What this code means
P0405 usually means the controller may be seeing the EGR position feedback lower than expected.
What the vehicle may do
- The vehicle may display an emissions-related warning message.
- A message or speed limitation may remain until the fault runs and passes.
- Driveability may be affected depending on how the vehicle responds to the EGR fault.
Possible fault areas
- Possible EGR position signal circuit issue
- Possible low reference circuit issue
- Possible reference voltage feed issue
- Possible connector or harness issue
- Possible EGR valve position sensing issue
- Possible control module side issue
Diagnostic path
Open and frame the fault
On this Silverado diesel, P0405 means the controller may be seeing the EGR position feedback lower than expected. The truck may turn on an emissions-related message, and in some cases a message or speed limitation may not clear until the fault runs and passes. This circuit family also has a high-voltage side, but here we’re staying focused on the low-voltage path: the EGR position signal, low reference, reference voltage feed, connectors, harness, the EGR valve, and the control module side of the circuit. Start with the basic system checks, then follow a structured diagnostic approach so you do not jump straight to a valve or module.
Know when the monitor can run
Before testing, understand the monitor gate. As a scan-data baseline, the normal parameter range is 0.25 to 4.8 V. A 5 V reference short-to-ground data point is listed as 5.0 V, and the signal short-to-ground data point is 0 V. For the code to run, the engine is running, ignition voltage is greater than 11 V, and once the running conditions are met, the monitor can run continuously. For P0405, the setting condition is an EGR Position Sensor short to ground for greater than 1 s. Treat those as the conditions you need to reproduce, not as a long checklist to read through.
Initial scan check before circuit testing
With ignition on and the vehicle in service mode, look at the EGR Position Sensor parameter. The expected verification range is 0.25 to 4.75 V. If it is outside that range, move into circuit testing. If it is inside that range, reproduce the operating conditions, or the captured conditions from the fault data, and confirm whether the code resets. If the code does not reset, the fault is not active at that point. If it does reset, continue into circuit testing.
Low reference checks
Power the vehicle and all systems off, then disconnect the EGR valve connector. For ground and low-reference continuity checks, give the vehicle time to go to sleep; it may take up to 2 min before the reading is accurate. First test between low reference circuit terminal 4 and ground. You are looking for less than 10 Ω. If that is 10 Ω or greater, disconnect the engine control module connector and check the low reference circuit from terminal 4 at the component harness to terminal 22 X2 at the control module harness. That circuit should be less than 2 Ω. If it is 2 Ω or greater, repair the open or high resistance. If it is less than 2 Ω, the diagnostic path points to the engine control module.
Reference voltage checks
If the low reference checks pass, turn ignition on with the vehicle in service mode and test the 5 V reference at the EGR connector: terminal 6 to low reference terminal 4. The expected range is 4.8 to 5.2 V. If it is low, turn the vehicle off, disconnect the control module connector, and check terminal 6 at the component harness to ground for infinite resistance. Anything less than infinite resistance points to a short to ground on that circuit. If resistance is infinite, check terminal 6 at the component harness to terminal 20 X2 at the control module harness for less than 2 Ω. If it is 2 Ω or greater, repair the open or high resistance. If it is less than 2 Ω, the diagnostic path points to the engine control module. If the 5 V reference is high, turn the vehicle off, disconnect the control module connector, turn ignition back on in service mode, and test terminal 6 at the component harness to ground. It should be less than 1 V. If it is 1 V or greater, repair the short to voltage. If it is less than 1 V, the diagnostic path points to the engine control module.
Signal circuit and sensor simulation
When the 5 V reference is in range, check the scan data again. With the EGR valve disconnected, the EGR Position Sensor should read greater than 4.8 V. If it is 4.8 V or less, turn the vehicle off, disconnect the control module connector, and check signal circuit terminal 2 at the component harness to ground for infinite resistance. Less than infinite resistance means repair the short to ground. If resistance is infinite, the diagnostic path points to the engine control module. If the scan tool shows greater than 4.8 V, use a 3 A fused jumper between signal circuit terminal 2 and low reference terminal 4. Now the EGR Position Sensor should drop to less than 0.2 V. If it does not drop, turn the vehicle off, remove the jumper, disconnect the control module connector, turn ignition back on in service mode, and check signal circuit terminal 2 at the component harness to ground. It should be less than 1 V. If it is 1 V or greater, repair the short to voltage. If it is less than 1 V, turn the vehicle off and check signal circuit terminal 2 at the component harness to terminal 21 X2 at the control module harness. That circuit should be less than 2 Ω. If it is 2 Ω or greater, repair the open or high resistance. If it is less than 2 Ω, the diagnostic path points to the engine control module.
Component decision and repair verification
If the signal responds correctly and drops below the specified value with the fused jumper, the remaining action in this path is to test or replace the EGR valve. After the repair, keep verification separate from the circuit testing. Verify the repair and confirm the code stays gone. With ignition on and the vehicle in service mode, check the EGR system event data. All of those parameters should display No. If any event still displays Yes, operate the vehicle under the running conditions for that event until it can run and pass. The key takeaway is simple: prove the low reference, prove the 5 V reference, then prove the signal circuit before condemning the EGR valve or the control module. For more diagnostic training, visit stepdiagnostics.com.
Final check
This code is often approached by proving the EGR position sensor circuits first, then verifying the repair under the conditions that allow the fault to run.
For more guided automotive diagnostics, visit STEP Diagnostics.
