My 2000 hyundai elantra is automatic it has a code p0734. which the mechanic explained me that i have to get the speen sensor replace and get new clutch because the 4 gear ratio is bad. pls advice what 2 do?

P0141 o2 sensor heater circuit malfunction bank 1 sensor 2
po455 evaporative emission control system leak Gross leak
po303 misfire cylinder three
p0732 gear two incorrect ratio
p0734 gear 4 incorrect ratio

Why did he or she say this?? Did he or she do the diagnostics for the fault codes or did they just guess??

Code 0734 : Gear 4 Incorrect Ratio. you may wanna take back to the Mechanic and tell him whats going on and let him deal with that. Good luck

P0734 means that your transmission is slipping. This being the case, it is not surprising that the gas mileage is poor. You will need to have your transmission checked out professionally. Take it to a trustworthy transmission mechanic or to a dealer. Get a second opinion before you have any work done.

Did you check the transmission fluid level? If necessary, add fluid. However you probably have a more serious problem. Take the car to a qualified transmission mechanic.

if it was me i would stop by a Hyundai dealership and talk to the service manger and he or she be able to get you this information wish you the best of luck Michigan Man.

YES!... There are many sensors on an automatic transmission which could pose a problem -- depending on what the problem actually is. For example, there are 'input' and 'output' speed sensors, there is a fluid temperature sensor, there is a vehicle speed sensor, and there are several solenoid valves located inside the transmission connected to the valve body. Each and every one of these items can be a problem -- again -- depends on what the problem ACTUALLY is. Agreed?

Same situation, Solenoids in the Transmission were throwing the codes, Lost O/D and had transmission replaced. No signs of problems after check engine light came on for close to 1000 miles later.

DTC P0734 is "4th gear incorrect ratio" - that's your torque converter 'LOCK UP Clutch" It's worn out (or you're using the wrong Trans Fluid required by Hyundai - SPIII OR Both). The P1624 is simply the Trans Control Unit (computer) communicating to the Engine Control Module (ECM computer) MIL - on request signal to ECM = asking it to turn on "Check Engine" light. MIL = Malfunction Indicator Lamp. When the torque converter 'lock up" clutch slips or doesn't engage, the trans automatically returns to 3rd gear and stays there (it's called 'fail safe' mode) until you cycle the ignition key, then everything resets until the malfunction occurs again.
You'll need to have an AT (Automatic Transaxel) tester hooked up to verify this. Hyundai has a Service Bulletin specifically for "gear incorrect ratio' codes. If when you changed the trans fluid, was it done at a Hyundai Service Center? If not, someone may have installed the wrong fluid.

Web site http://www.hmaservice.com/ - register and you'll have access to more info than you can imagine. Crea me.