Methods and systems are provided for estimating water storage in a charge air cooler (CAC). In one example, engine operation may be adjusted responsive to water storage parameters at the CAC, the water storage parameters based on an output of a first oxygen sensor positioned downstream of the CAC and a second oxygen sensor positioned upstream of the CAC. Further, operation of the first oxygen sensor and the second oxygen sensor may be diagnosed during certain engine operation conditions wherein no condensate is forming in the CAC.