Own a 2003 Toyota Highlander V6 AWD with 137K miles. Original Plugs, AF Sensors, Oxy Sensor, and recently CEL VSC lights came on the dash. P-Code is P0420. Checked MAF and it was dirty. Cleaned MAF with MAF spray and reset codes. Also checked Fuel Cap, and Hoses around air cleaner. P0420 returned day later. Should all three Sensors, 2 AF and 1 Oxy be replaced as well as plugs, or should I have a dealer check the diagnostics first before replacing all three sensors? Also, about the same time as the code appearing, I noticed a low rumbling sound coming from the front end. Sound starts around 20 MPH and stays constant until I make a right turn, making the right turn causes the sound to disappear completely but returns when straightening the wheels or turning left. Could the sound be one of the CV Axles starting to bind, and which one and are they replaced in pairs, or the hub bearing starting to go out, or is it related to the P0420 code?
Country: United StatesMake: ToyotaModel: Highlander Limited AWDYear: 2003Engine: V6
Cleaned MAF sensor, replaced front breaks, checked Fuel Cap and Hoses around Air Cleaner Box.
Welcome! I'm Ron Z! I will do my best to answer all of your questions. For easy access to this thread, bookmark it now! Please read everything!
The P0420 is a fairly straight forward code pointing to a deteriorating Catalytic Converter. The oxygen sensors measure the amount of emissions in the exhaust going into, and then out of the converter. When they see the converter is not "cleaning" the exhaust enough to me specs, the converter is deemed "below threshold" and the P0420 is set. Most commonly in this case, the converter is going to need to be replaced in the near future.
The "rumble" noise that is directly related to speed, and tends to stop when turning, is the classic sign of a worn Wheel Bearing. If the noise eases up when turning right, then this is the faulty side, as the bearing will quiet when pressure is applied. On this vehicle, the wheel bearing is "pressed" into the hub assembly.
Thanks Ron for the quick response, I have heard of other owners having the same P0420 problem as mine with this vehicle, and after replacing the converter the code reappeared. Is there some way to test the converter before it is replaced? I have heard of tapping the converter from below the vehicle and listening for rattling sounds. Also, one owner replaced the AF sensor closest to the firewall and that cleared the code, and another added SeaFoam to a full tank of Shell Gas Premium which eliminated the code. Not sure if either was long term. Lastly, will an exhaust leak near the exhaust manifold cause a P0420 code. I thought I heard a slight hissing sound comming from the back of motor near the firewall when I was checking the exhaust system.
The tap test will only work if the internal workings of the converter have been completely compromised. More commonly with the P0420 is simply the chemical composition of the converter is breaking down, and as a result, the emissions are not being converted enough to meet specs. On Toyota, when an Oxygen Sensor (aka A/F Sensor) is the fault, you would recieve the p-code pointing to said faulty sensor (ie. P0135 would be A/F Sensor Bank1 Sensor1 - the side closest to the firewall). SeaFoam will more than likely make matters worse, as the is a top-end cleaner. Yes! A leak in the exhaust system CAN set a P0420. Might be a good idea to have the leak fixed first, clear the codes and see what happens from there.
Thanks Again Ron for the fast response, if I find out is it not an exhaust leak, which of the converters do I replace: manifold (Bank 1 or 2), or downstream converter?
The P0420 is for "Bank1" this will be the one mounted to the exhaust manifold, on the side of the engine closest to the firewall.
18+yrs experience. State Inspector and Toyota Diagnostics