Glow plugs. Things that are often forgotten until running or starting issues arise. Hard cold starts, cold no starts, smoky when cold, rough running when cold, all are common concerns that could be associated with a failed or failing glow plug system. As always, before we dive into common issues, codes, etc., remember that what we’ll discuss are common things, but not necessarily what may be happening with your truck. Low compression, frayed wires, bad injectors, and other things can cause the same running issues. Use all the data you can to diagnose your specific issues. 

How do the systems work? There are three different systems: OBS and early 1999 7.3; 1999.5+ 7.3; and 6.0. However, they all generally work the same way, with some added diagnostic and cycling capabilities in the newer trucks. Each system consists of eight glow plugs, a relay (OBS and early 1999) or module (1999.5+ 7.3 and 6.0), and the PCM. For each system, the PCM determines run time based on Engine Oil Temperature (EOT), sending a signal to the Glow Plug Relay (GPR) or the Glow Plug Control Module (GPCM) telling it to energize the glow plugs. This happens at every cold engine key-on cycle and can stay on (or turn back on) based on temperatures seen by the PCM. We’ll include the full list of temperatures and related on-times at the end.

The removal of the relay and the addition of its replacement, the GPCM, allowed for better diagnostics by monitoring each glow plug’s current flow and also controlled cycling of the plugs, on and off, depending on need. The GPCM for the most part was introduced in late 1999 model year trucks and beyond. However, some still used a GPR and it seems to be random as to which trucks got the new GPCM. We've seen 2002's with a GPR. So take a look under your hood. If you have two relays above/in the valley that look like starter relays, you have a GPR. If you have only one relay and a module with two large electrical connectors attached, you have the newer GPCM.

Diagnosing Issues: We’ve listed all codes and on-times associated with their respective temperatures at the end of this article. Those codes and any associated tips may help with diagnosis. Generally speaking, if you are having rough starts when cold, rough running when cold, smoking when cold, or no start issues when cold, the glow plug system could be suspect. GPCMs and relays are the most common fail items, followed by the glow plugs themselves. Glow plugs do have a service life; they won’t last forever. As these trucks age, the wiring becomes a common failure item as well. The Under Valve Cover Harness  (UVCH) is detrimental to good working glow plugs. They fail often. GPCM harness connectors also commonly fail. Pull the plug on the harness and look for any burnt or melted areas, as this is common. Fuses and fusible links were used in all years to power the relay and GPCM. If you’re not getting power to either, check the fuse box and fusible links nearby.

The Data:

Temperatures and duration of on-time.
It’s important to note that the Wait To Start (WTS) light on all trucks is just a dummy light. It doesn’t necessarily mean the glow plugs are on or off. In most cases, it’s off well before the glow plugs are actually turned off.
Ford used many different calibrations in the PCMs, so the times listed are not always exact. Yours will vary from what's listed, especially for GPCM-controlled engines.

1994.5 through early 1999 7.3 
Notes: Glow plugs generally turn off during cranking, but the PCM may decide to turn them back on after the engine starts. Post-start glow plug activation generally only happens with EOT below 104-122°F and can last up to 120 seconds.

EOT below approximately 14°F:
WTS Light: 10-12 seconds.
On-Time Prior To Cranking: Up to 120 seconds.

EOT between approximately 14°F and 32°F:
WTS Light: 8-12 seconds.
On-Time Prior To Cranking: Up to 90 seconds.

EOT between approximately 32°F and 50°F:
WTS Light: 5-10 seconds.
On-Time Prior To Cranking: Up to 60 seconds.

EOT between approximately 50°F and 104°F:
WTS Light: 2-6 seconds.
On-Time Prior To Cranking: Up to 30 seconds.

EOT above approximately 104°F to 122°F:
WTS Light: 0-2 seconds.
On-Time Prior To Cranking: 0 seconds.

1999.5+ 7.3 
Notes: Glow plugs may remain on after startup when EOT is below 131°F for up to 120 seconds.  How long depends on RPM, EOT, and other factors. This is the case whether you wait for the WTS light to go out or not, before cranking.

EOT below approximately 14°F:
WTS Light: 10-12 seconds.
On-Time Prior To Cranking: Up to 120 seconds.

EOT between approximately 14°F to 32°F:
WTS Light: 8-12 seconds.
On-Time Prior To Cranking: Up to 90 seconds.

EOT between approximately 32°F to 50°F:
WTS Light: 5-10 seconds.
On-Time Prior To Cranking: Up to 60 seconds.

EOT between approximately 50°F to 95°F:
WTS Light: 2-6 seconds.
On-Time Prior To Cranking: Up to 30 seconds.

