“The potential for damaging your engine is huge,” warns RepairPal’s Bodas, who recalls a customer who decided to do their own timing-belt replacement on a Volvo. The customer very quickly got over his head and through a set of mishaps turned a $600 major-maintenance job into engine damage and a nearly $3,500 repair job.
Transmission maintenance or repair. Automatic transmissions contain thousands of small parts, precise tolerances, and narrow passageways for hydraulic fluid. It’s a smart move that leave transmission repair to specialized transmission shops, and keep maintenance like flushes and fluid changes to skilled mechanics, not to your own workshop or oil-change places, recommends the AAA’s Calkins, who says that improper cleaning around connectors or gaskets during transmission flushes could spell doom for your household budget some days, weeks, or months later. “One grain of sand could wipe out your transmission,” he cautions.
Overheating. With some level of training and the right tools, cooling-system maintenance—like replacing hoses and the thermostat—is one of the more advanced repairs that a skilled home mechanic can do. But if the engine’s already overheated, forget about it—there’s just too much risk involved (your engine and thousands of dollars are on the line), and you need to have a proper mechanic diagnose the issue and do some damage control.
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AAA's Calkins also notes this type of issue as a money pit for shade-tree mechanics because they don't understand that there may be pages of diagnostics, and other tests to be performed, for every code. “One particular parts store even says, ‘Come on down and we’ll read the code.’” mentions AAA's Calkins. “Most DIY mechanics just replace the oxygen sensor when they see an oxygen sensor code,” he says. “And that’s when the money starts adding up.”
Replacing suspension components. Suspension pieces like struts, arms, and bushings look deceptively easy to replace, but there are plenty of ways this kind of repair can go wrong in the hands of the uninitiated. Beginners might not understand the tremendous force that's in a compressed coil spring (when replacing struts, for instance); they might do things in the wrong order; or they might not realize that part of the suspension supports the front subframe—and the engine. It's impractical and cost-prohibitive for a home mechanic to own some of the specialty equipment (like a wheel-alignment rack) needed to get the job done right, while labor costs aren't all that high for typical suspension repairs, so you might as well leave them to the pros.
“Working with springs is dangerous—always has been, always will be,” said AAA's Calkins.