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Altima Check Engine Light

Q. I have a 1995 Nissan Altima.

  • 4 cyl, 2.4L
  • Automatic
  • 90,000 miles
  • Fuel injected
  • p/s, a/c, cruise control
  • Rack & pinion

The problem I am having is for the last 3,000 miles the check engine light is coming on. I used a manual to put the Electronic Control Module in diagnosis system inspection mode. The fault code I got was 3 long flashes followed by 3 short flashes. According to the Haynes manual this indicates that the oxygen sensor circuit is open. Once I reset the ECM the light goes away, but it reappears a few hundred to a thousand miles later. This has happened a few times.

What do you think is the problem? Is it the Oxygen sensor? Why is the check engine light not coming on right after I reset the ECM ? With the light on I am not seeing any effect on the car's performance.

Your thoughts will be very much appreciated.

Thank You.

A. A common misconception is that when a ECM throws a code, in this case a 33 indicating a fault in the O2 sensor circuit, that it must be a bad O2 Sensor. While this may very well be the case, the whole circuit must be checked before any parts are replaced.

A code 33 could result from a broken wire, a connector coming unplugged or a bad O2 sensor. Nissan uses a three wire O2 sensor. One wire is for the signal input to the computer and the other two are for the O2 sensor heater. See my article on Mixing the Fuel. It could also indicate a malfunction in the ECM itself. If it was a "hard" fault, the check engine light would come on again as soon as you reset the ECM and started the car. Since it takes a while to come back on, it is an intermittent problem and I would tend to look for a loose or broken wire somewhere in the circuit. If you want to try and check it yourself, I would get a manual with a wiring diagram for the EFI circuit. In this case, I would go to a Nissan dealer and buy an Altima service manual there. The manuals they sell are the same ones the Technicians use in the shop. It will give you the exact, step by step procedure for checking the O2 sensor circuit. If you follow it exactly as outlined, you will find the problem.

In 1987 Nissan started using a "smarter" ECM. The ECM would "learn" how you drove and stores that information in case of a malfunction. When there is a malfunction, it will throw a code, turn on the check engine light and use the stored values to replace the missing information coming from the malfunctioning sensor. Since the O2 sensor is a fine tune for the fuel mixture, it is unlikey you would notice any decrease in engine performance.

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