Every time you start your RAV4, many of the warning lights on the instrument panel light up and then go out. This is part of a self-test to make sure that these important lights are functioning properly. But what happens when some of these lights DON'T turn off? What should you do if some of these lights come on while driving? Of course, in the case of the oil pressure or A/T Temp light you should stop the car before any serious damage is done. However, in the case of the Check Engine Light (CEL), there's no need to panic! The CEL is only telling you that the Engine Control Module (ECM) has detected a fault. It could be something simple, like a loose gas cap or it could be something more serious.
Generally speaking, there are 2 different groups of lights that show up. The first scenario is when the CEL, VSC, and 4WD (TRAC in 2WD models) lights come on. The second scenario is when you see the VSC, ABS, and BRAKE lights on. Sometimes the 4WD or TRAC lights also show up. So why do all these lights come on at the same time and what do they mean? Let's take a look at each of these scenarios:
1. CEL, VSC, and 4WD/TRAC (Check Engine Light, Vehicle Stability Control, and 4-Wheel Drive/Traction Control
The CEL indicates that the ECM has stored a diagnostic trouble code (DTC) and means something is not quite right in the engine. It could be any of a hundred different things such as a bad spark plug or coil causing a misfire to a bad oxygen (O2) sensor or yes, even a loose gas cap. Fortunately, the code can be read from the ECM with an OBDII tester and will usually point to the area of the problem and often to the specific part. But why would a CEL shut down the VSC or 4WD systems? Many of these features depend on the engine running at peak performance. The VSC for example, uses not only the braking system, but also controls the throttle to help get the RAV out of tricky situations. If the engine is not running right, the ECM shuts down those systems that would affected.
2. VSC, ABS (Antilock Braking System), BRAKE, and sometimes TRAC (skidding car icon)
This series of lights indicate a problem with the braking system. The BRAKE light by itself means either the parking brake is on or the brake fluid level is low in the reservoir. But when the BRAKE light is on with these other lights, it's an ABS failure of some sort. In the vast majority of cases, this is usually caused by a bad or dirty wheel speed sensor. These sensors look somewhat like a bolt with wires on one end. The speed sensors detect the wheel speeds and send appropriate signals to the skid control ECU. Speed sensor rotors have rows of alternating N and S magnetic poles, and their magnetic fields change as the rotors turn. The speed sensors detect those magnetic changes and send pulse signals to the skid control ECU.
The ECU monitors the wheel speeds through these pulse signals to control the ABS control system. But if there is snow/ice or mud on the sensor, it may not detect the pulse, and a code is set. HOWEVER, this code is not set in the ECM, it's set in the Skid Control ECU. Many code readers can't see these codes, because they're only designed to look for engine codes in the ECM. That's why a lot of people get frustrated when a mechanic or auto parts place tells them they have no codes even with 3 or 4 lights glowing on the dash! The wheel sensor isn't always the problem in this scenario--some owners have reported a problem with the brake pedal switch or a trailer wiring harness caused these lights to show up.
I went back through many of the old threads and compiled a listing of all the RAV4World users who reported these error lights. Out of the 41 users who had the first scenario with the CEL, 31% reported the cause as a bad or defective charcoal canister filter. A defective O2 or A/F sensor accounted for 24% of these reports. The infamous loose gas cap was attributed to 19% of the reports. As for the second scenario, I only found 8 reports and 63% of those turned out to be broken or defective wheel speed sensors. To view the listing which shows the user name, lights, codes, and eventual fix, download it here:
RAV4 Diagnostic Lights.pdf
Many thanks to fellow user RickL for compiling the list into a nice PDF for easy viewing.
Next, I'll show how to read and clear the diagnostic trouble codes with or without a code reader.