My '00 HX (manual transmission) keeps getting two P1491 error codes but only under certain conditions. The engine has to be cooled down (stopped for 4 hours or more). The pair of codes only occur at the 2 1/2 to 3 mile point (I assume that's at the end of its warmup cycle). At that point the car's throttle response is not smooth (almost surges) and the codes register. It's fine after that. I stop the car and clear the codes and even though I have an 80-mile commute the next time they come back is when I go home for the day, and again at the 2 1/2 to 3 mile point after leaving work. Outside temperature seems to make no difference. I have the factory service manual and have tried the troubleshooting procedure several times but have yet to come up with a definitive answer. I have 2 EGR valves and the same thing happens with both.