I recently had an evap leak on my ford, code p0442, then p0445.
Small Leak= P0442
Large Leak= P0445

Replaced the purge and vent solenoids, but getting a new code of p0443.
Short in the Purge Circuit = P0443

I checked the ohms in the purge value on the terminals and its reading 35ohms, the connector is reading 10.8 to 11.8 volts can't remember which right now. I have disconnected the pcm wiring harness and checked the volts on the wiring harness and not getting anything on the pins or the purge valve. So do I need to replace the whole pcm harness just for that short to the evap circuit? Or should I trace all the wires from the evap to the harness and look for any torn wires there? Could it be a faulty fuel pressure sensor?

PS: I have also checked the fuses and they are good. I would like to check the ohms on the purge valve itself but don't know how since its all plastic

Thanks guys

 

The 443 code is a monitor detected failure for either a short or an open to the purge valve.
If you take the PCM connection off you won't read any voltage which would be normal.
Place a shunt (short) on the PCM connector terminals (not the PCM) and measure for that shunt at the purge terminal end with an ohm meter function.
If you see it, the circuit is good in that respect.
Remove the shunt and it clears at the purge end, the circuit is complete.
That leaves the purge valve or it's connector or even a shunt to ground from the positive side.
Check each side of the circuit to chassis ground.
The ground side should read near zero ohms a short, the hot side should read open if the circuit is good.
.
The PCM reads the circuit condition by internally checking the voltage drop across a resistance. The value read is the monitor function.
A large voltage drop indicates a short or to much current flowing.
No drop indicates an open. This how the PCM knows what the condition is.
This is about all there is to the general operation for this circuit check.
The issue has to be detected on one of these checks or your missing it.
Good luck.

 

Ok I just did some more check and what I got is this
VMV = 34 ohms
VMV Connector = 11.8V, now its 11.5v after some poking around
PCM Connector = steady 12.4v for pins 56(VMV) and 67(Vent Canister),
the ohms on it were fluctuating between 400 and 1900 my multimeter
only goes to 2000

I take my reading by shoving two sewing pins into the connector as a
way of measuring my connectors.

I think its the VMV Connector just because why would voltage drop
between the PCM and the VMV. I started the truck and the voltage
on the VMV connector went up to 13.2v. Think I'm on the right
path?

Got myself a test light and a wiring diagram and think I found the problem.
I wire that goes from the VMV to the fuses was reading 12.4v, and 12.4v
when connected to the test light. The wire going from the VMV to the
PCM was reading 12.4v, but dropped to 7.3v when I hooked the test light
to it. I disconnected the PCM and tested that pin and got the same result.

Now I just don't know how to replace that wire. I should probably just
get a whole new PCM harness. Ideas?