Reasons for a Check Engine Light on a Lexus

by Richard Rowe
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Lexus benefits from parent company Toyota when it comes to engineering, reliability and diagnostics, and Toyota's warning systems monitored catalytic converter and individual cylinder performance years before such things were required by the federal government. While the company's On-Board Diagnostic system is advanced, the Check Engine Light by which it notifies the driver of failure is far from specific.


Generally speaking, a Lexus' Check Engine Light will trigger any time the computer detects that one or more of the vehicle's systems are far enough out of their engineered parameters to cause a problem. The monitored systems are not limited to the engine itself; a check engine light can also be triggered by a malfunction in the transmission or emissions system.

Problem Identification

The car's computer keeps a log of all trouble codes encountered, which can be read by plugging an OBD-II scanner into the diagnostic port (located beneath the steering wheel). Any auto parts retailer will have a basic scanner that can read active codes (those which accompany a currently illuminated Check Engine Light), but you'll need to head to a specialized shop to check stored codes. There are hundreds of different codes, but a few examples are: P1200 (Fuel Pump Relay/ECU Circuit Malfunction), P1349 (VVT System Malfunction, Bank 1) and P1645 (Body ECU Malfunction).

Evaporative Emissions

All cars have their quirks, and Lexuses are no different. For instance, one common Check Engine Light trigger on 1995 and up cars is the notorious "Evaporate Emissions Control" malfunction, which simply implies that the fuel cap is open. This irritating warning can be dealt with simply by taking the gas cap off for 30 seconds, an then replacing it. Turn the cap clockwise until you hear three clicks and a sharp crack, which sounds remarkably like something breaking. The warning light should extinguish itself within a day or two.

Converter Malfunction

One problem endemic to Lexus' LX line is so-called "Converter Under-Heating." This condition tends to occur where temperatures regularly dip below freezing, but is not unheard of in warmer areas. The converter itself is not malfunctioning; the sensors only think it is. There is no known long-term fix for this problem, one can only reset the computer and hope it doesn't return.


After repairs have been made, the computer must be cleared of codes to restore the factory settings. Resetting the computer will always cause the Check Engine Light to self-extinguish, but you can count on the light to rear its ugly head again if the problem hasn't been solved. There are a number of different ways to reset the computer; you can disconnect one terminal on the battery, but pulling the ECU fuse is quicker and easier. After disconnecting the computer, wait 15 minutes for its memory to reset and you're good to go.

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