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Mitsubishi Eclipse Codes 12, 21, 51, 55

Q. Hi Vincent, I am really impressed by your site and wonder how could there be a guy so good and helpful on auto repair DIY. My car is a 1997 Mitsubishi Eclipse. 2.0 liter non-turbo with manual transmission and 75,000 miles. About 1½ months ago the Check Engine Light (CEL) is on for a few days and then went out a few weeks.

Mitsubishi Eclipse Codes 12, 21, 51, 55

Then on again for a few days and disappear again, but the car was running perfectly okay. Starting from two days ago, it has got a problem: The engine "choke" (feel like it's going to stall) when the car is accelerating and reach about 40 mph at fourth gear, and when the car is accelerating at third gear at 20 mph.

I performed the ignition key ON-OFF-ON-OFF-ON check and got the trouble codes 12, 51, 21 and 55. What do the codes represent and how can I check what the problems are? Do you think I can check and repair it by myself?

A million thanks!
Paulus

A. Here's what the codes mean:

  • 12 - Direct battery input to controller disconnected within the last 50 ignition key-on cycles.
  • 21 - Indicates an HO2S malfunction (P code would specify which HO2S.)
  • 51 - Fuel Mixture lean.
  • 55 - Completion of fault code display on CEL.

We can disregard codes 12 and 55. And I'm assuming someone disconnected the battery in an attempt to clear the codes. With the malfunction in one of the Heated Oxygen Sensors (HO2S) that should also cause the code 51. So the relevant problem is the code 21.

Auto Zone or some of the other larger auto parts stores will scan the system for free and you can then find out exactly which HO2S is causing the problem.

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