Rough Idle - CEL On - Dreaded P0340

I'm new to Mustangs. If you don't mind me asking, why do you not believe it's CPM related?

I wouldn't always trust those free checks from the parts stores. What voltage are you putting out at idle and if you rev it to 1,500 rpm's? Had it been raining when you noticed the issue? Have you tried unplugging, cleaning, and putting a little dielectric grease on the connector? Maybe check the wires to the CMP for any signs of chaffing?

I don't know what is causing your problems, but I really doubt the alternator is. It pretty much just charges the battery. The poor running can be a cam position sensor, spark plugs, dirty fuel filter and other things.

As for as not trusting the parts houses... The alternator checks aren't the best. If an alternator fails when warm, the parts house can't detect that. But for checking engine codes, they are simply reading the cars computer. Where ever you go to have them read, the actual code will read the same. But a good tech may have a better idea what really caused the code. Most of the time, it is what it is, but sometimes for instance a rich O2 sensor and a bad MAF may only be the bad MAF causing a rich fuel mixture so the O2 is reading right and would stop reading rich when the MAF is fixed.

Still doesn't help with your problem, but the alternator is not the likely problem.

This DTC is triggered when when the PCM can no longer detect the signal from the camshaft position (CMP) sensor in bank 1

Possible causes are:

• CMP circuit open
• CMP circuit short to ground
• CMP circuit short to voltage
• Signal return open (VR)
• CMP ground open (HALL-effect sensor)
• CMP circuit short to CMP2 circuit (if equipped)
• CMP incorrectly installed (HALL-effect sensor)
• Damaged CMP sensor shielding
• Damaged CMP sensor
• Damaged PCM

Also check harness routing, harness alterations, improper shielding, or electrical interference from improperly functioning systems as they may have intermittent impact on the CMP signal.

Did they simply check it for output voltage and amps, or did they look at a sign wave?

When one of the diodes goes bad, AC voltage leaks past and on the 99-04GT's plays havoc.

I would not rule out testing the CMP first.

The cam position sensor is located on the top/front portion of the valve covers.

i had the same issue !! They said the alternator was fine but replaced it anyway ...sure as shti fixed the problem!