People often say a product is “defective” without saying how. In Illinois law the how matters, because a flaw in one unit and a flaw in the whole design are proven in different ways. This guide explains the difference between a manufacturing defect and a design defect, the two tests Illinois uses for design, and what each kind of case usually requires.
Two different problems
Manufacturing defect
Something went wrong when this particular unit, or one batch, was made. It does not match the maker’s own design.
Design defect
The product was made exactly as intended, but the design itself makes it unreasonably dangerous, so every unit shares the problem.
Manufacturing defects
The Illinois pattern jury instructions describe a manufacturing defect as a flaw in a particular unit that comes from a problem in the making of it. The proof usually starts with the actual product: how it differs from the maker’s specifications and from other units. Because one bad unit or one bad production run can be the cause, these cases often turn on inspection records, quality-control data and testing. (IPI Civil 400.00 introduction)
Design defects
A design defect is different. The product worked as designed, and the design is the problem. Illinois lets a plaintiff prove it in two ways: by showing the product failed to perform as safely as an ordinary consumer would expect when used in an intended or reasonably foreseeable way, or by showing the design proximately caused the injury and the defendant cannot prove that the benefits of the design outweigh its risks. (IPI Civil 400.06 and 400.06A)
In Mikolajczyk v. Ford Motor Co., 231 Ill. 2d 516 (2008), the Illinois Supreme Court held that both tests remain available and adopted an “integrated” risk-benefit test in which consumer expectations are one factor among many. (Mikolajczyk opinion) In Calles v. Scripto-Tokai Corp., 224 Ill. 2d 247 (2007), it said the obvious nature of a danger is only one factor and refused to create a “simple product” exception. (Calles opinion)
How a design defect is usually built
- Get the productThe product itself is preserved and inspected before anything is changed.
- Show how it failedExperts explain how the design led to the injury.
- Show a safer designEvidence of a practical alternative helps weigh risks against benefits.
- Show it was not a one-offRecords of other similar incidents can show a pattern in the design.
Why the difference matters
A manufacturing defect case asks what went wrong with this unit. A design defect case asks whether the whole product line should have been built differently, which usually means experts, testing and a hard look at the company’s own documents. Courts also treat other incidents carefully: an Illinois appellate court has said a plaintiff offering prior accidents to show a particular hazard must show they were substantially similar to the one in the case. ({ia(S(SL[“cost”]),”We explain why this makes product cases expensive”)}.)
A third kind of defect involves warnings and instructions. It can overlap with the other two, and we cover it in our guide to warning and warranty claims. Our attorneys also discuss the main claim types in Episode 3 of Navigating Negligence.
What to do if you think a product was defective
Keep the product and everything that came with it. Do not repair or alter it, and do not let anyone else test it without a lawyer present. Photograph it, write down how it was used, and get medical care.
Frequently asked questions
Can a product have more than one kind of defect?
Yes. A claim can allege a design defect, a manufacturing defect and a warning defect for the same product.
Do I need to prove a safer design existed?
Not always, but under the risk-benefit test a practical alternative design is often central evidence.
Does an obvious danger end the claim?
Not automatically. The Illinois Supreme Court has said obviousness is one factor in the balancing, not an automatic bar.
Related guides: Strict liability in Illinois product cases; Failure to warn and breach of warranty claims; Why product liability cases are expensive; Illinois product liability lawyers.
Statistics and law reviewed October 23, 2026. This article is general information, not legal advice. Statutes, rules and court decisions change, and every case depends on its facts.
Hurt by a product you think was defective? Call (312) 346-4262 for a free consultation. There is no fee unless we win.
