Joe here's what the shop manual says:

Mike


Code 3-1-5 (EVAP System)
1) Check for other codes. If Code 5-4-1 is also present,
check that code first. If Code 5-4-1 is not present, go to next step.

2) Start and run engine at idle. Remove vacuum hose between
EVAP valve and intake manifold. If vacuum is present in hose, go to
next step. If vacuum is not present in hose, check for obstruction.
Also check hose connection to intake manifold.

3) Start and run engine at idle. Connect hose between EVAP
valve and intake manifold to EVAP valve. Remove hose between EVAP
valve and canister from EVAP valve. If there is no vacuum in EVAP
valve when valve is not activated, go to next step. If there is
vacuum, retest using new EVAP valve.

4) Start and run engine at idle. Disconnect hose between EVAP
valve and canister at EVAP valve. Check if there is vacuum in EVAP
valve when valve is activated. If there is vacuum, go to next step. If
there is no vacuum, retest using new EVAP valve.

5) Start and run engine at idle. Connect vacuum hose between
EVAP valve and canister to EVAP valve. Remove vacuum hose between EVAP
valve and canister from canister. Check if there is vacuum in hose
when EVAP valve is activated. If there is vacuum, check if all
canister inlets and outlets are okay. Repair as necessary. If inlets
and outlets are okay, fault is intermittent. Go to next step. If no
vacuum is present, check for obstruction in hose between EVAP valve
and canister.

6) Check hose between EVAP valve and intake manifold, and
between EVAP valve and canister for an obstruction. Check canister
connections to ensure they are not clogged. Check EVAP valve for dirt
or deposits. Repair as necessary. If no faults can be found, test
system using new EVAP valve.