P0138
Bad O2 sensor, bank 1,sensor 2? Using torque, the voltage constant at 1Volt. Bank 1 sensor 1 is fluctuating (1volt or less). I do have a rich smell at the exhaust. Pretty obvious, but I should start by replacing the O2 sensor?
Edit: O2 sensor replaced and fixed!

P0500
Vehicle speed sensor. I replaced the ear transaxle speed sensor, but I cannot locate the transmission one. Where is it? Google has zero info
1998 Dakota, 5 speed manual


P0455
EVAP leak. No gas lines or tube leakage at the venting system. Gas cap not an original, could it be the wrong one?

 

http://www.obd-codes.com/p0138 replacing the sensor is a good bet.
The VSS is on the rear of the transmission on the drivers side.
There's no way to tell if it's the right gas cap. Put a new one on, clear the codes, and see if they come back.

 

p0455- look at the short hoses that come off the back of the valve covers both sides. They are part of the pcv system and go to the air cleaner with one big hose going under the throttle body. You probably know about the under hood sticker that shows all the vacuum lines

 

BobbyB an AX-15 manual trans has no such speed sensor

Where is it? Google has zero info ?? I wonder why maybe it doesn't have one did you clean out the Diff?

Get with the program BobbyB list your Dakota info so we all know what year, engine and trans you have.

 

Diff cleanout is postponed, possibly till next weekend. Will update info and I'll add a signature with truck stats.

 

Thanks for bringing me up-to-date on the transmission speed sensor question.