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I have a 2000 Nissan Altima and the odometer (not trip meter) reads 86186 miles when I first start the car. As it's driven, the odometer seems to work correctly, incrementing miles. But if I turn off the ignition the odometer resets to 86186 miles again.

What could possibly cause this to occur?

I searched the internet to see if there was possibly a computer bug - but so far have found nothing.

  • Is the clock and trip meter also resetting too ? – Arka Patra Nov 1 '15 at 5:44
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    Mileage is stored in several places within the vehicle computer system. The main one (whatever/where ever that may be) is telling the dash system the mileage is as described and resets it when power is turned off. I don't know which part actually stores the information, so leaving this as a comment. No clue how to get it right, either. This is a strange problem which I've not heard of before, but anything is possible! – Pᴀᴜʟsᴛᴇʀ2 Nov 1 '15 at 10:58
  • Is the 86186 the mileage you expect, or has a random number just appeared? – HandyHowie Nov 1 '15 at 16:48
  • @Paulster2 The data is stored in the odometer unit itself. There is a c-mos battery it it , just like in a computer motherboard. It helps to keep the data saved. Probably that is causing the problem , given the car's age. How long the battery on the motherboard lasts ? – Arka Patra Nov 1 '15 at 18:13
  • @ArkaPatra - Yes the mileage is stored in the odometer. In most vehicles its stored in several places in the computer system. Its designed that way to help alleviate tampering, but people have found ways around it. If someone resets it one place to indicate a lower mileage, the computer will take over at some point and reset it to the proper mileage. Car lots will reset the mileage and an undetermined amount of time later the dash display will reset it self to the proper mileage, which completely befuddles the unsuspecting vehicle buyer. – Pᴀᴜʟsᴛᴇʀ2 Nov 1 '15 at 20:02
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Cause

Your car (like most cars) saves the milage in a small memory chip inside the instrument cluster. That chip is called an EEPROM. Like most memory units EEPROMs are subject to wear. If you rewrite the data often enough the chip will fail to save the new data or save corrupted data. This could be the case here.

So, why is the "correct" (i.e. new) value displayed? Because the value is saved somewhere else (in RAM) temporarily before it gets saved to the EEPROM for long-term storage. This temporary value does not survive your

Process of odometer display / save

  • Start of car: Read last value from EEPROM

  • Continuously: Calculate new value (by adding miles to last value). Show current value in display.

  • Every x seconds or miles: Save new value to EEPROM

Repair

Your can

  • replace the EEPROM in your instrument cluster. This is a rather complicated procedure which requires some electronics and computer knowledge. You have to desolder the old chip, buy a new, copy data from old to new and solder the new chip in. If you haven't done this before chances are you are breaking your instrument cluster. Besides you probably have to the buy special tools to do it.

  • replace your instrument cluster. This is a far more easy way of fixing the problem. Buy a used instrument cluster and swap it with your old. The new cluster's odometer will start from a wrong milage, but it will count correctly. If you plan to sell the car you should inform the buyer of the differing milage. In order to obtain the offset, maybe you can read the correct milage from the ECU using a OBD2 dongle or someone at a Nissan workshop can tell you using their diagnostic tool.

Bonus: EEPROM wear

EEPROMs last many erase and write cycles. The number of life-time cycles are in the 100000s or millions. That sounds much but considering a 20 year usage of a car, it is not.

If you use your car for 250k km (~150k mi) and the car saves the new value every 0,25 km thats 100000 writes. If you drive with an average of 50 km/h (~30 mph), it takes you 5000 hours to drive 250k km. If your car is not updating the values by distance but by time and it does that once every 10 seconds, you have 1,8 million writes. I don't know which of the two intervals is used to update the storage, but I guess it's a combination of those two: Each x miles, but at least every y seconds.

While most chips exceed their life expectancy, some don't. This can be a result of higher operating and storing temperatures or just a random fail.

  • Thanks for a very informative and detailed answer. I still have the car but want to get rid of it now. But I'm in a bad situation as the DMV requires accurate reporting of the mileage. And I cannot in good conscience do what the last seller did to me. I posted on Craigs and disclosed the speedometer issue - now for two weeks. But looks like no one wants to touch it. Maybe time to consider donating it to Father Joe's. Thanks Nissan. – docscience Feb 11 '16 at 22:23

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