EOT between approximately 95°F to 131°F:
WTS Light: 1-3 seconds.
On-Time Prior To Cranking: Up to a few seconds.

EOT approximately 131°F and up:
WTS Light: 0-2 seconds.
On-Time Prior To Cranking: 0 seconds.

6.0L 
Notes: During cranking, the GPCM will keep the glow plugs on only if a pre-crank glow plug cycle was initiated.
For all temperatures, once the engine is started, the PCM may decide to keep the glow plugs on (often cycling them on and off) until it decides they are no longer needed.

EOT below approximately 0°F:
WTS Light: 10-15 seconds.
On-Time Prior To Cranking: Up to 120 seconds.

EOT between approximately 0°F to 32°F:
WTS Light: 5-10 seconds.
On-Time Prior To Cranking: 60 to 120 seconds.

EOT between approximately 32°F to 60°F:
WTS Light: 2-5 seconds.
On-Time Prior To Cranking: 15-60 seconds.

EOT between approximately 60°F to 122-140°F (calibration dependent):
WTS Light: 0-2 seconds, possibly not illuminating at all.
On-Time Prior To Cranking: 0-15 seconds, or not activated at all if the EOT is on the higher end of this range.

EOT above approximately 122-140°F (calibration dependent):
WTS Light: 0 seconds (will not illuminate).
Total Glow Plug On-Time: 0 seconds. Ford’s documentation states that most (not all) calibrations will not activate the glow plugs above 131°F.

 

Related Codes: (OBS and early 1999 trucks will not have all these codes available)
P0380: Glow Plug/Heater Circuit "A" Malfunction
This is a more generic OBD-II code indicating a general problem with the glow plug system.

P0381: Glow Plug/Heater Indicator Circuit Malfunction
This code relates to the "Wait to Start" light circuit. If your WTS light isn’t working, you’ll likely see this code.

P0670: Glow Plug Control Module Control Circuit Malfunction
Usually indicates a fault within the GPCM itself. Can also be an issue with the PCM's ability to command the GPCM.

P0671: Cylinder 1 Glow Plug Circuit Malfunction
P0672: Cylinder 2 Glow Plug Circuit Malfunction
P0673: Cylinder 3 Glow Plug Circuit Malfunction
P0674: Cylinder 4 Glow Plug Circuit Malfunction
P0675: Cylinder 5 Glow Plug Circuit Malfunction
P0676: Cylinder 6 Glow Plug Circuit Malfunction
P0677: Cylinder 7 Glow Plug Circuit Malfunction
P0678: Cylinder 8 Glow Plug Circuit Malfunction
These codes (P0671-P0678) indicate a problem with the circuit for the specific glow plug mentioned (e.g., open, short, high resistance). This is often a bad glow plug, or an issue with the UVCH.

P0683: Glow Plug Control Module to PCM Communication Circuit Malfunction
This means there's a problem with the communication link between the GPCM and the Powertrain Control Module (PCM). Possibly the harness or its connectors.

P0684: Glow Plug Control Module to PCM Communication Circuit Range/Performance
Similar to P0683, but this suggests the communication is happening, but it's erratic, intermittent, or the data being exchanged is not within the expected parameters. This can also be due to wiring, connectors, module, or even PCM issues.

P1391: Glow Plug Circuit Low Side Bank 1 (Cylinders 2, 4, 6, 8 - Passenger Side)
Indicates an issue with the low side (ground side) control for multiple glow plugs on bank 1. Could be the GPCM, wiring, or multiple bad plugs/harness issues on that bank.

P1393: Glow Plug Circuit Low Side Bank 2 (Cylinders 1, 3, 5, 7 - Driver Side)
Same as P1391, but for bank 2.

P1395: Glow Plug Monitor Fault Bank 1
The GPCM is detecting an issue while monitoring the glow plugs on bank 1.

P1396: Glow Plug Monitor Fault Bank 2
The GPCM is detecting an issue while monitoring the glow plugs on bank 2.

P1397: System Voltage Out of Self-Test Range (KOER Glow Plug Monitor Test)
This code can pop up during a Key On Engine Running (KOER) self-test if the system voltage is too low or too high for the GPCM to accurately test the glow plugs. Often indicates battery/charging system issues rather than a direct glow plug fault, but it impacts the glow plug monitor test.

***Disclaimer: The information provided on this blog is for informational purposes only. We share our knowledge and experience, but we are not liable for any damages, injuries, or losses that may occur as a result of using this information. Situations are rarely cut and dry in the automotive world. Your situation will likely be somewhat different than what we describe here.  Use your best judgment and always consult a qualified professional for automotive repairs and modifications. Your safety is your responsibility.