The P1408 trouble code appears when the ECM detects that the exhaust gas recirculation (EGR) system is flowing either too much or too little compared to the expected value. Unlike P1407 (EGR No-Flow), P1408 typically means the flow is present, but outside the acceptable range.
Because EGR performance directly affects NOx emissions, reference material from the EPA Vehicle Emissions Testing Guide is useful when understanding how flow range is monitored.
1. What “Flow Out of Range” Means
The ECM continuously monitors:
- MAP sensor response
- MAF airflow drop during EGR activation
- DPFE pressure differentials
If the commanded EGR position doesn’t match the expected airflow change, the ECM sets code P1408.
2. Common Symptoms of P1408
- Check Engine Light
- Occasional rough idle
- Hesitation during acceleration
- Higher combustion temperatures on long drives
- Sulfur smell or exhaust reduction issues
3. Most Likely Causes
- Partially blocked EGR passages
- Weak or sticking EGR valve actuator
- DPFE sensor reading incorrect pressure levels
- Excessive carbon buildup in intake runners
- Vacuum leak (on vacuum-type EGR valves)
- Electrical control circuit resistance out of spec
4. Technician Diagnostic Workflow
Step 1 — Scan Live EGR Data
With a full-featured diagnostic tool like iCarsoft CR Ultra P, check:
- Commanded EGR % vs. actual flow readings
- DPFE voltage change during EGR activation
- MAP sensor drop when EGR opens
- Idle stability with EGR forced open
Step 2 — Perform Bidirectional EGR Testing
CR Ultra P allows you to force the valve open/closed at different increments. A sluggish or inconsistent response points to mechanical issues.
Step 3 — Inspect the Valve and Passages
- Remove valve and check movement manually
- Look for carbon blockage at ports
- Verify solenoid and connector integrity
Step 4 — Evaluate Sensors
DPFE and MAP sensors are critical. A failing sensor often leads to “flow out of range” even if the valve is working correctly.
Step 5 — Clean or Replace Components
Most P1408 issues can be resolved with a full EGR system cleaning, but valves that stick after cleaning should be replaced.
5. Recommended Tool: CR Ultra P
For accurate EGR diagnostics, the CR Ultra P provides:
- Bidirectional EGR activation tests
- DPFE/MAP/PIDs real-time graphing
- Full-system scanning across all modules
- Touchscreen interface and WiFi lifetime updates
- Supports all 4.0-series brands + two additional OEMs
- Ships to U.S. and U.K. customers
Need help choosing the right tool? Contact us via the site chat box or email service@icarsoft-us.com.
6. FAQ — Common Questions About P1408
Q1. Does P1408 always mean the EGR valve is bad?
No. Many times the valve opens fine, but the airflow sensors give incorrect data.
Q2. Can carbon buildup cause “flow out of range”?
Yes — especially if only one EGR port is partially blocked.
Q3. Will the engine run hotter with P1408?
Some engines will run slightly hotter because combustion temperatures increase without proper EGR flow.
Q4. Is DPFE sensor failure common?
Very. Ford, Mazda, and some Toyota models frequently show P1408 caused by a failing DPFE sensor.
Q5. Can the CR Ultra P reset EGR-related adaptions?
Yes. Resetting adaptions after repair is recommended for a clean baseline.
7. Summary
P1408 indicates that EGR flow is happening, but outside the acceptable range. A combination of scan data comparison, physical inspection, and sensor verification usually pinpoints the issue quickly.
