Direct absorption infrared spectroscopy of gases suffers from low sensitivity and requires large optical path lengths. By using photothermal spectroscopy (PTS) that relies on the thermal induced effects in the sample as consequence of photon absorption, we can achieve significantly higher sensitivity with low (mm range) optical path lengths. We present an interferometric cavity-assisted photothermal spectroscopy (ICAPS) operated with an active cavity locking that achieves a detection limit of 1 ppm. In ICAPS, a Fabry-Pérot Interferometer (FPI) and a probe laser (NIR diode) are used as optical transducer to detect refractive index variations, induced by gas thermal expansion [1]. We used a DFB-QCL emitting at 1900.5 cm-1 to detect NO in a 2f-wavelength modulation detection approach. The reflected NIR intensity was measured by a photodetector (APD). The APD signal was fed to both LIA 1, for locking of the probe laser, and LIA 2 to retrieve the 2f-ICAPS signal. A schematic of the experimental setup is provided in Fig.1(a). Our new active locking mode compensates for drift of the FPI by locking the probe wavelength to the inflection point of the resonance profile, where the slope of the reflectivity curve is maximized. To achieve this operational mode, the probe wavelength is modulated at a frequency fp, while the reflected intensity signal is fed to LIA1 and demodulated at 2fp. The demodulated signal is provided to a PID to act on the probe current and keep the 2f-signal at the zero-crossing. The results of the NO ICAPS-sensor are shown in Fig.1(b).