HVAC Heater and HVAC Delay Relay (PartTerminologyID 3445): Where Combination Heater Supply and Post-Ignition Timer, Dual-Function Failure Modes, and Differentiation from Discrete HVAC Heater Relay
Written by Arthur Simitian | PartsAdvisory
PartTerminologyID 3445, HVAC Heater and HVAC Delay Relay, is a combination relay module that integrates two distinct functions into a single housing: the heater supply contact that powers the HVAC heating circuit during normal operation, and a timer circuit that continues to supply the HVAC heater for a calibrated period after the ignition is switched off, allowing heat to continue circulating through the cabin during engine cooldown or allowing an auxiliary heater system to complete its heating cycle before shutting down. This combination module replaces two discrete relay functions on applications where the manufacturer chose to consolidate both into one replaceable unit, and it is distinct from the HVAC Heater Relay (PartTerminologyID 3444), which performs only the heater supply function without a delay timer. The defining diagnostic challenge of PartTerminologyID 3445 is that either function can fail independently of the other, producing two symptom patterns that point to the same component from different operational phases of the HVAC heating cycle.
What the HVAC Heater and HVAC Delay Relay Does
Heater supply contact and normal operation function
During normal ignition-on operation, the combination module's heater supply contact functions identically to the discrete HVAC Heater Relay (3444), supplying battery voltage to the HVAC heating circuit at the command of the HVAC control module. The heater supply contact activates when the ignition is on and the HVAC module commands heat output, and it de-activates when the module commands heat off or when the ignition is switched off. A failed heater supply contact within the combination module produces the same symptom as a failed discrete HVAC heater relay: blower operates correctly with normal airflow but the air delivered to the cabin is cold at all heat settings. The diagnosis path for this failure mode is identical to the HVAC Heater Relay (3444) diagnosis, with the added step of confirming the application uses the combination module rather than a discrete relay before ordering the correct replacement.
Post-ignition delay timer and cooldown operation function
The combination module's delay timer circuit monitors the ignition state and begins a countdown when the ignition transitions from on to off. During the countdown, the module holds the heater supply contact closed, continuing to supply the HVAC heating circuit after ignition-off for the calibrated delay duration. This delay function serves applications where continued heater operation after ignition-off provides a comfort or thermal management benefit, such as diesel auxiliary heater systems that require a cooldown cycle before shutting off, or cabin pre-conditioning systems on certain European platforms that continue heating for a fixed period after the driver exits the vehicle. A failed delay timer that cannot hold the contact after ignition-off produces heater operation that stops immediately when the ignition is switched off, with no post-ignition warm-up period. A failed delay timer that cannot release the contact after the delay period expires holds the heater on indefinitely after ignition-off, drawing battery current until the battery is discharged.
Dual-function failure mode identification
Because the combination module contains two independently failable circuits in one housing, the first diagnostic step is identifying which function has failed. A heater that does not work during ignition-on operation has a failed heater supply contact or a failed HVAC module output. A heater that works during ignition-on operation but does not continue after ignition-off has a failed delay timer. A heater that continues after ignition-off without stopping has a stuck-closed timer contact. The failure mode identification from the symptom description determines which internal circuit to test and prevents replacing the combination module for a delay timer fault when the buyer actually has a heater supply contact fault, or ordering a discrete HVAC Heater Relay (3444) when the application requires the combination module with the delay function.
Top Return Scenarios
Scenario 1: "Heater works with ignition on but stops the moment ignition is turned off"
The combination module's delay timer circuit has failed. The heater supply contact is functioning correctly during ignition-on operation, confirmed by normal heat output while driving. The timer circuit is not holding the contact after ignition-off. This is a timer circuit failure within the combination module and requires module replacement. Confirming the application's expected post-ignition heat duration from the service manual establishes whether the loss of post-ignition heat represents a real functional loss or whether the application never had this feature and the symptom is being attributed to the module incorrectly.
Prevention language: "Confirm whether the application is designed to provide post-ignition heat before diagnosing a delay timer fault. Not all applications with PartTerminologyID 3445 have a meaningful delay duration. If the application is designed to provide post-ignition heat and it stops immediately on ignition-off, the combination module's timer circuit has failed."
