The 5.9L 24-valve Cummins warms its intake air for cold starting with a grid heater, an electric heating element in the intake, rather than the glow plugs used by some other diesels. When that grid heater or its control circuit faults, you get hard cold starts, white smoke and rough running until the engine warms, and sometimes a check-engine light. Because the Cummins relies on the grid heater instead of glow plugs, understanding this system is key to curing cold-start complaints on the 24V.
Why a 24V grid heater faults
The grid heater draws heavy current through relays to warm intake air before and just after a cold start. Faults occur in the element, the relays, or the control and wiring. The likely causes on the 24V, ordered from most to least common:
- Failed grid heater relay(s) - the most common failure; the high-current relays wear or fail and the element never energizes.
- Grid heater element failure - the heating element itself burns out or cracks and stops warming the air.
- Wiring, connection, or ground fault - corroded or loose high-current connections interrupt the heavy draw.
- Control circuit or sensor input fault - the ECM decides when to cycle the heater; a bad input or control keeps it from operating.
- Blown fusible link or power feed issue - loss of power to the heater circuit.
- Element loose or shorting in the intake - least common, but a loose or shorting element can cause faults and even damage.
How to diagnose a grid heater fault
Confirm whether the grid heater is actually energizing on a cold start. On the 24V you can watch for the current draw and voltage drop as the heater cycles, and listen for the relays clicking. A heater that never draws current points to a failed relay, a burned element, or a power or ground fault. Because the relays are the most common failure, they are a logical first check, followed by the element and its connections.
Inspect the high-current wiring and connections for corrosion and looseness, since the grid heater pulls a lot of amperage and a poor connection interrupts it or causes heat. Scan for any grid heater or intake air heater codes, which point to the control side. Remember the Cummins uses a grid heater, not glow plugs, so do not go looking for glow plugs that do not exist on this engine; the fix lives in the grid heater element, relays, and their circuit.
Parts and checks by cause
- Grid heater relay(s), the most common fix
- Grid heater element, if it has burned out or cracked
- High-current wiring, connections, and grounds if corroded or loose
- Control circuit or power feed diagnosis if codes point there
Not sure which part your truck needs? The team at sales@coredieselparts.com can match the correct OEM or reman component to your VIN and engine before you order.
Frequently asked questions
Does a 24V Cummins use glow plugs or a grid heater?
A grid heater. The 5.9L Cummins warms its intake air with an electric grid heater element in the intake, not glow plugs. That is why cold-start problems on the 24V are diagnosed through the grid heater element, its relays, and the control circuit rather than by looking for glow plugs.
What are the symptoms of a bad grid heater?
Hard cold starting, white smoke and rough running until the engine warms up, and sometimes a check-engine light. In mild weather the truck may start fine, with the problem showing only when it is cold enough that the intake air needs preheating to ignite cleanly.
What usually fails in the grid heater system?
The high-current relays are the most common failure, followed by the heating element itself and corroded or loose high-current connections. Confirming whether the heater actually draws current on a cold start quickly points to which of these is the problem.
Need the part?
We stock the parts these trucks need. Shop 5.9L Cummins 24V Grid Heaters →