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Frequently Asked Questions
What are vehicle diagnostic trouble codes (DTCs)?
- Diagnostic trouble codes tell you the systems or parts that malfunctioned or failed, causing the CEL to come on.
- Codes display as a string of letters and numbers you may need to look up online to “decode.” The ODBLinkMX+ app deciphers DTCs into easy-to-understand language.
- Although using a code reader or scan tool sounds simple, it’s doesn’t always precisely identify the issue. An O2 “lean” DTC can mean the sensor is dead, the air/fuel mixture is lean, or you’ve got a vacuum leak or fuel problem.
- Armed with this information, you can decide whether to fix the problem yourself or take your vehicle to a pro.
Can you find engine codes without a scanner?
- Maybe. According to Simes, depending on the make, model and year, cycling the ignition key on and off three or five times will display DTCs if the vehicle has a multi-display unit. “On some Toyota’s, codes can be accessed through the radio display,” he says.
- Never “jumper” across the data port pins with a paper clip to read codes. All you’ll do is damage the terminals, making it impossible to use a code reader in the future.
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How do you clear engine codes?
After you fix the problem causing the codes and CEL to light up:
- Use the code reader to clear the codes and check the “readiness monitors” status. A scan tool or code reader cannot reset readiness monitors. Or:
- Disconnect the battery for 10 minutes. Remove the negative/black (-) cable first, then the positive/red (+). Follow all safety steps when working on or near a battery, including wrapping the positive cable terminal/end with tape to prevent a short circuit.
Where can I get vehicle diagnostic trouble codes checked for free?
- Many local and national auto parts stores.
- Some auto repair shops. If you just need the codes checked, there should only be a minimal or no fee charged, especially if you’re a loyal customer.
- Ask your neighbors. If you’re lucky, someone has a code reader they’re willing to share.
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What are the most common engine codes, and what do they mean?
P0119: Engine Coolant Temperature (ECT) Sensor Circuit sending erratic or intermittent data to the ECM. Affects engine drivability and other operations. Threaded into an engine coolant passage, you can replace the sensor yourself after draining the coolant system.
P012X/P022X: Throttle/Pedal Position Sensor (TPS) Circuit malfunction or inaccurate voltage readings. Affects fuel delivery causing fluctuating engine idling speed, surging and stalling. Try cleaning the throttle body yourself.
P0128: Coolant Temperature Below Thermostat Coolant Temperature. This means the ECM detects the engine running colder than it should be. Affects engine warm up and fuel delivery. Most likely causes are a bad thermostat or air bubbles trapped in the cooling system. “Burping” the coolant system or replacing the thermostat are DIY-able.
P0137-P0147: Oxygen Sensor (O2) Circuit means the engine air-fuel mixture is running too rich (burning more fuel than air) or too lean (burning more air than fuel). Either will cause poor performance, rough idle and reduced fuel economy, as well as excess smog. This may be one for the pros. Check for an exhaust leak or replace the O2 sensor yourself if it has more than 50,000 miles on it.
P0171-P0177: Same as above, air-fuel mixture too rich or lean. However, the most likely culprits include clogged air or fuel filters or dirty fuel injectors or Mass Air Flow (MAF) sensor. Replacing the filters and cleaning dirty injectors and MAF are DIY repairs.
P030X: Random/Multiple Cylinder Misfire Detected. The last code number refers to a specific engine cylinder; P0305 means cylinder No. 5 is misfiring. This can be caused by anything from bad spark plugs to low engine compression. If you feel comfortable and have the proper tools, you can make repairs yourself.
P0340-P034B: Camshaft Position Sensor (CMPS) Circuit. A CMPS malfunction means the ECM cannot precisely control fuel delivery and ignition (spark) timing. Symptoms include hard- or no-start condition, lack of power or engine misfire. Leave this repair to the pros.
P0400-P040E: Low Exhaust Gas Recirculation (EGR) Flow Detected means the ECM is not sensing enough exhaust gases flowing into the engine to lower combustion temperatures. Symptoms include rough idling and stalling. It’s most likely a clogged or failing EGR valve you can decarbonize yourself.
P0446-P0448: Evaporative Emission Control System (EVAP) Vent Control Circuit means the ECM thinks there’s an issue with the EVAP vent valve. Unfortunately, there are many possible causes for this code, from a loose fuel cap (try retightening or replacing the gas cap) to a bad ECM.
