ZTech Engine Thermostat Volvo VNL/D13 | 21613426 21412639

You have 30 calendar days to return an item from the date you received it. To be eligible for return all parts must meet the following conditions:
• The item must be new, unused, and uninstalled;
• The product must be in salable condition, with no signs of installation (e.g., grease, scratches, or missing components);
• Original packaging is required, especially for branded parts;
• Any item that has been installed, modified, or misused is not eligible for return;
• Unacceptable returns may be shipped back to the customer upon request, at the customer's expense;
• Your item needs to have the receipt or proof of purchase.
We have a Restocking Fee which applies for return items with damaged package. Restocking Fee is 25%-50% of initial purchase.
• Do not tape the original packaging box — Restocking Fee will apply;
• Do not stick any shipping label/sticker on the original packaging box — Restocking Fee will apply;
• We recommend using stretch film to wrap the original packaging box. Use an additional shipping box or envelope to return the item.
You will be responsible for paying your own shipping costs for returning your item. Shipping costs are nonrefundable.

Standard Shipping — rates calculated at checkout (3-5 business days)
Express Shipping available at checkout (1-2 business days)
Guaranteed Overnight Shipping available at checkout — order before 2 PM PST (Monday–Friday)
International shipping available to Canada and EU — rates calculated at checkout.
All orders processed within 1 business day (excluding Sunday and holidays).

The ZTech Engine Thermostat (21613426 / 21412639) regulates coolant flow in Volvo VNL trucks with D13 and Mack engines, controlling engine warm-up time and maintaining optimal operating temperature. A stuck-open thermostat causes slow warm-up, poor fuel economy, and increased emissions. A stuck-closed thermostat causes rapid overheating and can result in head gasket failure within minutes. ZTech thermostats are calibrated to OEM temperature specifications for reliable operation in heavy-duty diesel applications.

Q: Which trucks and engines does this thermostat fit?
A: Fits Volvo VNL trucks with D13 engines and compatible Mack trucks. Replaces OEM part numbers 21613426 and 21412639.

Q: What temperature does this thermostat open at?
A: Calibrated to OEM D13 temperature specifications — consult product specifications for exact opening temperature rating.

Q: How do I know if my thermostat has failed?
A: A stuck-open thermostat keeps the engine cold and reduces fuel economy. A stuck-closed thermostat causes rapid overheating — both require immediate replacement.

Q: Is this OEM or aftermarket?
A: ZTech aftermarket thermostat calibrated to Volvo D13 OEM temperature specifications.

Q: Should the thermostat housing gasket be replaced at the same time?
A: Yes. Always replace the thermostat housing O-ring or gasket when replacing the thermostat to prevent coolant leaks.