Fast start-up crystal oscillators (XOs) based on energy injection with an auxiliary oscillator (AO) are efficacious in improving the energy consumption of duty-cycled the Internet of Things (IoT) devices. Nonetheless, the frequency mismatch ( ΔF ) between the injection frequency and the crystal’s resonance frequency restrains the injection efficiency and start-up improvement. This work presents an automatic phase-error correction (APEC) technique for automatically correcting the phase error incurred due to ΔF up to 104 ppm and safeguarding the XO start-up performance. An inflection detector (ID) based on a dynamic comparator (DCMP) traces the crystal resonant signal’s peak points and selects the appropriate signals for injection. We fabricated the proposed fast start-up XO in a 40-nm CMOS process. It achieves a start-up time of 17.5 μ s with a 16-MHz crystal and a 1.0-V VDD . Furthermore, the start-up time only varies by ±1.42% over a ΔF of ±104 ppm, and ±4.5% amid temperature variations (−40°C to 85 ∘ C), evincing the robustness of the proposed technique. The power consumption and phase noise in the steady state are 84 μ W and −139 dBc/Hz at 1-kHz offset, respectively.