I can recall two possibilities:
1) Emmission blocked. The blockage gradually backed up to prevent fuel and air being sucked in. Typically the catalytical converter overheated and fused. A simple vacuum test will tell if it is this problem;
2) EGR system problem. The PCM is working in closed loop mode after engine temperature reached target level. The the emmission control will modulate EGR to blend the exhaust gas with intake fuel/air mix, to control the combustion temperature to reduce NOx together to increase the thermal mass to increase the fuel efficiency. However, if the mix ratio is incorrect, it will get the fuel too lean and the engine is starving. However, most time the OBDII system will set CEL.
Start with vacuum test first.
1) Emmission blocked. The blockage gradually backed up to prevent fuel and air being sucked in. Typically the catalytical converter overheated and fused. A simple vacuum test will tell if it is this problem;
2) EGR system problem. The PCM is working in closed loop mode after engine temperature reached target level. The the emmission control will modulate EGR to blend the exhaust gas with intake fuel/air mix, to control the combustion temperature to reduce NOx together to increase the thermal mass to increase the fuel efficiency. However, if the mix ratio is incorrect, it will get the fuel too lean and the engine is starving. However, most time the OBDII system will set CEL.
Start with vacuum test first.
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