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Thomas Bus Detroit DD5, DD8 DEF Head Unit

THOMAS BUS DETROIT DD5, DD8 DEF HEAD UNIT

Shipping ICON

ORDERS PLACED BEFORE 3PM EST WILL SHIP SAME DAY

WARRANTY ICON

WARRANTY
1M MILES OR 1 YEAR

$270.00

CORE CHARGE:

$250.00
iCORE DISCLAIMER: The CORE charge is a 30 day hold and will not be processed as long as the CORE is returned within 30 days. If you do not have a CORE or fail to return the CORE within 30 days, the CORE charge hold amount will be processed at that time.

AVAILABILITY IN STOCK
OEM NUMBER 04-33656-000
PART TYPE DEF Head Unit
BRAND Tamerx Diesel Products
ENGINE MAKE Detroit
ENGINE TYPE DD5, DD8
YEAR 2021-2024
PART NUMBER 2327-20-3
APPLICATION NOTES Thomas Bus
ADDITIONAL NOTES DEF Header, Analog, SAE, Detroit, 20"

PRODUCT DESCRIPTION

This is a Remanufactured, Tamerx DEF Head Unit, Tamerx part number 2327-20-3 ( OEM Part Number 04-33656-000 ).This part is primarily made for a Detroit DD5, DD8, production years 2021-2024, and comes with a One Million Miles or One Year Warranty. For additional warranty info Click Here

Additional Notes: DEF Header, Analog, SAE, Detroit, 20"

OEM NUMBERS

Primary OEM04-33656-000
Additional OEM04-33656-000

ADDITIONAL DETAILS

CONDITIONRemanufactured
UPC / Barcode00840490541702
WEIGHT8.00 LBS

SHIPPING OPTIONS

Ship Weight : 8.00 LBS
UPS Ground, 2nd Day Air, Next Day Air

SHIPPING POLICY

Tamerx Diesel Products ships via UPS Ground. Next day air shipping can be arranged for a reasonable price of anywhere from $50.00 - $100.00 depending on the weight of the product and your location. We will do everything we can to make sure that you get the service you need. We understand how important it is to receive your order as quickly as possible.
In most cases Tamerx Diesel Products only needs one address, as long as your product is shipping to the same location as your billing information. In order to eliminate credit card fraud, your billing information must match the credit card you provide.

The terms, conditions, and details of our warranty vary from product to product. Specific warranty details can be found below.

Tamerx Diesel Products provides a replacement warranty on all of our parts. The duration and mileage limitations of the warranty vary by product and are specified on the product's detail page/item listing.

Please note that the warranty covers replacement of the part and the cost of shipping the replacement part to you only. It does not cover any shop, labor, towing, or any similar or related costs. This is not a "money back guarantee".

If you experience a problem with your part or it is not functioning properly, the correct warranty procedure is to call us at (510) 257-1777 (8:00 AM - 5:00 PM ET Monday through Friday). Be sure to have your order number and/or other relevant order information ready so we can easily locate your order and resolve your issue as quickly as possible. From that point, we will review with you any troubleshooting steps that may need to be taken and other areas or systems that can be checked to rule out external problems contributing to your part's issue. If a replacement is determined to be necessary, we will then arrange for your replacement part to ship out ASAP and issue any necessary return shipping labels and/or documents.

As is the case for the majority of the diesel products sold on Tamerx Diesel Products, your part's warranty is active as soon as you install it. In some other cases, warranty activation is contingent upon you sending us your core (i.e., your old part). Tamerx Diesel Products will provide you a return shipping label for parts that have a warranty with a core return condition. When we have received your core return, the part's warranty coverage begins. We reserve the right to void the product warranty if your core is not received within the allotted time, or if it isn't received at all. If the warranty for the diesel part you are purchasing has a core return requirement, it will be clearly noted on the product detail page/item listing.

What is a Core Charge?

A core charge (also called "core deposit") is similar to the deposit you might pay for a can or bottle of soda and is a standard requirement in the auto parts industry. For example, in many states, to promote recycling, you pay a deposit when you purchase a can of soda and receive your deposit back when you return the empty can. Many automotive parts have a core charge, or core price, that works in essentially the same way as a soda can deposit.

30 Day Core Charge Hold

You authorize Tamerx Diesel Products, to hold your credit card for the specified core charge amount and that you agree to return your core with 30 days of receiving the purchased part. When we have received your core return, the hold is released and the core charge payment is canceled.

The customer, or parts buyer, is responsible for any shipping costs associated with core returns unless otherwise specified in the product listing/detail page. Tamerx Diesel Products offers prepaid UPS shipping labels (SKU numbers UPS-CR1 and UPS-CR2) for use in returning your core that are available for purchase on our website during your shopping/order process.

