I took my 99 S70 in for an inspection last week and it failed because the Check Engine light would not go out after the test. They hooked an OBD scanner up to the car and told me t was reading 2 codes. 0171 and 0506. The scanner siad that meant the engine (bank 1) was running lean and that the idle was lower than expected. The car had been running fine prior to the emissions test. Afterward the idle was a little rough. As the car was over the 50,000 mile mark I decided to do go ahead wit some maintenance I had planned and replaced the plugs and air filter and cleaned the throttle body. It was not bad, but had a little residue around the flap. Afterward the rough idle was gone and the car was back to its original form. We reset the computer and things seemed fine. Now 70 miles later the light is back on and the codes are the same. Any ideas? I am just starting to learn how to work on my Volvo and would love to hear what else I should check.