Diagnostic Aids:

Code "EO48" is designed to set if there is an EGR system fault. The test for code "EO48" is performed under conditions where EGR is normally allowing exhaust gas to flow into intake.

To perform test, ECM turns off EGR flow to engine and monitors oxygen sensor (closed loop) integrator. With EGR turned off, the integrator should swing to a higher value reflecting leaner air fuel mixtures. If not, ECM assumes that either EGR was turned off before test started, or EGR is flowing and ECM does not have ability to turn it off. The ECM tests EGR 5 times in a given key cycle. If EGR does not respond 3 or more times in 5 tests, code "EO48" is logged.

1. Many codes disable EGR and inhibit "EO48" test. Repair other DFI codes before diagnosis of code "EO48".
2. Check for control system ability to provide vacuum to valve, and cause to valve to lift.
3. Check for flow through valve. At idle, EGR flow causes rough engine operation. If engine does not idle rough or stall with vacuum applied to valve, then EGR is not flowing sufficient exhaust gas to intake.
4. Vacuum pump provides source vacuum from EGR valve. In output cycling, pump should operate and provide sufficient vacuum to lift EGR valve.
5. In output cycle, EGR solenoid should bleed vacuum to atmosphere when energized and should pass vacuum pump vacuum to EGR valve when de-energized.