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Decoding Engine Trouble Codes - Part 1

If you're a fan of the show "Big Bang Theory," you know that Penny's "Check Engine Light" is always on – to Sheldon's consternation. He thinks it means the end of the world while she dismisses it as a minor annoyance, but who's right?


As it turns out, that annoying little light switches on when something triggers a diagnostic trouble code (DTC) in the engine's computer. Depending on the type of fault, such as a problem with the fuel delivery system, the light may remain on, flash, or come and go. Since the light is so ambiguous, there's no way to know what's going on without running a diagnostic test. If no other warning lights are triggered and the engine seems to be running OK, then you can probably continue driving for the moment, but you should get the problem investigated sooner rather than later. But if you notice other warning signs – such as funny smells or sounds – then stop driving until the problem is repaired.

Reading Trouble Codes

Plug your scan tool into the OBD II connector underneath the instrument panel, by the steering column. Or, you could go to an auto parts store and see if they'll do it for free.
Write down any code number(s) that pop up, but do not erase them until you're sure you have them recorded safely! These codes indicate which systems or sensors have a problem. If your scan tool doesn't define the code for you, then look it up online or in your shop manual. This will show you where to start troubleshooting.

Clearing Trouble Codes

The best method uses a scan tool; this instructs the computer to erase the codes without changing other learned settings that are needed for normal operation. Do not try to clear the codes by disconnecting the battery; this will erase the codes plus any other settings that the engine needs to function most efficiently and effectively.

Be aware that clearing the codes will not turn off your "Check Engine Light" permanently; eventually, an unfixed problem will trigger the codes again and switch on your warning light.
What Problems Do Trouble Codes Indicate?
Here's a sampling of what trouble code diagnostics, but by no means is this list complete:

P0100-P0104, P0171-P0175 – Mass Airflow (MAF) Sensor
P0115-P0119 – Coolant Sensor
P0130-P0147 – Oxygen Sensor
P0171, P0174 – Lean Fuel Mixture
P0172, P0175 – Rich Fuel Mixture
P0300 – Random Misfire
P0301-P0312 – Engine Misfire
P0335-P0339 – Crankshaft Position Sensor
P0340-P0349 – Camshaft Position Sensor
P0400-P0409 – Exhaust Gas Recirculation (EGR)
P0420-P0439 – Catalytic Converter
P0440-P0457 – Evaporative Emission Control (EVAP)

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