Friday, June 3, 2011

1998 Toyota Corolla VE upstream o2 sensor error code?


What error code is showing, check that bank one sensor one or bank one sensor 2.
As per that sensor has to be replaced.If any of the one is just replaced, then, try removing that sensor, then clean that sensor and reconnect and check.If same problem, then its other sensor issue.You can also test both sensors by meter to confirm, which is faulty.

A P0420 diagnostic trouble code is a "generic" fault code that is set when the Onboard Diagnostic II (OBD II) system sees a drop in converter efficiency. The OBD II system monitors catalyst efficiency by comparing the switching activity of the upstream and downstream oxygen sensors in the exhaust. The upstream O2 sensor in the exhaust manifold reflects the condition of the exhaust gases as they exit the engine. The downstream O2 sensor in or behind the catalytic converter reflects the condition of the exhaust after it passes through he converter.

The OBD II system can't actually measure the concentration of HC or CO in the exhaust, so it compares the upstream and downstream O2 sensor readings to estimate how well the catalyst is actually doing its job of removing pollutants from the exhaust.

The upstream O2 sensor will typically show a lot of switching activity because the engine computer is constantly adjusting the fuel mixture between rich (more fuel) and lean (less fuel). When the engine is first started, the catalyst is cold and doesn't do much. During this time, the switching activity of the upstream and downstream O2 sensors are essentially the same because nothing is happening inside the converter.
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If the downstream O2 sensor is bad (heater circuit not working, loose or corroded wiring connector, contaminated sensor element, etc.), the OBD II system should detect the fault and set an oxygen sensor code. The same goes for a bad upstream O2 sensor. In either case, the presence of an O2 sensor code should prevent the catalyst monitor from running and setting a false P0420 code. Of course, this is ideally speaking and nothing is ever ideal. Sometimes a faulty O2 sensor is not bad enough to set an O2 sensor code but is off just enough to affect the accuracy of the catalyst monitor.

To minimize this risk, it's a good idea to check the operation of the O2 sensors with a scan tool or scanner software. You should see normal switching activity in both sensors shortly after the engine is started, with the O2 sensor voltage switching back and forth between rich (over 0.8 volts) and lean (less than 0.3 volts). A flat line O2 sensor reading or one that shows little switching activity is a bad sign. The downstream O2 sensor should slow down and go flat when the converter lights off (if the catalyst is working). If it keeps on switching like the upstream sensor, it tells you the catalyst is probably bad and the converter needs to be replaced.

You should also inspect the exhaust system and converter for leaks. "False" air can enter the exhaust through leaks and upset the O2 sensor readings, causing them to read leaner then normal.


As for the voltage ranges on your particular model year - the upstream sensor (pre-cat) should read between ~0V - ~5V, the downstream (post-cat) should read between ~0V - ~1V. The reason for the difference, is your upstream sensor is not an O2 sensor, it is an Air/Fuel sensor - very different beast. It acts more like a wideband O2 sensor than the typical O2 sensor (90% are narrow band). You can buy the right ones aftermarket or at the dealership - but they would be considerably more expensive than the older versions.
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For the P0420 CEL - you will be looking at the downstream sensor - the one after the car. You should be able to tap the wiring by pulling back the carpeting under the passenger seat. I can't remember if you can get at the rear O2 sensor without pulling any up or not. Might have to remove the chair and center console to get more room - remember to disconnect the negative terminal on the battery first,
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