O2 sensor error..but which one?

I have a 1996 Dodge Dakota SLT 4X4, V-8 auto. I recently had my check engine light diagnosed and was told I have a bad O2 sensor. The mechanic told me I should replace both because his reader will not tell
him which sensor is bad. The symptom is that sometimes the truck starts real hard and doesn't want to stay running. If I stay in the gas for a minute, it usually straightens out. Does anyone out there know of a way to determine which sensor is bad? If it means anything the check engine light will be on for a few days and off for a few days. The problem seems to be more apparent when the light is off. Greatly appreciated.
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snipped-for-privacy@vermontel.net wrote:

Not knowing the exact system you have I'll try.
You may have up to 3 - O2 sensors on your exhaust. The book on my 01 4.7 showed 3, but I only had 2. Anyway, since they are easily replaced I would change the one that is just foward of the Cat converter. If I am wrong, move it to the other side.
Hank
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here. if its OBDII like I'm thinking it should be on a 96 the code should be P followed by a 4 digit number. like P0301. maybe your mechanic has already told you this code number, the code number should tell witch sensor is causing the fault.
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That has nothing to do with the O2 sensors. The O2 sensors aren't used on initial startup. The PCM is operating in something called 'open-loop' mode - fuel amounts are determined by a set of pre-determined fuel maps in the PCM. It's only when the engine warms up that the PCM enters 'closed-loop' mode, and modulates fueling based on O2 sensor feedback.
Your symptom of having to stay on the gas to keep the engine running during start-up is indicative of a problem with the IAC (idle air control) motor, which could be because of a bad IAC itself, or could be the cause of a bad battery (not supplying enough voltage to the PCM, causing all sorts of problems due to invalid sensor readings).
First, have the battery load-tested, and replace it if it shows any signs of not holding a full charge. If that checks out, remove the IAC from the throttle body, give it good cleaning with TB or carb cleaner, re-connect it to the electrical connector while still out of the TB, and turn on the ignition. The IAC pintle should cycle in and out freely and smoothly. If it doesn't, replace the sensor.
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Tom Lawrence wrote:

That is interesting because sometimes it turns over real slow, and it is always when this problem is happening.
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jf3489 wrote:

THAT (couple with the other problem) sounds like either a sick battery or cruddy battery terminals. As Tom alluded, both are first (and easiest) to check & correct. Bryan
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