Services were delayed by up to half an hour between South Hylton and Pelaw as a result of the problem.
The fault was rectified by about 6.30am, but delays have still been reported since then.
An initial tweet by Metro said: "Delays of up to 20 minutes between Pelaw and South Hylton, and up to 30 minutes between South Hylton and Pelaw due to a technical fault."
The service later updated: "The earlier fault in the Sunderland area has been fixed.
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"There are still some delays but trains are frequent and running to all destinations."
It was later confirmed that there would be a 'gap in service' between the two stations.
Metro tweeted: "There will be a gap in service between Pelaw and South Hylton.
"The 7.23am South Hylton to Airport is the train affected. Apologies."