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What other symptoms?
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Wed, 21 Jan 2004 12:54:49 Share Post by Email
In Reply to: help please-had auto zone read cel codes for me...., a12, Tue, 20 Jan 2004 16:43:13
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Engine fault codes are pretty dumb. They can't take everything into account. It still takes a human.

First off, what other symptoms do you see? As patprice says, is the temperature gauge right? Do you see the car warm up properly? If the temp gauge is stuck low and you're not getting good heat, suspect a bad thermostat.

However, if the engine heats up right, it still might be the sensor circuit - just not the sensor. Did you measure the sensor resistance at the sensor or at the ECU? It could be as simple as a broken wire or a poor connection. Measure the resistance at the ECU. That doesn't rule out a problem in the circuitry inside the ECU, but it's a start. Also check the sensor at another temperature. 20C is fine, but the engine doesn't operate there. The sensor temperature coefficient could have shifted.

The O2 sensor is most likely a different problem. But again, an O2 sensor code doesn't mean the sensor is bad. The range of the O2 sensor is very limited - it operates right around stochiometric, and a little bit lean gives the same signal as a lot lean. The O2 sensor tell the ECU to adjust the fuel. It can (depending on model & year) adjust about 25% of the fuel flow. Lets say the O2 sensor reads lean. The ECU will add fuel, up to a point, until the O2 sensor reads the desired value. If the ECU adds fuel but the O2 sensor still reads lean, then the ECU will decide the O2 sensor is bad. But-
Let's say you've got a vacuum leak (most engine have at least one). That will lean out the mixture. The ECU will add fuel, but if the vacuum leak is bad enough, the ECU can't add enough fuel to get the O2 sensor 'off the stop', so it keeps reading lean. The ECU flags the O2 sensor for being bad, but all the sensor is doing is telling the truth. The same would happen rich if you had a leaky fuel injector, or the Fuel Pressure Regulator (FPR) had the fuel pressure too high.

Error codes are a good place to start, but they aren't sophisticated enough to take without a little more troubleshooting and some smarts.

posted by 192.249....
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