P0404

Code

ISUZU P — Powertrain

Exhaust Gas Recirculation Control Circuit Range/Performance

Brand: ISUZU

Type: P — Powertrain

Views: UK: 12 EN: 28 RU: 26

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Page language: EN

Causes

Faulty EGR valve (stuck, carbon-bound or mechanically failed)
Failed EGR position sensor or differential pressure sensor (DPFE) where used
Faulty EGR control solenoid/actuator (electrical or vacuum type)
Wiring harness damage: opens, shorts to power or ground, or poor connectors
Corroded or loose connector terminals at EGR components or PCM
Clogged EGR passages/intake manifold preventing expected flow

Symptoms

Malfunction Indicator Lamp (MIL) illuminated
Rough or erratic idle
Engine hesitation, stumble or reduced drivability under load
Poor fuel economy
Increased NOx emissions / failed emissions test
Possible engine pinging or knock under light load

What to check

Retrieve freeze frame and stored data with a scan tool; note engine conditions when code set (RPM, load, temp, commanded EGR %, feedback).
Check for related codes (P0400–P0409) or sensor/circuit codes that can point to wiring or inputs.
Visually inspect wiring and connectors at EGR valve, EGR solenoid, DPFE/EGR sensor and PCM for damage or corrosion.
Backprobe connector and monitor live data: commanded EGR vs feedback (position voltage or differential pressure).
Measure supply voltage and ground at the EGR actuator/solenoid and at the sensor with ignition on.
Measure resistance of EGR solenoid/actuator coil (specs vary by model) and continuity to PCM.

Signal parameters

EGR position sensor or DPFE typical signal: ~0.5–4.5 V (varies by model). Signal should change smoothly when EGR is commanded.
EGR command: duty cycle or percent open from 0–100% (or 0–5 V analog command depending on system).
EGR control solenoid coil resistance typically in the tens of ohms (e.g., 20–60 Ω) — manufacturing values vary; compare to service spec.
Battery voltage at component: ~12–14 V with ignition on; ground continuity near 0 Ω to chassis ground.
No-load continuity between harness and PCM pin should be present; open circuit indicates wiring fault.

Diagnostic algorithm

Connect a capable scan tool, confirm P0404 and related codes, record freeze frame and live data (engine speed, load, commanded EGR, EGR feedback).
With key on engine off, visually inspect EGR valve, solenoid, sensors and harness for damage, corrosion or disconnected plugs.
Backprobe the EGR feedback sensor and the command line. Verify supply voltage and ground at the component. Compare feedback to expected idle/rest values and verify it changes when command changes.
Command EGR ON/OFF or vary commanded duty from the scan tool while monitoring feedback. If command changes but feedback does not, suspect valve, sensor or mechanical blockage.
Measure resistance of EGR solenoid/actuator and continuity to PCM. Repair any open/shorts in wiring or connectors.
For vacuum EGR systems: apply vacuum to the valve (using a hand pump) to confirm movement and leakage. For electronic valves: gently verify movement when energized (follow safe procedures).
Remove and inspect EGR valve and passages for carbon build-up. Clean or replace valve and clear passages if restricted.
If wiring and mechanical components check good but feedback remains out of range, test or replace the EGR position/DPFE sensor. Re-test.
If all component and wiring tests pass, consider PCM software update or PCM replacement after verifying manufacturer bulletins and confirming failure with lab-level tests.
After repair, clear codes and perform a road test and monitor live data to confirm commanded vs feedback agreement and that code does not return.

Likely causes

Carbon-clogged EGR valve or ports preventing movement/flow
Bad EGR position sensor or DPFE producing out-of-range signal
Open/short/poor connection in EGR actuator control circuit
Faulty EGR control solenoid (electrical or vacuum leak)
Wiring or connector corrosion at EGR valve or PCM

Fault status

⚠️ Status

EGR Control Circuit Range/Performance — PCM detected EGR feedback outside expected range or not matching commanded position/flow.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-3.0 hours

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P0404 — Exhaust Gas Recirculation Control Circuit Range/Performance