Glow Plug Control Module Control Circuit High


This code indicates that the PCM has detected a malfunction in the glow plug heater circuit. Frequently, other glow plug circuit codes will accompany this code. Repair codes in the order that they appear.

Code Set Parameters

Variations in system reference voltage that exceed 10-percent of that recommended by the manufacturer will result in a stored code and an illuminated service engine soon lamp.


Typical symptoms include diesel engines that experience extended time required to start (especially in cold weather), a glow plug indicator lamp that either remains illuminated constantly or stays on too long, increased white smoke from exhaust on start-up, increased engine noise at start-up, a stored code, and an illuminated malfunction indicator lamp.

Common Causes

Common causes of this code may include (but are not limited to) faulty glow plugs, a faulty glow plug relay, a bad glow plug timer or module, shorted or open wiring in the glow plug heater circuit, loose or corroded electrical connectors in the glow plug circuit. Blown fuses are also a common problem but there is usually an underlying problem that caused a fuse to blow. Repair codes in the order that they appear on the code reader or scanner. Freeze frame data can also be helpful in diagnosing this condition.

Common Misdiagnosis

Glow plug timers and relays are often replaced before testing system wiring and connectors, or the glow plugs themselves. This can lead to an incorrect diagnosis.


  • Diesel engines utilize the glow plug heater in order to pre-heat individual cylinders
  • The individual glow plugs (one per cylinder) are heated up using battery voltage
  • The glow plug is actually placed in sequence with a circuit that is rated at battery voltage (normally between 12.6 and 13.8 volts)
  • When the circuit is energized, voltage heats the glow plug up until it is glowing red hot for a short period of time
  • Once the glowing red hot plug increases cylinder temperature, diesel fuel is easier to ignite and burns more efficiently
  • This decreases white smoke at start up, as well as reducing exhaust emissions and engine noise. The PCM typically controls glow plug heater circuit activation
  • Some models use a glow plug timer or separate glow plug module that interacts with the PCM
  • The PCM may allow the heater to remain operational for several moments after the engine is started on some occasions, especially during brutally cold conditions. Since the glow plug heater circuit is fairly simple, you can usually test the entire system using a scanner (or code reader) and a digital volt/ohmmeter
  • Begin with a visual inspection of all wiring and connectors
  • Repair or replace damaged, disconnected, shorted, or corroded wiring, connectors, and components as necessary
  • Always retest the system after repairs are completed to ensure success. If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data
  • This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
  • Continue by clearing the code and operating the vehicle to see if it returns
  • This will help to determine whether or not the malfunction is intermittent
  • After the codes are cleared, test drive the vehicle to see if the code returns
  • If the code fails to immediately return, you may have an intermittent condition
  • Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made
  • Check an individual glow plug connector to make sure that a voltage and ground signal are present when the glow plug heater lamp is illuminated
  • If voltage and ground are both present, then replace the glow plugs and retest the system
  • A quick method for testing glow plugs (after they have been removed from the engine) is to apply battery voltage (from a fully charged battery) to one end and ground to the other end
  • Be careful though, the glow plug should heat up to a glowing orange within a few seconds
  • If the glow plug fails to heat up or only heats up to a “brick” red color, it is faulty and must be replaced
  • You may also test glow plugs using the digital volt/ohmmeter
  • This is done by testing resistance values and comparing your findings to manufacturer’s reference values. If no voltage is detected at the glow plug connector, then begin by testing system fuses to make sure that they are in proper working order
  • Remember to test fuses with the glow plug heater activated to ensure that they are not breaking down under load
  • Replace fuses as required and attempt to activate the glow plug heater
  • If the glow plug heater fuse blows again, then you know that there is a short in the glow plug heater system circuitry
  • Repair the short and replace the fuse as needed
  • Test the glow plug relay, timer, or module according to manufacturer’s recommended specifications and compare your findings with the predetermined reference voltage and resistance figures
  • Replace the glow plug relay if all input voltage and ground values coincide with battery voltage and it fails to activate the glow plug heater circuit. Visually inspect glow plug heater circuit wiring and connectors for signs of broken or corroded wiring or connectors, bent pins in electrical connectors, or burnt marks on system harnesses
  • Repair damaged or faulty wiring and connectors as required and retest
  • If system wiring and connectors appear normal, then disconnect the PCM connector and test circuit resistance using a digital volt/ohmmeter
  • Repair open or shorted wiring as needed and retest
  • PCM failure is rare