Service soon light on code 1391 keeps appearing

most codes are generic and then their are codes that the manufacturer will have as specific 1391 for a mopar may be different for a toyota or on a ford its a glow plug problem etc --------That code for mopar for the crankshaft position sensor or the CMP sensor once the CKP sensor goes your car wont start at all-----

generic p1391 is CMP or CKP Sensor Signal Intermittent Conditions.
p1391 code Ask com YouTube Search this link is codes more for chrysler ---the following codes pertain to your vehicle p1393 is Glow plug circuit low input, bank #2 (left) Open/short/miswired circuit, ... ---- p1391 is Glow plug circuit low input, bank #1 (right) Open/short/miswired circuit, faulty relay, glow plugs.
An Introduction to the Power Stroke Glow Plug System this link will explain and assist in repair

Not sure I am following your story.
Code 351 is a problem with the primary OR secondary circuit for the coil.
Code 1391 is a problem with the ABS system.

HI,
http://engine-codes.com/p0671_gmc.html

It's a Glow Plug.

Possible causes- Cylinder #1 glow plug circuit is open or shorted
- Faulty cylinder #1 glow plug
- Faulty glow plug module Help with thisSymptoms- Engine Light ON (or Service Engine Soon Warning Light)
- Lack/Loss of Power
- The engine may be harder to start
- Engine hesitation

Read more: http://engine-codes.com/p0671_gmc.html#ixzz28UT1VO75

OBD Code P0673refers to
The Engine Control Module (ECM) has detected a fault in the cylinder #3 glow plug circuit SymptomsPossible sumptoms of OBD code P0673
- Engine Light ON (or Service Engine Soon Warning Light) - Lack/Loss of Power - The engine may be harder to start - Engine hesitation CausesPossible causes of OBD code P0673
- Cylinder #3 glow plug circuit is open or shorted - Faulty cylinder #3 glow plug - Faulty glow plug module

not always the golw plugs can be bad and the engine can be low on compression. does it start alot better if you add a gallon of kerosene to 5 gals of desiel? Or a quaart of gas? the glow plugs can be checked with a test light the connectrs are in plain veiw and the wires going to them are big. Follow the wires back to the relay they go straight there, the relay is real close to the battery as it draws alot of power follow the big wire on the battery that doesn't go to the starter and thats the glow plug relay. they last good but the connections get dirty, reconnect all the connectors on itand see with the key on if the glow plug light comes on on the dash that wire that lights the light is the same wire that lights the light fires the glow plugs. You can take the wire off the glow plug( you need to do it anyway it's a yearly service anyway and with a test light connected to power touch the disconnected glow plug with the lights probe and the light should light, not all of them are likly to be bad but if you find two or more just change them all as the ones that aren't out are likly to fail anytime and if you want to drive though the winter you need all the help you can gt. the battery cables and the one going to the glow plug relay are likly to fail at the battery so go over this good It might start fine with just two glow plugs working I'd pick the two center cyclinders or the two outboard ones as once the engine starts it'll keep running long enough to start the other ones. low voltage as your cranking can effect the glow plugs but the relay is suposed to turn the glow plugs off at cranking and time out after they've been on long enough. dirty connections are almost always the problem so clean everthing before you replace anything. Needs cleaned before yu put on new parts anyway and the glow plugs need to be taken out with the right tool or you may end up with a broken off glow plug in your cyclinder head.

HI. This is an indication of a failing glow plug. It will be necessary to test each plug to isolate the issue, or run a diagnostic scan on the CPU. During the scan, you should be issued a code that will isolate the actual glow plug that is malfunctioning. If you insist on testing each plug, use my procedure below to assist. You can find the resistance specs in your operators manual.


A test light can be used to determine if the glow plugs are working.


1. With the test-light grounded, insert the test light probe into a glow-plug wire to check for continuity.

2. If the light comes on, check the other cylinders in the same manner. If the test light failed to light, check for continuity at the glow-plug relay.

3. If no power is present, check the control circuit of the glow plug relay.

4. If the control circuit is working, the relay may be defective (FYI, manufacturers may have the system fused and fusible links may be located throughout the system).

5. If the glow-plug circuit is working, the glow plugs themselves may be defective.

Checking glow-plug resistance:(Remove the plug for this test)

1. With an ohmmeter, connect one lead to the terminal and the other lead to the body.

2. Read the resistance and compare to specifications.

3. A visual inspection may also be necessary to check for excessive carbon buildup and physical damage to the tip.

4. A glow plug that has excessive carbon deposits on its tip can give the correct ohmmeter reading but still not work. Clean the tip with carbon cleaner.(DO NOT USE A BRUSH TIP CLEANER)

HI. Usually, when this light illuminates, it will signal that one, or more of the glow plugs are malfunctioning. I recommend to test all glow plugs, so that you may isolate the defective plug.

A test light can be used to determine if the glow plugs are working.


1. With the test-light grounded, insert the test light probe into a glow-plug wire to check for continuity.

2. If the light comes on, check the other cylinders in the same manner. If the test light failed to light, check for continuity at the glow-plug relay.

3. If no power is present, check the control circuit of the glow plug relay.

4. If the control circuit is working, the relay may be defective (FYI, manufacturers may have the system fused and fusible links may be located throughout the system).

5. If the glow-plug circuit is working, the glow plugs themselves may be defective.

Checking glow-plug resistance:(Remove the plug for this test)

1. With an ohmmeter, connect one lead to the terminal and the other lead to the body.

2. Read the resistance and compare to specifications.

3. A visual inspection may also be necessary to check for excessive carbon buildup and physical damage to the tip.

4. A glow plug that has excessive carbon deposits on its tip can give the correct ohmmeter reading but still not work. Clean the tip with carbon cleaner.(DO NOT USE A BRUSH TIP CLEANER)

Above the Relay Panel is the Auxiliary Relay Panel. Has thirteen positions in it.
6 across the top, 7 across the bottom.

Across the bottom the Glow Plug Relay is the fourth one over from the left. It is Number 10.
(180)

The garage should be able to test the glow plugs without removing them. All the need to do is use an ammeter or amp clamp and measure the current drawn by the glow plugs. the glow plugs should draw 16 - 20 amps each. much less than that and they want replacing.