We present 33 transit minimum times of 20 transiting planets discovered by the CoRoT space mission. These have been obtained from ground-based observations since the missionís end in 2012, with the objective to maintain the ephemeris of these planets and to identify potential transit time variations. Twelve of the observed planets are in the CoRoT fields near the galactic center and the remaining eight planets are in the fields near the anticenter. We detect indications for significant transit timing variations in the cases of CoRoT 3b, 11b, 13b, 27b. For two more planets (CoRoT 18b and 20b) we conclude that timing offsets in early follow-up observations led to ephemerides in discovery publications that are inconsistent with timings from follow-up observations in later epochs. In the case of CoRoT-20b, this might be due to the influence from a further non-transiting planet. We also note that a significant majority (23 of 33) of our reported minimum times have negative O-C values, albeit most of them are within the expected uncertainty of the ephemeris. All acquired light curves are available at the Strasbourg Astronomical Data Center (CDS).