How to Write a Supplement Letter for OEM Parts (And Win)

The OEM vs. Aftermarket Parts Battle

Insurance companies have a financial incentive to use aftermarket parts: they're cheaper. The difference between an OEM front bumper cover and an aftermarket equivalent can be $200–$600 per part. Multiply that across thousands of claims and the savings are significant — for the insurer.

For the shop and the vehicle owner, aftermarket parts often mean fitment problems, finish quality issues, and potential safety concerns. For structural components, the difference can be even more significant: OEM parts are engineered and tested to absorb and distribute crash energy in specific ways. Aftermarket structural parts may not perform the same way in a subsequent collision.

The good news: you have strong legal and contractual grounds to demand OEM parts in many situations. Here's how to make the case.

When OEM Parts Are Required

OEM parts are required — and you have the strongest grounds for a supplement — in the following situations:

• Structural components: Any part that is part of the vehicle's structural system (frame rails, pillars, rocker panels, floor pan sections) should be OEM. Aftermarket structural parts are not tested to OEM crash standards.
• Safety systems: Airbag components, seatbelt pretensioners, and ADAS sensor housings should always be OEM. The vehicle owner's insurance policy typically requires that safety systems be restored to pre-accident condition.
• Vehicles under warranty: Using aftermarket parts on a vehicle that is still under the manufacturer's warranty can void the warranty. This is a powerful argument for OEM parts on newer vehicles.
• State regulations: Many states require insurers to disclose when aftermarket parts are being used and to obtain the vehicle owner's consent. Some states prohibit aftermarket parts on vehicles under a certain age.

The OEM Parts Supplement Letter

Your OEM parts supplement letter should include:

1. The Specific Part and Why OEM Is Required

Don't just say "we need OEM parts." Identify the specific part, explain why the aftermarket alternative is inadequate, and cite the standard or regulation that supports your position.

Example: "The proposed aftermarket front bumper reinforcement (part #12345) has not been tested to FMVSS 581 bumper standards and does not carry the same energy absorption characteristics as the OEM part. Installing an untested structural component creates liability for the shop and fails to restore the vehicle to its pre-accident condition as required by the insured's policy."

2. Warranty Argument (If Applicable)

If the vehicle is under warranty, include the VIN and the warranty expiration date, and cite the Magnuson-Moss Warranty Act, which generally prohibits manufacturers from voiding warranties solely because aftermarket parts were used — but many manufacturers' service agreements do require OEM parts for warranty repairs.

3. State Regulation Citation

Look up your state's regulations on aftermarket parts. Many states have specific disclosure requirements and restrictions. Citing the applicable state regulation in your supplement letter puts the insurer on notice that you know your rights.

4. Vehicle Owner Authorization

Have the vehicle owner sign a statement indicating that they want OEM parts and understand their rights. Include this with your supplement letter. An insurer who ignores a vehicle owner's explicit request for OEM parts is on much shakier ground.