Chrysler Repair: 1995 dodge neon: codes 21, 25, loss of speedo, oxygen sensors, dodge neon

Question
When driving the car, the speedometer abrupty quits working and the car becomes quite sluggish.  The following codes were recorded:  12, 21, 25.  Is replacing the oxygen sensor a simple job or do I need a trained mechanic?  Also, where is the oxygen sensor located?  Thanks very much for any assistance.


Answer
Hi Joe,
There are two oxygen sensors, one on the outlet flange of the exhaust manifold and one on the outlet of the catalytic converter further downstream. Unfortunately the code doesn't say which one is at fault. If you want to check the voltage on each you will find the signal wire to be tan/white on the harness that connects to the catalytic and black/dark green on the manifold. If the voltage on either of those wires while the engine is being heated stays fixed it is probably the bad one. In general I would expect the upstream (exhaust manifold) sensor to be bad.
The sensor looks like a spark plug and changes out the same way. You may want to buy a tap (18mm x 1.5 +6E) to clean the threads of the hole in the manifold after you remove the old sensor. You may want to spray the old one at the thread/manifold interface with penetrating oil and let is soak in overnight before trying to remove the old sensor. Also run the engine to heat the manifold and perhaps make the threads release more easily when you try to remove it. The new sensor comes either with pre-applied thread grease of a tube of such to apply to the threads. Tighten the sensor about that same as you would a spark plug being careful not to damage the delicate wires.
The 25 code is about the autimatic idle speed motor on the throttle body. The speedometer not working is usually a sign that the output speed sensor on the transmission is intermittent and that too will impact the engine function because its data is needed to properly obtain the fuel mixture.
Roland