Like Poolman said,PO172 is probably a vacuum leak.There is a multi-port fitting on the intake manifold sort of under the master cylinder.Check for cracked rubber fittings or leaky hoses.The manifold gasket could be leaking,about a 2hr job to change it.My 96 960 showed code PO410 today.When the ECU calls for air pump operation on cold start-up,it closes a relay in the fuse box,and the air pump runs,sounding like a vacuum cleaner.The ECU also powers and opens a valve supplying vacuum to a check valve in air line from air pump to top of engine.The check valve on my car would not hold vacuum.Retail at dealer almost $115.The special tools needed were a $20 vacuum pump and a $10 voltmeter.A '96 or newer car will not pass E-test with CE light on in most states,and must be run through a "drive cycle "so ECU will show ready for test.Good luck,Bill