P0446 Code (Evaporative Emission System Vent Control Circuit) - ??

grey2112 · October 12, 2019

My vehicle:

2004 Suburban 2500 LT, 4x4, 8.1L engine, Whipple supercharger. Bought from original owner with 208,000 miles.

Have had some Check Engine light codes thrown at me since buying it - have replaced both O2 sensors on the drivers side, gotten new catalytic converters, exhaust manifold, tuneup.

Drove for a week with no Check Engine light showing - yay!

But the other day, I hooked up my boat trailer and plugged in the 7 pin.

Started up the vehicle and the CEL is back.

Towed just fine, no issues.

Took it to get the code pulled: P0446 Code (Evaporative Emission System Vent Control Circuit)

Person at the auto place said it could just be a wiring issue throwing a random code. Tried clearing it (they told me to disconnect the neg terminal from the battery and wait 15 minutes) but the CEL is still on after I did that.

Thoughts?

Formulabruce · October 13, 2019

It could need a new gas cap. Have seen this before a few years ago. I remember it being something simple like the gas cap. Good luck !

grey2112 · October 13, 2019

12 hours ago, Formulabruce said:

It could need a new gas cap. Have seen this before a few years ago. I remember it being something simple like the gas cap. Good luck !

Thank you - got one on order and hopefully that is all that it is the problem.

tbarn · October 13, 2019

Gas cap won't help. P0446 is for a partially or fully plugged evap system. Stuck closed vent valve, pinched line, plugged vent valve, etc.

Jon Kruse · February 15

What was the solution @grey2112?

carkhz316 · March 17

On 10/13/2019 at 2:27 PM, tbarn said:

Gas cap won't help. P0446 is for a partially or fully plugged evap system. Stuck closed vent valve, pinched line, plugged vent valve, etc.

This isn't really true either. The code is when the PCM that it can't control the circuit and determines that the wiring/ circuit is either open, shorted to 12V, or shorted to ground. I just dealt with this on a Grand Prix, but the circuit design is the same for all GM vehicles of this era. The EVAP vent solenoid receives 12V B+ through a switches ignition feed. Then, the PCM grounds this circuit to turn the vent solenoid on. When the PCM doesn't see 12V when it expects to, it triggers this fault. In my case, it was a broken control wire that had been chaffed on the engine harness before the PCM.

tbarn · March 17

DTC P0446

System Description

This DTC tests the evaporative emission (EVAP) system for a restricted or blocked EVAP vent path. The control module commands the EVAP canister purge solenoid valve Open and the EVAP canister vent solenoid valve Closed. This allows vacuum to be applied to the EVAP system. Once a calibrated vacuum level has been reached, the control module commands the EVAP canister purge solenoid valve Closed and the EVAP canister vent solenoid valve Open. The control module monitors the fuel tank pressure (FTP) sensor for a decrease in vacuum. If the vacuum does not decrease to near 0 inches H2O in a calibrated time, this DTC sets.

The following table illustrates the relationship between the ON and OFF states, and the Open or Closed states of the EVAP canister purge and vent solenoid valves.

The only voltage checks for P0446 would be for the fuel pressure sensor.

Diagnostic Aids

An intermittent condition could be caused by a damaged EVAP vent housing, a temporary blockage at the EVAP canister vent solenoid valve inlet, or a pinched vent hose. A blockage in the vent system will also cause a poor fuel fill problem.
For intermittent conditions, refer to Intermittent Conditions.
An EVAP canister, vent hose or vent solenoid valve that has restricted flow may cause this DTC to set. Using purge solenoid valve command with a scan tool, will allow vacuum to be applied to the system instead of pressure. With the EVAP canister vent solenoid valve open and the EVAP canister purge solenoid valve commanded to 100 percent, vacuum should not increase to more than 9 inches H2O