Why would check vent light go on after replacing sensor and computer

Did you clean the exhaust duct from end to end?
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Sound like you need to replace the heating element.
If you are in the market for an Dryer Vent cleaning check out Dryer Vent Cleaning Dallas Tx.

Mass air low sensors can be cleaned. If a new one was installed you should be ok as far as metering the air, sensor has very sensitive and fragile wires, and should only be cleaned by mass air flow sensor cleaner. Unless they left it unplugged, or the computer is not picking up signal from one dead out of the box. The all points check may have dislodged a vacuum line. But if you are at the dealer, they need to be answering your questions, after that 4K tune-up.

You have a short somewhere. As long as it was put back together properly it is most likely a bad element that is touching the heater box. Check for a short there by testing each terminal of the element to ground.

Burner parts need yearly cleaning, air intake needs periodic cleaning, burner parts cannot be exposed to trace amounts of chemical without damaging parts, no water should be dripping down from vent, vent should be checked for updraft, ample combustion air is required especially in closed basements, tight houses, and hot attic, bleed gas line to remove air, check that gas control thermostat is functioning.
Open following links for pilot light help:
http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot
http://waterheatertimer.org/pdf/Pilot-light-will-not-stay-lit.pdf

You're probably in for a service call and/or possibly water heater replacement if unit is more that 5-6 years old.
Your water heater plugs into outlet. Check outlet with cheap circuit analyzer to make sure the unit is receiving correct polarity without line interference.
If unit is in the attic, then add more venting to soffit and roof so pilot is not being snuffed out by hot air. If unit is in basement, then open some doors and a window and see if unit needs more air.
Most gas water heaters require yearly cleaning and service.
Clean the combustion parts and tubes.
Clean the air intake.
Have gas company check gas pressure.
Open following link for possible illustrations:
http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot

The problem can be bad gas control valve, air in gas line, low gas pressure, gas shut off handle not fully parallel with gas line, dirty combustion parts, lack of combustion air, improper venting. Or combination of all things. It's an electronic gizmo, anything can be bad or shorted, and repair may not last long.

Without a trained service technician to ohm-test the parts using correct service manual, then we are guessing at the cause.

Power vent water heater implies that you have side wall venting which must be replaced with another side-wall vented unit -or- it implies rapid reheating of water.

If water heater is more than 6 years old, you might be money ahead by installing a new one from local box store. Avoid the power vent and electronics. Local box store will have passive roof-venting type water heater BUT box store does not sell side-wall power vent. If your power vent goes up through the roof, then box-store variety gas water heater will work. Total cost $350 do-it-yourself for 40 gallon gas.
Or get 40 gallon 240Volt electric for $220 and connect to circuit breaker box. Requires no venting, and is easy to repair for years to come.
Or small 120Volt electric that plugs into outlet to tide you over until gas repairs can be made.
Look for scratch-n-dent models and always offer 10% off.
http://waterheatertimer.org/How-to-install-gas-water-heater.html
Shower uses 5-9 gallons hot water.

Clean all lint out of the exhaust vents tubes You will be surprised to see it work again
A moister sensor detects lint build up and shuts the heat off or nearly off at all times

SUUNDS LIKE TERMOTSTATS HAVE BLOWN.

you have to understand that there is a leak in the system other than the gas cap for the evaporative system. The system needs to be professionally closed, and smoke introduced into to find the leak, and a through component check, eg.. purge valve vapor canister vent solenoid all functioning correctly