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Corolla 2010 engine 2ZR-FE-0521336 model1.8P GX HATCH 5 4A..120000km..on low steady throttle (generally on slight downslope) the engine surges and revs drop from about 1800 to 1400rpm then returns. the engine warning light came on recently but after a general service is OK. otherwise the car seems to function normally with no apparent misfire.. they did a computer check and one coil had given a fault code once.

Hi there,
Generally the engine check light is a key to potential faults, and might only come on if that fault happens long enough to log the fault, which is not to say it doesn't happen more often.
I recommend having your coils and sparkplugs checked to rule out the missfire possibility. If the problem is intermittent, they are more difficult to locate and fox and it may not happen often enough to replicate of test after some work has been done.