Serpentine belt: the $30 part that can strand you if it snaps
How to inspect your serpentine belt for wear, what happens when it breaks, and why replacing it proactively is one of the cheapest insurance policies in automotive maintenance.
The serpentine belt is a single ribbed rubber belt that drives the alternator, power steering pump, water pump, and AC compressor. If it snaps while driving, you lose all of those systems simultaneously: the battery stops charging (engine dies within minutes), the power steering locks up (hard to steer), the water pump stops (engine overheats immediately), and the AC quits.
Modern serpentine belts are made of EPDM rubber that wears differently than older neoprene belts. Instead of cracking visibly, EPDM belts wear like tire tread — the ribs get shallower over time. A belt gauge ($5 at parts stores) measures rib depth: new belts measure about 0.090 inches; replace at 0.060 inches. Most belts last 60,000-100,000 miles.
Other signs of a worn belt: a squealing noise on cold start (the belt slips on smooth pulleys), visible glazing on the rib surface (shiny instead of matte), or any fraying or chunks missing from the edges. If the belt has any missing ribs, replace it immediately — it will fail soon.
The belt itself costs $25-40. A shop will charge $80-150 for replacement, which includes relieving the spring-loaded tensioner, routing the new belt, and verifying proper tracking. Many cars have a belt routing diagram on a sticker under the hood. If you're DIY-inclined and have a 3/8-inch ratchet to release the tensioner, this is a 20-minute driveway job.
Information is for educational purposes only. Always consult a qualified mechanic for diagnosis and repair. AutoAt's knowledge base is AI-assisted and regularly updated but may not reflect the most current manufacturer specifications. Always verify critical specifications with official service manuals. Repair procedures described here may require professional tools, training, and certifications. Attempting repairs beyond your skill level can be dangerous.