Learn professional methods to test O2 sensors with multimeter, OBD2 scanner, and visual inspection. Step-by-step diagnostics for upstream/downstream sensors, signs of failure, tools needed, and common testing pitfalls. Expert mechanic guidance for accurate results.

Comprehensive guide to check engine light causes - from loose gas caps to catalytic converters. Learn DIY fixes, repair costs, code meanings, and when to see a mechanic. Includes troubleshooting tables and cost breakdowns.