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1999 Dodge Ram 2500 Cummins P1688 Code: 48RE Transmission Only Shifts in Low and Reverse

Model: 1999 Ram 2500 Cummins Fault Code: P1688 Posted: 2017-07-19 22:25

I own a 1999 Dodge Ram 2500 with a Cummins engine and a 48RE transmission. The truck only shifts into low gear and reverse—no first or second gear. I've already replaced the governor solenoid and transmission sensor, but the issue persists. After pulling diagnostic codes, I received only one: P1688. Is this code directly responsible for the transmission not shifting properly? If not, what should I check next? The truck runs well and starts easily, so I'm hesitant to replace the injection pump or transmission control module without confirmation.

Related fault codes

P1688

Comments (3)

Anonymous 2017-07-20 07:34

P1688 is typically an injector-related code—unlikely to be the root cause of limited shifting. Did you use a proper OBD-II scanner? I suspect there may be additional codes present. Also, note: you mentioned a 47RE in the title but referenced a 48RE in the post—this discrepancy should be clarified for accuracy.

Anonymous 2017-07-20 17:38

Apologies—I meant to say it's a 48RE. I did use an OBD-II scanner, and that’s the only code returned. If P1680 isn’t related to the transmission control, where should I look next? I really don’t want to replace the injection pump or transmission control module since the truck performs well otherwise.

Anonymous 2017-07-21 06:47

My recommendation is to focus on diagnosing the transmission first. Check if your scanner can display live data—such as gear ratios, solenoid pressures, and shift timing—to identify any abnormal behavior during operation.