DTC 421 — Cylinder 1 – High Side to Low Side Open — International DT466E
Diagnostic reference for International DT466E (1996–2003) Navistar three-digit code 421. Focus on injector circuit diagnosis for cylinder 1 high-side to…
What it means
The ECM detected an open between the high-side and low-side of the cylinder 1 injector driver circuit. On the DT466E HEUI system this is an injector-circuit fault for cylinder 1; codes 422-426 cover cylinders 2-6.1
Most likely causes
- Open or damaged injector coil on cylinder 1 — The injector solenoid winding is open, breaking the high-to-low side path the ECM monitors.1
- Corroded, loose, or damaged injector connector at cylinder 1 — Terminal fretting or moisture intrusion at the valve cover pass-through or injector pigtail creates an open in the high/low side circuit.1
- Chafed or broken wiring in the injector harness for cylinder 1 — Harness rub-through on the valve cover, bulkhead, or near the ECM can open the high-side or low-side wire to cylinder 1.1
- Faulty injector driver/ECM output for cylinder 1 bank — Internal ECM driver fault or connector pin issue for the cylinder 1 injector bank can appear as a high-to-low side open.2
How to diagnose
- Verify code with a scan tool or flash code read; confirm DTC 421 is active and note freeze-frame conditions.
- Inspect cylinder 1 injector connector and pigtail for corrosion, oil intrusion, bent pins, and proper seating; compare to adjacent cylinders.
- Perform a visual and wiggle test of the injector harness from the valve cover to the ECM, looking for chafing, heat damage, or oil saturation.
- Check injector circuit resistance/continuity at the ECM harness connector for cylinder 1 high-side to low-side per Navistar service specs; an open indicates wiring or injector fault.
- If wiring tests good, isolate the injector by disconnecting it and rechecking circuit resistance at the harness side; a persistent open points to harness/ECM, a restored path points to the injector.
- Inspect related injector driver codes (e.g., DTC 525) and cylinder contribution codes (DTC 461) for additional clues before condemning the ECM.
Parts and tools you may need
- Digital multimeter
- Navistar-compatible scan tool for flash/Navistar codes
- Terminal release tools
- Dielectric grease
- Service manual wiring diagram for DT466E injector circuits
Tools you’ll need
- OBD-II scan tool — Read and clear DTC 421 and view freeze-frame data to confirm the fault is active on cylinder 1.
- digital multimeter — Measure injector coil resistance and continuity of the high-side to low-side circuit for cylinder 1 per Navistar specs.
- inspection mirror — See the cylinder 1 injector connector and harness routing under the valve cover where direct line of sight is limited.
- wire piercing probes — Test continuity and resistance at the ECM harness connector without cutting insulation during the wiggle test.
Rough cost and difficulty
| Likely repair | Parts | Shop labor | Who usually does it |
|---|---|---|---|
| Wiring or connector repair | $10–$100 | 1–4 hrs | DIY with tools and a manual |
| Fuel injector (each) | $250–$500 | 1.5–3 hrs | Shop |
| Engine wiring harness section | $100–$500 | 2–5 hrs | Shop |
| ECM/PCM replacement (programmed) | $600–$2,000 | 1–3 hrs | Shop |
Typical US independent-shop figures; labor runs about $140–$200 per hour (dealers charge more). Confirm the diagnosis before buying parts.
When to see a pro
If circuit tests are inconclusive, intermittent faults persist after harness repair, or ECM driver fault is suspected, have the system evaluated by a qualified diesel technician with Navistar diagnostic capability. Do not attempt high-pressure oil or injector removal without proper training and safety controls.
Check for recalls
Recalls are tied to your truck’s VIN, so check yours directly: enter the VIN at NHTSA’s recall lookup. Open safety recalls are repaired free at a dealer.
Sources
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