set evaporate and cat monitors. But if it’s due for a smog evap can be incomplete but not cat monitor. I check all data to see if I can find the driving conditions for this truck??

No check engine light. I don't think the monitor will run because vent control circuit driver is bad in pcm. They've been trying to get monitors to go through for a couple months now from what I was told. I had attempted driving procedure first and decided to check system when Monitors wouldn't run for me.

I guess it's possible you've got a stuck driver in the PCM? :huh: But I'm really surprised it doesn't have a circuit code to go along with it. And even if it didn't set a circuit code, it should have set an EVAP large leak code.

No codes set during your testing with the vent solenoid unplugged? That should have at least set a circuit low code, IF everything was working correctly. :silly: How were you testing the wiring at the vent solenoid? Test light, DMM, Power Probe?

Has anyone tried disconnecting the battery and touching the cables together? Just in case the PCM is logic locked into activating that driver.

I did try disconnecting the battery and holding terminals together. Vent was stuck in on position with koeo driver stuck on & still unable to control circuit with scan tool. I tested first unplugging solenoid with dmm & had solid power & ground. Then with incadescent test light then cut control wire by pcm connector & tested control wire coming out of pcm same.

If the control wire is always grounded I would suspect a bad driver. I would check powers and grounds and possible bent pins at the pcm before condemning.

Matts Auto wrote: I did try disconnecting the battery and holding terminals together. Vent was stuck in on position with koeo driver stuck on & still unable to control circuit with scan tool. I tested first unplugging solenoid with dmm & had solid power & ground. Then with incadescent test light then cut control wire by pcm connector & tested control wire coming out of pcm same.

Way ahead of me! Sounds like it's smoked to me. If you feel like the replacement vent solenoid isn't a junk part, then you're probably good for a new PCM.

Maybe check the current draw of the vent solenoid? Just to make sure it's not part of the problem. I'd expect less than 1A.