P1391

DTC P1391

CIRCUIT DESCRIPTION
The rough road sensor is a vertical low G-acceleration sensor. By sensing vertical acceleration caused by rough road conditions, the engine control module (ECM) can determine if the changes in crankshaft speed are due to engine misfire or are driveline induced. If the rough road sensor detects a rough road condition, the ECM misfire detection diagnostic will be de-activated. The rough road sensor at rest output should be between 2.30-2.65 volts. During a rough road condition, the voltage output can vary between 0.5 volts and 4.5 volts. The rough road sensor is a capacitive type sensor with 3 circuits:
- A 5-volt reference circuit
- A low reference circuit
- A signal circuit

The ECM provides the rough road sensor with 5 volts on the 5-volt reference circuit and a ground on the low reference circuit. Vibration to the rough road sensor from rough road conditions in the road, provides the ECM with a signal voltage from less than 1 volt to more than 4 volts. If the ECM detects that the rough road sensor input is not within a calibrated limit of the estimated value, this DTC sets.

DTC DESCRIPTOR
This diagnostic procedure supports the following DTC:
DTC P1391 Rough Road Sensor Performance

CONDITIONS FOR RUNNING THE DTC
- The engine is running.
- The vehicle speed is less than 5 km/h (3.1 mph).

OR

- The engine is running.
- The vehicle speed is between 30-70 km/h (19-43 mph).
- DTC P1391 runs continuously once the above conditions are met.

CONDITIONS FOR SETTING THE DTC
The ECM detects that the rough road sensor output is not within range for more than 22.5 seconds.

ACTION TAKEN WHEN THE DTC SETS
- The control module stores the DTC information into memory when the diagnostic runs and fails.
- The malfunction indicator lamp (MIL) will not illuminate.
- The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Failure Records.
- The driver information center, if equipped, may display a message.

CONDITIONS FOR CLEARING THE DTC
- A current DTC Last Test Failed clears when the diagnostic runs and passes.
- A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other non-emission related diagnostic.
- Clear the DTC with a scan tool.

TEST DESCRIPTION

Steps 1-3:

Steps 4-8:

Steps 9-13:

Steps 14-16:

The numbers below refer to the step numbers on the diagnostic table.
2. The rough road sensor will give correct voltages only if it is level and mounted securely to its bracket.
12. This step tests for poor connection at the ECM or the harness connections for backed-out terminals, improper mating, broken locks, improperly formed or damaged terminals, and a poor terminal-to-wire connection.