The shipping label you will need to use for the core return is dependent upon what part you are purchasing. UPS-CR1 is good for cores weighing up to 20 pounds. UPS-CR2 is appropriate for shipping cores weighing 21 pounds or more. You must include enough information with the core return that clearly identifies it as a.) a Core Return, and b.) contains enough information we can use to clearly associate the core return with your order. Including a copy of the invoice or packing slip that came with the original shipment, clearly marked "Core Return", is ideal. Alternately, you can include a note marked as "Core Return" that includes your order number, full name and address, and phone number or other contact information. Failure to include this information with your returned core will most likely delay any due core credit being issued and/or delay any applicable warranty activation. Core returns need to be for the exact part purchased and cannot be damaged. Substituting another part in place of the part you have purchased is not acceptable.

All fluids must be drained.

If you have purchased a set or a parts package, you will need to return cores for all components of the set. For example, if you have purchaed a high pressure oil pump with an IPR valve, you will need to return both HPOP and IPR valve cores to receive proper credit. If you have purchased an EGR cooler/EGR valve package, your core return must consist of an EGR cooler core and an EGR valve core, or core credit will not be given.

Core Charges are often mandated by state legislature, and, as such, the actual core charge and taxes on the charge may vary depending on the state in which you buy your part. The Tamerx Diesel Products core charge presented to you online today with your item is representative of the most common core charge.


The Thomas Bus Detroit DD5, DD8 DEF Head Unit OEM 04-33656-000 is a factory-original component designed for model years 2021–2024. Specifically built to meet Detroit Diesel and Thomas Bus specifications, this head unit ensures proper Diesel Exhaust Fluid system function for consistent and reliable operation. Its precise fit and OEM-certified quality help maintain emissions compliance while supporting smooth engine performance in daily use. With a durable design, it withstands the demanding conditions of commercial and school bus applications, delivering long-lasting service. For operators seeking a dependable replacement, this DEF head unit provides the assurance of OEM quality and trusted reliability.

FITMENT CHART

YEARMAKEENGINE(S)
2021Detroit DD5 DD8
2022Detroit DD5 DD8
2023Detroit DD5 DD8
2024Detroit DD5 DD8

PEOPLE ALSO ASKS | BLOG(S)

From 2021 to 2024, Thomas Bus vehicles powered by Detroit DD5 and DD8 engines have relied on the DEF Head Unit OEM 04-33656-000 as a key part of their emission systems. This assembly, often referred to as a DEF header, combines multiple sensors, ports, and electronic features into one compact unit mounted directly to the DEF tank.

Key Components

  • Housing / Header Body
    • The main structure that attaches to the DEF tank and holds all internal elements.
  • DEF Level Sensor
    • Detects the amount of Diesel Exhaust Fluid in the tank and communicates that data to the engine control module (ECM).
  • Temperature Sensor
    • Monitors DEF temperature to ensure proper operation and prevent freezing.
  • Heating Circuit / Coolant Interface
    • Maintains DEF in usable liquid form during cold weather by providing heat through coolant flow or built-in elements.
  • Pickup Port
    • Draws DEF from the tank and delivers it to the dosing system.
  • Return Port
    • Allows unused DEF to flow back into the tank for balance and efficiency.
  • Internal Filter / Strainer
    • Keeps impurities and crystallized particles from entering the dosing system.
  • Electrical Connector
    • Relays real-time sensor data to the ECM for monitoring and diagnostics.

By integrating these components, the DEF Head Unit OEM 04-33656-000 ensures that Thomas Bus vehicles remain compliant with emissions regulations while protecting the aftertreatment system from damage.

The DEF (Diesel Exhaust Fluid) head unit is a vital part of the emissions system on Thomas Bus vehicles equipped with Detroit DD5 and DD8 engines. From 2021 through 2024, the OEM replacement for this component is part number 04-33656-000.

What This DEF Head Unit Does

This unit monitors:

  • DEF level
    • Ensures accurate fluid readings
  • DEF quality
    • Confirms proper fluid concentration
  • DEF temperature
    • Protects against freezing or overheating

When it fails, drivers may experience inaccurate readings, dashboard warnings, or derate mode that limits vehicle operation.

Why Choose the OEM 04-33656-000 Unit?

  • Exact fit
    • for Thomas Bus DD5 and DD8 engines (2021–2024 models)
  • OEM reliability
    • with factory specifications
  • Sealed design
    • to withstand heat, vibration, and contamination
  • Compliance assurance
    • with EPA emissions standards

This DEF head unit is not serviceable or repairable. When it fails, replacement with the correct OEM part is the only reliable solution. Always handle DEF fluid with care during installation to prevent contamination of the new unit.

If you own a Thomas Bus with a Detroit DD5 or DD8 engine, model years 2021–2024, the correct DEF head unit replacement is OEM part 04-33656-000. Replacing a failing unit quickly ensures continued compliance and keeps your vehicle operating without costly downtime.

Yes, false or missing readings from the unit can trigger dashboard warning lights.

Using high-quality DEF fluid and keeping the tank clean can help protect the sensors.

The engine control module uses the data to adjust emissions operations and prevent faults.

Yes, it can be installed by technicians during scheduled maintenance if needed.

Detroit DEF Head Units

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