Fault Code P1137

Hi All
Last week driving home from work, a short journey of approx 4 miles I noticed my EM light on.
I own a VW ProScan so I plugged it in and got the following reading
17545 - 226 01/01
KWP 2000
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(http://www.passatworld.com/forums/42-volkswagen-passat-b5-discussion/352932-fault-code-p1137.html#)Fuel Trim : Bank 1
(add) : system too Rich : P1137
sporadic 002 - Lower Limit Exceeded

The car had not lost any performance and had no mpg issues
I reset fault code
Last night driving home again it came on again, I have driven over 200 miles since it first came on with no problem
until last night. Rechecked and the same reading again.
I have had my Passat 2.0 Sport (petrol) since new (2002- mileage 62,000) and this is the first time I have had the EM light on.
Has anyone else has had this problem. The car is performing well even with the light on, but I do not want to damage the engine. should I get a VW dealer to check it out. The car is serviced every year
Thanks
Paul

I would check and clean the mass air flow sensor. Also have a good top engine clean and injector cleaning done followed by an oil change.
Also might be time for a front oxygen sensor replacement

Thanks For your help
I will clean the mass airflow sensor if I knew where it is, I will check manual. Also I will replace the front oxygen sensor, and clean injectors. Can you use the injector cleaner you add in the petrol tank or is it a strip down of injectors ?
Once again Thank You for your help
Paul