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Super Moderator
1966 Crown Coupe, 2016 200 S AWD, 1962 Lark Daytona V8.
23,848 Posts
An OBD I code reader would be able to narrow it down and be more specific about the O2 failure. Something like a DRB III (or equivalent) with the body connector cable. One bank may show rich or lean fuel trim adaptives.
The code 21 is just a general O2 fault, it won't tell you if the O2 signal is high or low or the the O2 heater is failing or which O2 is failing.
If the right or left O2 signal is the problem and you don't have a reader, you could measure the O2 signal wire with a voltmeter to see if one is pegged high or low to determine which one is failing or measure heater element resistance to see if one heater is open or not.
The signal should swing constantly between about 0.2 and 0.8 volts. You never want it to sit still or peg at 0 or 1.0 volt. The heater is thermistor-controlled and will show less or more of a voltage drop depending on a cold or warm engine. As long as there is a nominal voltage drop, the heater should be operating. It should ramp up heat quickly on a cold engine and level off as the engine warms.
The 1995 Neon (PL) and 'cloud cars' (JA) were the first Chrysler OBD II cars. The 1995 LH was still OBD I.
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