Can a check engine light reset itself when a scan tool is connected?

Question

I have a Toyota RAV4 2002 4WD.

I was driving to the car wash when suddenly the check engine light come on when I accelerated hard. After the wash, it still was on. I drove home, shut it off, turn it back on, light still on. Went and got my scan tool, pluged it in and fired up the engine and now the light suddenly went away.

I know these models have problems with Air/fuel sensors, O2 sensors and ECUs.

Can something have happend to my ECU? Or is this "normal"? I can still see live data, and the light has not come on again. Everything seems to be normal. There are no stored codes in the ECU either.

I am using a Bluetooth OBD-II reader and the Torque app.

Answer 1

Might be an intermittent issue. Before taking the car to the dealership, I would wait to see if the check engine light turns on again. Newer cars have typically more intelligent systems. and therefore, false alerts are rarer than in older cars (and similarly, the probability that the light will automatically turn off is lower). In your case, however, the light turned off which indicates it was an intermittent issue.

I used to have a 1989 Opel Vectra that occasionally had a trouble code related to incorrect voltage from the air flow sensor. The problem always went away after driving for a while. I eventually got rid of the car for different reasons than a diagnostic trouble code.

Merely connecting an OBD reader should not automatically reset the code (although, in theory it is possible to design an OBD reader that automatically resets the code before even printing the code, but why would anybody design such a stupid device?). What probably reset the code was that the car itself figured out that the problem was intermittent and has went away.

By the way, have you checked that the gas cap is tight? A loose gas cap is one of the most common reasons for a trouble code, and if the gas cap is only very slightly loose, there is the possibility that the trouble code goes away automatically.

Answer 2

Assuming you've replaced that sensor by now.

Also, did you have or do you know if that ECU/PCM was re-flashed due to false catalytic codes?

Some documentation from the factory manual that may help others.

DTC P0141: HEATED OXYGEN SENSOR HEATER CIRCUIT (BANK NO. 1 SENSOR NO. 2) ... Diagnosis & Repair

1. Remove glove box and passenger's side kick panel (if necessary) for access to ECM with electrical connectors still installed on ECM. See Fig. 2 . Turn ignition on. Using voltmeter, check voltage between body ground and terminal No. 3 (Red/Black wire) at ECM electrical connector E5. See Fig. 4 . This is the HT1B terminal on ECM. Voltage should be 9-14 volts. If voltage is not within specification, go to next step. If voltage is within specification, replace ECM.

2. Disconnect electrical connector for heated oxygen sensor (bank No. 1 sensor No. 2). See Fig. 7 . Using ohmmeter, check resistance between +B and HT terminals on electrical connector for heater on heated oxygen sensor (bank No. 1 sensor No. 2). See Fig. 9 .

3. Resistance should be 11-16 ohms at 68°F (20°C) and 23-32 ohms at 1472°F (800°C). If resistance is within specification, go to next step. If resistance is not within specification, replace heated oxygen sensor (bank No. 1 sensor No. 2). See HEATED OXYGEN SENSOR under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - 4-CYLINDER article.

4. Check wiring harness and electrical connectors between ECM and heated oxygen sensor (bank No. 1 sensor No. 2), and between heated oxygen sensor (bank No. 1 sensor No. 2), EFI3 fuse (10-amp) and EFI main relay. See WIRING DIAGRAMS article. EFI main relay may also be referred to as EFI relay. EFI3 fuse and EFI main relay are located in fuse/relay box at driver's side front corner of engine compartment.

Answer 3

I have an 06 scion tc. N it is a 2.4litre 4cylinder, i believe nearly identical engine/motor to a rav 4. Yesterday my check engine light came on. Today went to use scan tool. After it read, there were no codes. So i went back and tried again. However this time i turned the car on, but the check engine light was gone. The car reacted as if i had disconnected the battery on the initial start up. When the car turns on, the idle goes low. After 2 to 4 start ups the idle is normal. I think when using the code reader, it must've cleared. So yes i think that can happen. Happened to 2 toyotas obviously. Last time it was gas cap and time before was alternator. 256k miles n still running strong. Still passing smog. Just keep oil in it they burn oil like crazy.