It ended up being , that I needed to reset the fuel trim with the laptop. Once I did this, it worked great. Thanks for the reply.

check the mixer (carb), it should have an screw where you can set the CO.

The trim valves are supposed to be set 50/50 (duty cycle), this way they can adjust mixture up or down, setting them to 50/50 value is done by the mixture screw. Diagnosis software is needed to set the duty cycle.
Also, you can adjust the range of mixture setting by adding thin plastic shims under the cone of the mixer.
Daewoo/Doosan had lots of issues with this in their 2-3 series, also different spring rates for the mixer and regulator where available at that timespan.

Hi,
The Doosan Forklift Truck showing error D4 and D6 within 10 second after engine start and also light On which for engine check on dashboard. Please help me for this.
Model: D80S-5
Serial Number: FDB0D-1270-02180
Type: D
Industrial Truck: 2F61

When you disconnect the trim valves the truck is designed to run rich so that's why it runs better-more fuel. I would check for a vacuum leak on the hoses. You can double check those FTV's with an ohm meter (26-28 ohms) and you should not be able to blow through them.