There's nothing wrong with working on your own car. The problem is when the work gets done with the wrong parts, the wrong tools, the wrong technique, or a level of confidence that didn't really match the skill level. Here's what mechanics find — and what to look for if you're buying a used car and wondering what the previous owner was apparently up to. If you end up at the shop after one of these discoveries, knowing how to describe the problem accurately will save everyone time.
The fastener situation
Bolts are not interchangeable. They have specific thread pitches, lengths, grades, and coating requirements. A brake caliper bolt that's been swapped out for a hardware-store equivalent of similar size might look fine and hold fine — right up until it doesn't, which is usually during a hard stop. Mismatched or incorrect fasteners are one of the first things an experienced mechanic notices when taking over a car with sketchy history.
Stripped threads are especially common after DIY oil changes. The drain plug gets overtightened once, strips a bit, gets overtightened again to compensate, and eventually the threads in the oil pan are gone. A repair that should have been a $25 drain plug becomes a $300 oil pan replacement. I've seen this more times than I can count.
JB Weld, RTV, and other creative applications
JB Weld is a genuinely useful product in the right applications. An exhaust bracket on a lawnmower. A cracked non-structural component. What it's not: a head gasket substitute, a cracked block repair, or a solution for an oil pan rail that's leaking. When a mechanic opens an engine and finds epoxy in places that should just be clean metal surfaces, it tells a story. Apparently not a short one.
RTV silicone has similar properties — useful in the right spots, a problem in the wrong ones. Too much RTV squeezed into an oil passage can break off and block oilflow to downstream components. The correct amount is a thin bead around a gasket surface. What sometimes gets put on looks more like someone was decorating a cake.
Wrong or mystery fluids
Brake fluid and power steering fluid look similar. Some automatic transmission fluid and power steering fluids look similar. Engine coolant comes in multiple incompatible formulations. There is a well-documented history of people putting the wrong fluid in the wrong reservoir, and it's almost always expensive to sort out.
Mixing incompatible coolants can cause gel formation that blocks the cooling system. Using the wrong hydraulic fluid can degrade rubber seals. Putting brake fluid in the power steering reservoir causes its own category of problem. When a mechanic drains something and it smells wrong, that's a story that rarely has a cheap ending. Turns out the previous owner apparently topped it up with whatever was in the garage.
Duct tape and wire nuts on automotive wiring
Automotive wiring uses specific solder-and-heat-shrink or quality crimp connectors designed to handle vibration, temperature extremes, and moisture over years of use. Household wire nuts are not rated for any of those conditions. Duct tape adhesive breaks down with heat and leaves a residue that attracts moisture. When someone spliced a wire with a wire nut and wrapped it in duct tape, it might work for a while. Until it doesn't, which is usually at night, in the rain, when the headlights decide they've had enough.
When you're looking at a used car: lift any rubber grommets near the firewall and have a look at the wiring in the engine bay. Original harnesses with intact factory connectors are good. Splices wrapped in electrical tape — especially multiple layers — are a reason to ask questions.
Parts installed backward or upside down
This sounds too basic to actually happen. It happens regularly. Brake pads installed with the friction material facing away from the rotor. One-way valves put in backward. Thermostats installed upside down. In most cases the failure announces itself pretty quickly — brakes that don't really brake, an engine that overheats despite a full coolant level (knowing what your warning lights actually mean is what prevents that from becoming something much worse). Customer states brakes feel weird since their buddy helped out. Turns out the buddy did some brake work, ALL by himself.
Parts removed because they made a noise
Heat shields, resonators, and secondary air injection components are commonly removed because they rattled or made noise. They're also there for a reason. Heat shields protect underbody components from exhaust heat. Removing them might stop the rattle; it may also gradually cook the components they were protecting. When a mechanic finds a heat shield missing, the question is always what it was shielding and whether that component has definitely seen some better days as a result.
Buying a used car: a pre-purchase inspection by an independent mechanic — not the selling dealer's shop — is worth every penny of the $100-150 it costs. They'll find things a test drive and a visual walkthrough will miss. Apparently "it runs great" is not always the full picture. For proof: the 14 funniest customer states moments from real repair orders illustrate exactly what "pretty good shape" can mean in practice.