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Discussion Starter #1
1995 Intrepid 3.3l - has two O2 sensors - one in each exhaust manifold on each side - car is throwing a code 21 02 sensor error with Check engine light coming on sometimes (not consistently ... yet). The code applies equally to L and R sensor - how can I tell which one is on the fritz? I dont have an OBD1 code reader - code reader connector is different than my OBD 2 and I don't think this car is ODB2...
 

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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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Thanks - that helps - still intermittent so I'll watch it for a while - I have had these clear on their own before (carbon on the sensor?).
 

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Well last last week it got to the point where the Check Eng light was on every trip. So I bought a sensor and went at it yesterday.
In case anyone else needs to diagnose one in the future:
Heaters in both sensors checked out - about 8 ohms - just over the spec of 5-7 ohms in the FSM. So I tried an analog voltmeter on the sensor signal - could not really read anything. So I got out the (rarely used) oscilloscope to see what that would show me. Hooked up a trace on one sensor then the other - both looked to be varying between about 0.2 and 0.8V as Crown indicated. It was only when I brought up traces on both sensors together on the scope (one on Ch1 and the other on Ch2) - then you could see - the right one showed a very regular pattern up and down between those voltages - almost sinusoidal - the left one was very irregular - going up and down almost randomly - and sometimes staying at the top or bottom of the trace for maybe 100ms - so I figured replace the left one which I did. I was surprised how easily it came out after 18 years in there (pretty sure it was original) - easily came out with an open end wrench. Last night when I went out to my hockey game - no check engine light - so that seems to have been it.
 
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