Malfunction Indicator Lamp (MIL) Inoperative

Circuit Description

There should always be a steady MIL with the ignition ON and the engine stopped. Ignition feed voltage is supplied directly to the MIL bulb. The Powertrain Control Module (PCM) turns the MIL ON by grounding the MIL driver circuit. No MIL with the key ON, engine not running and DTC P1671 set suggests an open in the MIL driver circuit.

MIL Operation

The Malfunction Indicator Lamp (MIL) is located on the instrument panel (CHECK ENGINE) or (SERVICE ENGINE SOON). The MIL performs the following functions:

^ It informs the driver that a problem has occurred and that the vehicle should be taken for service as soon as possible.
^ As a bulb check and system check, the MIL will illuminate with the ignition switch ON and the engine not running. when the engine is started, the MIL will turn OFF. If the MIL remains ON, the self-diagnostic system has detected a problem. If the problem goes away, the MIL will turn OFF in most cases, but a diagnostic trouble code will remain stored.
^ If the MIL is illuminated, then the engine stalls, the MIL will remain Illuminated so long as the ignition switch is ON.
^ Now, if the MIL is not illuminated and the engine stalls, the MIL will not illuminate until the ignition switch is cycled OFF, then ON.

Perform the Powertrain On-Board Diagnostic System Check first, when the following conditions are present: Initial Inspection and Diagnostic Overview

^ When the MIL does not turn ON when the the ignition switch is turned to the RUN position
^ When the MIL remains ON while the engine is running
^ When you suspect a driveability problem

Diagnostic Aids

An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside the insulation. Check for the following items:

^ Inspect the PCM harness and connectors for improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.
^ If the engine runs OK, check for a faulty light bulb, an open in the MIL driver circuit, or an open in the instrument cluster ignition feed.
^ If the engine cranks but will not run, check for an open PCM ignition or battery feed or a poor PCM to engine ground.

Number(s) below refer to the step number(s) on the Diagnostic Table:

4. A no MIL condition accompanied by a no start condition suggests a faulty PCM ignition feed or battery feed circuit.
7. Using a test lamp connected to B+, probe each of the PCM ground terminals to ensure that a good ground is present.