1996 Nissan Truck error code/computer question

I’m getting a p0446 code for the evap canister vent valve. The valve is new and works fine with 12V directly from the battery. It also works fine jumping it from the positive pin at the valve plug to frame ground, but the valve does not work if grounded from the plug positive pin to the computer pin (which is the ground of the circuit and that circuit does show 12v with voltmeter.) The harness from the plug to the computer has continuity as well.

The only thing that makes sense to me is that there’s high resistance through the computer that causes the valve not to work even though the circuit is closed with 12V through it. I have cleaned and sanded all the ground straps. No other codes have been thrown


Thank you.

What is the resistance to ground through that pin? I.E. How many ohms to a good chassis ground?

From your description, that would seem to be the problem, no ground through the connector to energize the valve. You need to trace the wire back to its connection to ground… a good wiring schematic would show where that is.

Most electrical problems on old cars are with bad, corroded grounds!

1 Like

The canister itself could be saturated or clogged. I had a 97 and this is one model that does not tolerate over filling of the gas tank as it will saturate the canister.


I’m getting about 600K Ohms from that computer pin to a reliable body ground. I assume it should be quite a bit lower? 600K Ohms is choking off the amperage needed to power the valve plunger?

When I first bought this truck, that had already happened. Loose charcoal made it up to the engine and packed the evap valves under the hood. I flushed all the lines really well and replaced all the packed valves and the charcoal canister. So far, the new canister charcoal is staying in the canister for several years now, and it’s not clogged and doesn’t appear to be saturated.

I am going to say that is too much resistance if it is from the valve connector pin.

If that wire leads all the way back to the computer, it is possible there is a resistor in the ECU to reduce current. Since you have powered the valve with 12v across the pins and didn’t burn the coil out, I would rule that out.

You have a corroded ground somewhere in that circuit.

That was with the computer unplugged of course.

Next measure the resistance in that ground control wire. Unplug the vent valve solenoid, back-probe the red wire with a pin and connect a ground wire. Now measure the resistance again from the PCM connector.

You mean the wire that goes from the vent plug negative pin back to the computer? That’s 0.8 Ohms.

That wire is good. If you have 12 volts at the power wire and the ground wire is good then the circuit seems to be good. Be aware this valve only closes to test the system for leaks so you may not see when it is in use.

This fault is not specifically for an open circuit, if the fuel tank has pressure or vacuum when the valve should be open then the computer determines that the vent valve is malfunctioning. Restrictions/plugged lines can be the cause of this fault. P0446 often occurs when there is an evaporative emission system leak but there is usually also a leak fault.

Thanks everyone for the input. The harness that plugs into the computer wasn’t plugged in all the way from the factory or prior owner, I assume. I made sure it was plugged in tightly and the code hasn’t returned. I assume one of the pins wasn’t making good contact, setting off the code.