P1391

DTC P1391 G SENSOR ROUGH ROAD RATIONALITY




Circuit Description
The G sensor is a vertical low g-acceleration sensor. By sensing vertical acceleration caused by bumps or potholes in the road, the powertrain control module (PCM)/engine control module (ECM) can determine if the changes in crankshaft speed are due to engine misfire or are driveline induced. If the G sensor detects a rough road condition, the PCM/ECM misfire detection diagnostic will be de-activated. The G sensor at rest output should be between 2.35-2.65 volts (+1G). During a rough road condition, the voltage output can vary between 0.5 (-1G) and 4.5 volts (+3G).

Conditions for Setting the DTC
- Engine is running.
- No vehicle speed.
- G sensor output indicates greater than 2.5 volts (+1G) or less than 1.5 volts (0G).
OR
- Vehicle speed is between 30 mph (50 kph) and 70 mph (112 kph).
- G sensor signal changes more than 0.002 volts each tenth of a second (changing faster than physically possible).

Action Taken When DTC Sets
- The Malfunction Indicator Lamp (MIL) will illuminate.
- The PCM/ECM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame and Failure Records buffers.

Conditions for Clearing the MIL/DTC
- The MIL will turn off after three consecutive ignition cycles in which the diagnostic runs without a fault.
- A history Diagnostic Trouble Code (DTC) will clear after 40 consecutive warm-up cycles without a fault.
- DTC(s) can be cleared by using the scan tool.

Diagnostic Aids
Check for the following conditions:
- Poor connection at the PCM/ECM - Inspect the harness connections for backed-out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal-to-wire connection.
- Damaged harness - Inspect the wiring harness for damage. If the harness appears to be OK, observe the G sensor display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the display will indicate the location of the fault.

Since the G sensor shares the PCM/ECM 5 v reference and ground terminals with the Throttle Position (TP) sensor, a damaged TP harness or sensor could cause a G sensor DTC to set. See Diagnosis By Symptom.
The G sensor will give correct voltages only if it is level and mounted securely to its bracket.
Reviewing the Failure Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.

Steps 1 To 7:

Steps 8 To 15:

Test Description
The number(s) below refer to step(s) on the diagnostic table.
1. The On-Board Diagnostic (OBD II) System Check prompts the technician to complete some basic checks and store the freeze frame and failure records data on the scan tool if applicable. This creates an electronic copy of the data taken when the malfunction occurred. The information is then stored on the scan tool for later reference.