I have 2 codes showing on my 1996 ford explorer p1451 and p1507 could you explain what must be changed in order to correct the problem

Question edited for clarity, 'evac' to 'EVAP' and toss in some CAPS.
Question moved to model category.

Better to test it than just replace. $30 on Amazon, $90 at Autozone.

https://www.google.com/search?q=2001+ford+f150+code+p1451


The P1451 code indicates that there's an issue with your vehicle's EVAP systems vent solenoid. The engine control module (ECM) looks for a change in the EVAP system pressures to validate that the EVAP vent solenoid has actually opened or closed the vent.

p1451 Code - Evap Emission Control Sys Vent Control Valve ...

Kelley Blue Book https://www.kbb.com > obd-ii > p1451
Replacement

https://www.google.com/search?q=2001+ford+f150+evap+systems+vent+solenoid+replacement

..

need to have it check for trouble codes...most auto parts will do it for free

take it to your local ford dealer there is a possible recall

http://www.motortrend.com/cars/2000/ford/explorer/recalls/

its the evap canister solenoid, fairly easy to diagnose and fix, common on fords-here is a link with description of problem and correction:
;

http://www.obd-codes.com/trouble_codes/ford/p1451.php

Technical Description:
EVAP Control System Canister Vent Solenoid Circuit Malfunction
The EVAP system monitors the canister vent (CV) solenoid circuit for an electrical failure.
The test fails when the signal moves outside the minimum or maximum allowable calibrated parameters for a specified canister vent duty cycle by PCM command.
Symptoms: You will likely not notice any drivability problems.
Causes: A code P1451 may mean that one or more of the following has happened:
*VPWR circuit open
*Damaged canister vent solenoid
*Canister vent solenoid circuit open or shorted to ground, power, etc.
*Damaged Powertrain Control Module (PCM)
*Possible SolutionsVerify canister vent solenoid - To verify normal functioning, monitor the EVAP canister vent solenoid signal PID EVAPCV and the signal voltage
(PCM control side).With the valve open, EVAPCV indicates 0% duty cycle and a voltage approximately equal to battery voltage.
When the valve is commanded fully closed, EVAPCV indicates 100% duty cycle, and a minimum voltage drop of 4 volts is normal.
Output test mode may be used to switch output on/off to verify function.
*Replace canister vent solenoid

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http://www.obd-codes.com/trouble_codes/ford/p1451.phpgo to this link and it will explain that code for you

P1451 means EVAP-vent valve circuit malfunction.Did you check wiring?

do it step by step
p0503
1- change vss sensor and recheck
2- check wiring of vss sensor
p1451
1- check wiring of EVAP vent
2- change Evap vent

look under hood . its on the front right.

that is true your firing order is wrong here is a link to a web sight that has all the 1998 explorers 6cyl just click on the one that your explorer is and it'll show you the correct firing order also make sure you have your plugs gaped right good luck.
http://autorepair.about.com/library/firing_orders/bl-ford-firing-98.htm