Circuit Description
The engine control module (ECM) provides 12 volts to the park/neutral position (PNP) switch. The switch consists of five separate circuits, four of which indicate gear selector position to the transmission control module (TCM), and one that indicates park/neutral position to the ECM. When the transmission is in PARK or NEUTRAL, the switch closes and pulls the ECM voltage low, 0 volts. When the transmission is not in PARK or NEUTRAL, the ECM voltage is high, 12 volts. The ECM uses the switch in order to enable starter operation.
If the PNP switch signal indicates low when the transmission is not in PARK or NEUTRAL for a specified length of time, DTC P0851 sets. DTC P0851 is a type C DTC.
DTC Descriptor
This Diagnostic procedure supports the following DTC:
DTC P0851 Park/Neutral Position (PNP) Switch Circuit Low Voltage
Conditions for Running the DTC
| • | No transmission serial data DTC U0100. |
| • | The ignition voltage is 9-18 volts. |
| • | The transmission gear selector serial data message has been received and is valid. |
| • | The engine speed is less than 8192 RPM. |
Conditions for Setting the DTC
The park/neutral switch circuit is high, 12 volts, for 3 seconds while the TCM is reporting the gear selector is in PARK or NEUTRAL.
Action Taken When the DTC Sets
| • | The ECM does not illuminate the malfunction indicator lamp (MIL). |
| • | The ECM uses the transmission range data to enable engine cranking. |
| • | The ECM stores the operating conditions when the Conditions for Setting the DTC are met. This information is stored as a Failure Record. |
| • | The ECM stores DTC P0851 in ECM history. |
Conditions for Clearing the MIL/DTC
| • | The ECM clears the DTC from ECM history if the vehicle completes 40 warm-up cycles without a non emission diagnostic fault occurring. |
| • | The ECM cancels the default action when the ignition is OFF long enough in order to power down the ECM. |
| • | A scan tool can clear the DTC. |
Step | Action | Value(s) | Yes | No | ||||
|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | |||||
2 |
Important:
Did you record TCM Failure Records for DTC P0705? | -- | Go to Step 3 | |||||
3 |
Does the PNP Switch display Park/Neutral? | -- | Go to Step 4 | Go to Step 8 | ||||
4 | Shift the transmission from PARK to DRIVE. Does the PNP Switch display "In Gear"? | -- | Go to Testing for Intermittent Conditions and Poor Connections | Go to Step 5 | ||||
5 | Disconnect the automatic transmission inline 20-way connector. Does the PNP Switch display Park/Neutral? | -- | Go to Step 6 | Go to Step 7 | ||||
6 | Test the PNP Switch signal circuit for a short to ground between the automatic transmission inline 20-way connector and PNP switch. Refer to Circuit Testing . Did you find the condition? | -- | Go to Step 11 | Go to Step 12 | ||||
7 | Test the PNP Switch signal circuit for a short to ground between the automatic transmission inline 20-way connector and the ECM. Refer to Circuit Testing . Did you find and correct the condition? | -- | Go to Step 14 | Go to Step 13 | ||||
8 |
Does the PNP Switch display Park/Neutral? | -- | Go to Step 10 | Go to Step 9 | ||||
9 | Test the PNP Switch signal circuit for an open between the automatic transmission inline 20-way connector and the ECM. Refer to Testing for Continuity . Did you find and correct the condition? | -- | Go to Step 14 | Go to Step 13 | ||||
10 | Test the PNP Switch signal circuit for an open between the automatic transmission inline 20-way connector and the PNP switch. Refer to Testing for Continuity . Did you find the condition? | -- | Go to Step 11 | Go to Step 12 | ||||
11 | Replace the transmission internal wiring harness. Refer to Valve Body and Pressure Switch Replacement . Is the action complete? | -- | Go to Step 14 | -- | ||||
12 | Replace the PNP switch. Refer to Park/Neutral Position Switch Replacement . Is the action complete? | -- | Go to Step 14 | -- | ||||
13 | Replace the ECM. Refer to Control Module References for replacement, setup and programming. Is the action complete? | -- | Go to Step 14 | -- | ||||
14 | Perform the following procedure to verify the repair:
Has the test run and passed? | -- | Go to Step 15 | Go to Step 2 | ||||
15 | With the scan tool, observe the stored information, capture info and DTC info. Does the scan tool display any DTCs that you have not diagnosed? | -- | System OK |
