Diagnostic Instructions

DTC Descriptors

DTC P1391: Rough Road Sensor Performance

DTC P1392: Rough Road Sensor Circuit Low Voltage

DTC P1393: Rough Road Sensor Circuit High Voltage

Diagnostic Fault Information

Circuit	Short to Ground	Open/High Resistance	Short to Voltage	Signal Performance
5-Volt Reference	P1392	P1392	P1393	P1391
Rough Road Sensor Signal	P1392	P1392	P1393	P1391
Low Reference	--	P1391	--	P1391

Typical Scan Tool Data

Rough Road Sensor

Circuit	Short to Ground	Open	Short to Voltage
Operating Conditions: Engine operating in Closed Loop Parameter Normal Range: 1.1-3.7 Volts
5-Volt Reference	0.0 Volts	0.0 Volts	5.0 Volts*
Rough Road Sensor Signal	0.0 Volts	0.0 Volts	5.0 Volts*
Low Reference	--	0.0 Volts	5.0 Volts*
* Internal ECM or sensor damage may occur if the circuit is shorted to B+.

Circuit/System 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 0.5-4.5 volts. The rough road sensor is a capacitive type sensor with 3 circuits:

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 greater than 4 volts.

Conditions for Running the DTC

P1391

P1392 and P1393

Conditions for Setting the DTC

P1391

The ECM detects that the rough road sensor output is not within range.

P1392

The ECM detects that the rough road sensor voltage is less than 0.1 volt.

P1393

The ECM detects that the rough road sensor output is greater than 4.9 volts.

Action Taken When the DTC Sets

DTC P1391, P1392, and P1393 are Type C DTCs.

Conditions for Clearing the DTC

DTC P1391, P1392, and P1393 are Type C DTCs.

Diagnostic Aids

Reference Information

Schematic Reference

Engine Controls Schematics

Electrical Information Reference

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Verification

1. Ignition ON, observe the DTC information with a scan tool. Verify DTCs P0641 or P0651 are not set.

⇒	If any DTC is set, refer to DTC P0641 or P0651.

2. Engine running, observe the scan tool rough road sensor voltage parameter. The reading should be between 1.1-3.7 volts.
3. Operate the vehicle within the Conditions for Running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

Circuit/System Testing

1. Ignition OFF, disconnect the harness connector at the rough road sensor.
2. Ignition OFF, test for less than 5 ohms of resistance between the low reference circuit and ground.

•	Hatchback low reference circuit terminal A

•	Notchback low reference circuit terminal C

⇒	If greater than the specified range, test the low reference circuit for an open/high resistance. If the circuit tests normal, replace the ECM.

3. Ignition ON, test for 4.8-5.2 volts between the 5-volt reference circuit and ground.

•	Hatchback 5 volt reference circuit terminal C

•	Notchback 5 volt reference circuit terminal A

Important: The 5-volt reference circuits are internally and externally connected at the ECM. Other component DTCs may be set. If other DTCs are set, review the electrical schematic and diagnose the applicable circuits and components.

⇒	If less than the specified range, test the 5-volt reference circuit for a short to ground or open/high resistance. If the circuit tests normal, replace the ECM.

⇒	If greater than the specified range, test the 5-volt reference circuit for a short to voltage. If the circuit tests normal, replace the ECM.

4. Ignition ON, verify the scan tool rough road sensor parameter is less than 0.1 volt.

⇒	If greater than the specified range, test the signal circuit for a short to voltage. If the circuit tests normal, replace the ECM.

5. Install a 3A fused jumper wire between the signal circuit terminal B and the 5-volt reference circuit. Verify the scan tool rough road sensor parameter is greater than 4.8 volts.

•	Hatchback 5 volt reference circuit terminal C

•	Notchback 5 volt reference circuit terminal A

⇒	If less than the specified range, test the signal circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the ECM.

6. If all circuits test normal, test or replace the rough road sensor.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.