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HOMESERVICE MANUALSPONTIAC1987GRAND PRIX BROUGHAM, 5.0 HREPAIR AND DIAGNOSIS (SINGLE PAGE)ENGINE PERFORMANCESYSTEM4.3L TBI "A" CHARTSCHART A2 - NO CODE 12, "SES" LIGHT ON STEADYWON'T FLASH CODE 12 "SERVICE ENGINE SOON" ("SES") LIGHT ON STEADY
1987 Pontiac Grand Prix Brougham, 5.0 H

Won't Flash Code 12 "Service Engine Soon" ("SES") Light On Steady

1987 Pontiac Grand Prix Brougham, 5.0 HSECTION Won't Flash Code 12 "Service Engine Soon" ("SES") Light On Steady

There should always be a steady "SERVICE ENGINE SOON" light, when ignition is on and engine stopped. Battery voltage is supplied directly to the light bulb. The ECM will turn the light on by grounding circuit No. 419 at the ECM. With the diagnostic terminal grounded, the light should flash a Code 12, followed by any trouble Code stored. A steady light could be a short to ground in light control circuit No. 419, or an open in diagnostic circuit No. 451.

NOTE: Test numbers refer to test numbers on diagnostic chart.
  1. If there is problem with the ECM that causes a "SCAN" tester not read "Serial Data", then the ECM should not flash a Code 12. If Code 12 does flash, be sure that the "SCAN" tester is working properly on another vehicle. If the "SCAN" tester is functioning properly and circuit No. 461 is okay, the Mem-Cal or ECM may be at fault for the NO ALDL.
  2. If the light goes off when the ECM connector is disconnected, then circuit No. 419 is not shorted to ground.
  3. This test checks for an open diagnostic circuit No. 451.
  4. At this point, the "SERVICE ENGINE SOON" light wiring is okay. The problem is a faulty ECM or PROM. If code 12 does not flash, the ECM should be replaced using the original PROM. Replace the PROM only after trying an ECM, as a defective PROM is an unlikely cause of the problem.
Fig 1: Chart A2 Schematic, No Code 12, "SES" Light On Steady
G49130
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