February 2001

Porsche

Emission Control Light

Operation

                                              By Steve Grosekemper

Modern cars with their computerized engine management systems will warn us about the slightest problem through the engine malfunction light (MIL). If an oxygen sensor or throttle position switch fails or is out of range, a big red light displaying ENGINE will come on the dash to tell us to see our Porsche service technician for immediate repair. In this system all critical engine management parameters are constantly monitored so we don't drive a car with a failure that could damage either the engine or catalytic converter.

In the genesis of Porsche emission control, things were not quite as sophisticated. Back in the 70's the first emission control lights were introduced on the 911, 914 and 924. These lights are not emission control failure lights as one would expect, but service reminder lights. These lights marked "EGR" or "OXS" tell us when 30,000 miles has elapsed since our last emission control service. The way that these lights are activated is through a counter box that lives behind the speedometer. No state of the art computer monitoring here, just a little black box that counts off the miles until your next emission service and flips on a light to remind you 30,000 miles has elapsed.

You may be wondering just what an emission control service involves. For early cars with EGR systems the check is quite simple. Find the EGR valve and apply vacuum to the fitting while the engine is idling. The idle will drop if the system is working. Cars with dual fitting valves should make the engine die completely with vacuum hooked up to one of the fittings. Then hook up a gauge to the vacuum hose going to the valve. It should read over 5in.Hg. at a cruise throttle position. If all checks OK, then you can reset the EGR service reminder light.

For cars with oxygen sensors their check is even easier. Un-plug the sensor wire and hook up a digital voltmeter to the sensor plug. With the car on, you should get a voltage reading of between .45-.55 volts. Now hook the oxygen sensor back up and check the voltage with the car at 2500 RPM. The voltage should swing rapidly from .20-.80 volts. If all is well, reset the OXS service reminder light.

Resetting the light is the easiest part of this procedure. Remove the clock and speedometer (on 911's and 912E's) and locate a white button on a small black box behind the speedometer. Push the button, when you hear a click the service light is reset and ready for another 30,000 miles. The box is located in the passengers foot well on 914's and in the engine compartment below the master cylinder on 924's.

Good Luck

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