'96 S-10/2 dr

CE light with a PO446 code.

Took it to a local mechanic for "diagnosis." But he actually didn't diagnose anything with certainty. Instead, he tells me it's "probable" that it's a bad vent valve. What a friggin' scam.

So he suggests that I have him install a new vent valve ($190). I say to him, "But you can't guarantee it's the vent valve, right? So what happens if it's not the vent valve? I pay you to install another part or parts until you get it right at my expense?"

Crap!!

So...Question: Does anybody here know if a definitive diagnosis of a bad vent valve can be made without having to install a new one to see if it solves the CE light problem?

 

This might help.
Got this info off the net.

P0446 Evaporative Emission Control System Vent Control Circuit Malfunction

The code P0446 is set when the engine computer recognizes a fault or restriction at the vent control part of the Evaporative emission control system (EVAP). The description of the fault varies between different car manufacturers, but one thing is common, the code P0446 doesn't directly point to a defective part; some testing might be required to pinpoint the faulty component.

What can cause the code P0446:

- faulty vent control valve
- blockage at the vent hose, vent filter or vent control valve
- open or short in the vent control valve circuit
- bad charcoal canister
- EVAP system leak
- faulty EVAP system pressure sensor
- stuck open purge valve (solenoid)

Looks like the mechanic wants to do the easy obvious fix.

What needs to be checked:

Vent control valve
The vent control valve and vent hose need to be checked for blockage; The vent control valve also needs to be tested; the vent control valve is an electrical solenoid/valve that closes when the voltage is applied. Unless an obvious problem is found (e.g. blockage at the vent control valve), the whole EVAP system might need to be tested with a special testing equipment known as a 'smoke machine'. A good scan tool with active test capability can also help. Not every auto repair shop has this equipment, you might need to visit a dealer or a larger well-equipped shop for repairs. Most places are probably guessing and throwing parts at it.
Hope this helps.
Maybe somebody that has had a similar problem can give you more guidance.

Mark K

 

Following the flow chart for P0446 will diagnose the problem with 100% accuracy. The flow chart can be found here: https://blazerforum.com/forum/diagno...plained-35143/

 

Hello Captain,

That's a detailed flow chart to follow.
Does one need a particular kind of scanner to
do some of the functions. It seems there are
steps that would require one.

I could see this being a costly thing to get
diagnosed in a shop. It seems time involved.


Thanks - Mark

 

To diagnose it properly, yes, a bi-directional scan tool is required. Any shop that does this type of work needs to have one. Point being, they are ~$10K to purchase, ~$1k annually to maintain current software, plus train someone to use it. $190 is a reasonable charge, IF they diagnose it correctly. Vent solenoid failure is fairly common due to where it's located, (under the vehicle). The solenoid is ~$40 and takes 20 minutes or so to replace it. It's just a guess, but for $40 it beats a $190 guess

 

If you can get to the connector you need to check for voltage on the red wire, backprobing between the wire and weatherpack seal with a paperclip or small gage stiff wire is best, with the connector plugged in, you should get 12 volts, (Use a meter, not a test light) if thats good then youve verified half the circuit is ok, the 12 volt supply is good. Now move your paperclip to the white wire, (DO NOT use 2 paperclips/backprobe pins, if you accidentally shorted both wires it could fry the circuit inside the PCM) you should still get 12 volts if the coil inside the solenoid is ok, the 12 volts on red is passing through the solenoid winding and then you are measuring the same voltage on the other side of the solenoid, if the coil is open you will not get the 12 volts on the white wire, you may get 0.5 volts or something like that.
The pcm senses circuit integrity basically by sampling to see if 12 volts is available on the white/ ground, when it grounds the white wire it gives the +12 a path back to the battery and thus energizes the solenoid.
The only thing you CANT test without the scantool is if the PCM is actually grounding the white wire. Chances of the PCM being faulty over a wire or solenoid problem is probably less than 5%. It's just easier if you can manually click it on and off and test that way.
Heads up i was going by memory here so wire colors may not be 100% accurate but GM tends to stick with same color wires and circuit design.
Hope this helps.

 

Nice job, Dime!

 

Thanks Captain,
Thanks Dime,

Your experience and knowledge shows through.
Money Savers.

Very appreciated.

Mark

 

Agree! Outstanding job! Like it says in Captain Hook's sig, you have to know how a system works before you can figure out why it doesn't! Too many parts changers out there simply throwing parts at problems based on codes! That is a sign of inexperience.

Working on cars these days is similar to modern medicine. Although definitive tests and equipment is available for nearly every illness/malfunction, cost of definitive diagnosis can be a big problem unless you are independently wealthy. You have to understand the system and rule out the simple things when you can. Then you need to make a judgement call on what is left as potential problems. It's a balancing act, especially if you are making your living at it.

In this case, the simple testing rules out the simple things. The judgement call is that the problem is highly unlikely to be the PCM, and this simplifies the diagnosis. Unfortunately, not everyone is up to this level (medical doctors as well)! It always irritated me that the general public tends to perceive auto technicians as not all that bright and often dishonest. In fact, a good auto technician is required to be extremely intelligent as well as extremely knowledgeable.

Les

 

Les, If there was a "Like" button on here, I'd push it.