Honda Odyssey 2000 P0170/P0172/P1491 and P0300-P0305 Codes: EGR Valve Failure & Intake Carbon Blockage Diagnosis

My 2000 Honda Odyssey (107,000 miles) has been exhibiting persistent engine trouble with DTCs including P0170 (Fuel Trim Bank 1 - System Too Rich), P0172 (System Too Rich Bank 1), P1491 (EGR Valve Lift Sensor Insufficient Flow Detected), and intermittent misfire codes P0300–P0305. The issue is consistent: when I drive at steady speeds between 50-70 mph on the highway, the DTCs reset and disappear. However, during stop-and-go driving or transitions through 3rd to 4th gear (or into overdrive), the check engine light and TCS warning light activate — a pattern that suggests an engine-related shift jerk rather than transmission failure. The vehicle runs less smoothly than it did at 25–50k miles, with noticeable roughness during acceleration. While I’ve used injector cleaner and cleaned the PCV valve and MAP sensor, no improvement has been seen. The EGR valve was previously serviced by a dealer at 35,000 miles and appears clean upon visual inspection; however, resistance testing shows normal values. My Dyno-Scan (Auterra) data reveals: O2 Sensor B1-S1 varies from 0–0.9V, while B1-S2 ranges from 0.1–0.9V but remains consistently high — indicating a lean-to-rich imbalance. Short-term fuel trim for both sensors fluctuates between -24% and 0%, confirming the rich condition. I’ve been told by dealers that an EGR valve replacement is needed due to P0170/P0172/P1491 codes, but I’m skeptical since the valve appears functional. However, a deeper analysis reveals that P1491 (insufficient flow) and P0170 are not isolated — they point to a dual failure: EGR valve malfunction AND carbon buildup in the upper intake manifold. After removing the intake manifold, I found only light carbon deposits. A port near the throttle body was partially blocked — a known issue on this model. The dealer previously installed an EGR sleeve kit at 35,000 miles; it has now failed due to wear and heat exposure. I replaced the EGR valve with a new one. After installation, idle quality improved dramatically — the engine runs smoother than ever before. No DTCs have been set after driving for several hundred miles. I will continue monitoring for 500+ miles before declaring full resolution. Key insight: The EGR valve was not just 'sticking' — it exhibited erratic behavior under voltage testing. When a variable DC source (3–4A) is applied to the solenoid terminals, it should open smoothly and gradually, not jerk or pop open. After prolonged operation (5–10 minutes), the old valve overheated and failed to open at all. This explains why codes appear during stop-start driving — when EGR flow is interrupted, the O2 sensor detects a rich condition, triggering aggressive fuel trim corrections (e.g., -100% short-term correction). The root cause appears to be: 1) EGR valve failure due to carbon buildup and mechanical wear; 2) Partial blockage in the upper intake manifold port near the throttle body — both contributing to poor EGR flow. This issue is especially common on older Honda Odysseys, where EGR components degrade over time despite passing basic functional tests. The DTCs reset at highway speeds because EGR flow is minimal under steady-state conditions, and only reappear when engine load fluctuates during acceleration or deceleration. Recommendation: Replace the EGR valve and inspect/replace the upper intake manifold port sleeve (if not already done). This fix resolves both P0170/P0172 and P1491 codes. For long-term reliability, periodic cleaning of the intake port is advised.