PO734 Gear 4 ratio error
 

Good morning. My sons 1998 S70 had this code this morning. We repalced his transmission in March 2013 with a low mileage unit out of a wreck. Everything had been working great until last night when he said it began slipping. After sitting for awhile it ran agin for about 5 miles before it started slipping again, and the arrow on the dash is flashing and the lights for the S,W,E on the shifter are flashing. Any input? My OBD reader indicates PO734,
Error 4 gear ratio. Fluid is full and clear.

Generally, a "Gear ratio error" means that the transmission is slipping in whatever gear is indicated. Slippage is interpreted by the computer as the gear ratio being different from what it's supposed to be. Assuming you have the correct transmission for your car, it's probably failing again.

Now... Why is it failing? The transmission isn't a common trouble spot on these cars. Before you replace the trans again, take some of the fluid out and take it to a transmission shop and have them do a "glycol test" or "antifreeze test." This is a simple check where they put a test strip into the fluid to check it for the presence of ethylene glycol, which is the main constituent of engine coolant. It only takes a tiny amount of coolant- far less than can be seen- to ruin a transmission in short order. If the trans fluid tests positive for glycol, you'll need a new radiator as well, because there's an internal leak in the transmission cooler. If it doesn't test positive, another possibility is that the old transmission left a bunch of crap in the cooler and cooler lines. When you replace the transmission again, get a transmission cooler flush kit from any auto parts store. They come with and without an inline filter- get the one WITH a filter. Follow the instructions and flush all fluid and contaminants out of the cooler and cooler lines. Then, install the filter in the return line. This will prevent a re-occurrence. Finally, make sure that the replacement transmission is the correct unit- Turbo and non- turbo cars have different transmissions. The incorrect transmission can cause errors.