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KBB
KBB, Technician
Category: Car
Satisfied Customers: 390
Experience:  3 YEARS OF EXPERIENCE ON DOMESTIC VEHICLES. EXTENSIVE GM TRAINING ON GMC,PONTIAC AND BUICK .
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I have trouble codes P1391 and P1393. These are both for are

Resolved Question:

I have trouble codes P1391 and P1393. These are both for are related to the "G sensor". What is a G sensor?
Submitted: 5 years ago.
Category: Car
Expert:  KBB replied 5 years ago.

HelloCustomerthe G-sensor is what monitors the rough road parameter. Rough road parameter would be like running the vehicle over bumpy roads. It tells the PCM if the misfire diagnostic should be enabled or disabled. I'm going to post information on the codes and the G-sensor below this should help you. I would replace the the G-sensor it sounds like the issue here.

 


DIAGNOSTIC TROUBLE CODE (DTC) P1393
G SENSOR HIGH VOLTAGE


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) 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 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 volt (-1G) and 4.5 volts (+30).

Conditions for Setting the DTC

  • G sensor signal is greater than 4.8 volts for a total of 12.5 seconds over a 25-second period of time.

Action Taken When the DTC Sets

  • The PCM will illuminate the malfunction indicator lamp (MIL) the first time the fault is detected.
  • The PCM will store conditions which were present when the DTC was set as Freeze Frame and in the Failure Records data.

Conditions for Clearing the MIL/DTC

  • The PCM will turn the MIL "OFF" on the third consecutive trip cycle during which the diagnostic has been run and the fault condition is no longer present.
  • A history DTC P1393 will clear after 40 consecutive warm-up cycles have occurred without a fault.
  • DTC P1393 can be cleared by using the scan tool "Clear Info" function or by disconnecting the PCM battery feed.

Diagnostic Aids
Check for the following conditions:

  • Poor connection at PCM - Inspect harness connectors 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 C 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 PCM 5V reference and ground terminals with the MAP sensor, a damaged MAP harness or sensor could cause a G sensor DTC to be set.
  • The G sensor will give correct voltages only if it is level and mounted securely to its bracket. If DTC P1393 cannot be duplicated, the information included in the Failure Records data can be useful in determining when the DTC was last set. If it is deter mined that the DTC occurs intermittently, performing the DTC Pi 394 Diagnostic Chart may isolate the cause of the fault.



DIAGNOSTIC TROUBLE CODE (DTC) P1391
G SENSOR PERFORMANCE


Circuit Description
The G sensor is a vertical low g-acceleration sensor. By sensing vertical acceleration caused by bumps or pot-holes in the road, the powertrain control module (PCM) 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 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 volt (-1G) and 4.5 volts (+3G).

Conditions for Setting the DTC

  • Engine running.
  • No vehicle speed.
  • G sensor signal indicates greater than 2.5 volts (+1G) or less than 1.5 volts (0G) for one minute.

OR

  • Vehicle speed is between 50 km/h and 132 km/h (30 mph and 80 mph).
  • G sensor signal changes more than .0002 volts each tenth of a second (changing faster than physically possible).

Action Taken When the DTC Sets

  • The PCM will illuminate the malfunction indicator lamp (MIL) after the second consecutive trip in which the fault is detected.
  • The PCM will store conditions which were present when the DTC was set as Freeze Frame and in the Failure Records data.

Conditions for Clearing the MIL/DTC

  • The PCM will turn the MIL "OFF" on the third consecutive trip cycle during which the diagnostic has been run and the fault condition is no longer present.
  • A history DTC P1391 will clear after 40 consecutive warm-up cycles have occurred without a fault.
  • DTC P1391 can be cleared by using the scan tool "Clear Info" function or by disconnecting the PCM battery feed. Reviewing the Failure Record 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.

graphic

 



G SENSOR (ROUGH ROAD SENSOR)
The PCM receives rough road information from the G sensor. The PCM uses the rough road information to enable or disable the misfire diagnostic. The misfire diagnostic can be greatly affected by crankshaft speed variations caused by driving on rough road surfaces. The G sensor generates rough road information by producing a signal which is proportional to the movement of a small metal bar inside the sensor.
If a fault occurs which causes the PCM to not receive rough road information between 50 km/h and 132 km/h (30 mph and 80 mph), DTC P1391 will set.

 

 

 

If this has helped please hit the accept button. Positive feedback is greatly appreciated!

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KBB, Technician
Category: Car
Satisfied Customers: 390
Experience: 3 YEARS OF EXPERIENCE ON DOMESTIC VEHICLES. EXTENSIVE GM TRAINING ON GMC,PONTIAC AND BUICK .
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