Scenario 2: "Heater continues running after ignition is off and drains the battery"
The delay timer contact is stuck closed and is not opening after the calibrated delay period. The HVAC heater continues to draw current after ignition-off until the battery is discharged. Removing the combination module from its socket should stop the heater immediately if the stuck contact is the fault. If the heater continues after module removal, the HVAC control module is holding the heater supply from a separate output and the fault is in the control module rather than the combination relay.
Prevention language: "HVAC heat that continues indefinitely after ignition-off indicates a stuck-closed delay timer contact. Remove the combination relay module to confirm: heater stops on module removal confirms a stuck-closed contact. If the heater continues after module removal, the HVAC control module is the fault source."
Listing Requirements
PartTerminologyID: 3445
dual function description: heater supply contact and delay timer in one module (mandatory)
delay timer function and application context (mandatory)
dual-function failure mode symptom differentiation (mandatory)
differentiation from discrete HVAC Heater Relay (3444) (mandatory)
application confirmation: verify application uses combination module vs. discrete relay (mandatory)
OEM part number cross-reference (mandatory)
FAQ (Buyer Language)
How do I know if my vehicle uses this combination relay or just the regular HVAC heater relay?
Check the fuse box relay label or the body electrical service manual for the application. A relay labeled as a heater delay relay, a comfort heater relay, or a post-run heater relay in the fuse box legend indicates the combination module is present. A relay labeled only as heater relay with no delay reference indicates a discrete HVAC Heater Relay (3444). The combination module and the discrete relay are not interchangeable even if they share a common relay footprint, because the discrete relay has no delay timer circuit to replace the combination module's timer function.
My vehicle's heater works fine but the post-ignition warmth stopped. Do I need the full module replacement?
Yes. The delay timer circuit is internal to the combination module and is not serviceable separately from the heater supply contact. If the delay function has failed while the heater supply function remains correct, the entire combination module must be replaced to restore both functions. There is no discrete delay relay available as a separate component on applications that use the combination module, since the manufacturer consolidated both functions into the single replaceable unit.
What Sellers Get Wrong About PartTerminologyID 3445
The most common listing error is describing only one of the two functions. A listing that describes only the heater supply function will attract buyers with a delay timer fault who install the module, find the heater supply works correctly, and conclude the part has not resolved their concern because the listing gave them no information about the delay function that was actually failing. A listing that describes only the delay timer function will attract buyers with a heater supply fault who search for a heater relay, find no heater relay description in the listing, and order a discrete HVAC Heater Relay (3444) instead. Both functions must be described with their distinct symptom patterns so buyers presenting either failure mode can identify the combination module as the correct component before ordering. The second error is omitting the discrete relay differentiation. Buyers who search for an HVAC heater relay will find both PartTerminologyID 3444 and 3445 listings in results, and without a clear statement that 3445 includes an integrated delay timer that 3444 does not, buyers will order based on price or availability rather than function match, generating returns from buyers who needed one and installed the other.
Cross-Sell Logic
HVAC Heater Relay (PartTerminologyID 3444): the discrete heater relay without delay timer; not a substitute for the combination module on applications that require the delay function, but the correct part on applications that use a discrete relay without delay
HVAC Heater Blower Relay (PartTerminologyID 3448): controls the blower motor independently of both the heater relay and the combination module; no airflow at any fan speed points to the blower relay, not the heater or combination module
HVAC Control Module: if no combination module coil activation is present from the control module during ignition-on heat commands, the control module output has failed and the combination module replacement will not restore heater operation
Final Take for PartTerminologyID 3445
HVAC Heater and HVAC Delay Relay (PartTerminologyID 3445) is the combination heater-and-delay module where dual-function description, per-function failure mode symptom differentiation, and discrete relay differentiation are the three listing attributes that prevent the most common wrong-component orders and incomplete-diagnosis returns in this narrow but technically specific category. The dual-function description is the non-negotiable baseline: a listing that describes only one function will generate returns from buyers whose failure mode corresponds to the undescribed function. Sellers who describe both the heater supply contact function and the delay timer function, pair each with its distinct failure mode symptom, and distinguish the combination module from the discrete HVAC Heater Relay (3444) give buyers the complete information needed to confirm the combination module is the correct component for their application and their specific failure mode before the order is placed.