RepairPal is your advocate for car care advice and guidance

Q: low gas mileage on 2002 Toyota Sienna

July 2010 mpg went from 18 mpg to 11 mpg. Toyota dealer said nothing wrong with engine, but computer that generates check engine light was bad and needed to be replaced for $1,000. Dealer said no harm done to just ignore it, so we did. Dealer said low mileage because car is "old." (That I can't believe!) Low mileage continued but check engine light went off. Dec 2010 still 11 mpg but check engine light came on. Again dealer says nothing is wrong except transmission fluid needs to be changed. What to do.
Get a Repair Estimate
Guaranteed by certified locations nationwide. Learn more
RepairPal estimates are guaranteed at over 1,700 quality certified locations nationwide. Learn more
well, first of all, you need a new shop. if the light is on, never ignore it. it is a call for help when a malfunction occurs.
you need a code number to help us isolate the system to give you direction.
fuel mileage has many factors involved but we need to address the light first.

I don't know what the code number is, but the dealer says it indicates that a new computer is needed. The code after that one indicates that a new catalytic converter is needed. Make any sense to you?
You really did not answer the ? of what trouble codes did the dealer retrieve. I'm assuming you paid for diagnostics, therefore you should demand what the codes are.
the cat code is an efficiency code. it needs to be replaced. it is hard to say about the fuel mileage without looking at the o2 sensor readings as they would directly effect fuel usage.

a shop that is versed in this type of repair is in order and I hope you get to the bottom of it. sorry i could be more help but i am in pa too far away to asess it for you.
good luck

invoice says "Hook up scan tool fuel trim is fine--tests both air fuel sensors and oxygen sensor OK. There is a PO420 code for catalitic convertors. There is a bulletin for ECM replacement first. Then retest. May need convertors also??"
Roy, I was asking the owner what the codes were not you.
Vehicle Application: 1995 Toyota Tacoma Regular Cab 2.7L, Vin U, Eng Cfg L4, Eng Des 3RZFE 2000 Toyota Solara 3.0L, Eng Cfg V6, Eng Des 1MZFE
1999 Toyota 4Runner 2.7L, Eng Cfg L4, Eng Des 3RZFE 2001 Toyota Camry 2.2L, CNG, Eng Cfg L4, Eng Des 5SNFE
1999 Toyota 4Runner 3.4L, Eng Cfg V6, Eng Des 5VZFE 2001 Toyota Camry 2.2L, GAS, Eng Cfg L4, Eng Des 5SFE
1999 Toyota Camry 2.2L, Eng Cfg L4, Eng Des 5SFE 2001 Toyota Solara 2.2L, Eng Cfg L4, Eng Des 5SFE
2000 Toyota Avalon 3.0L 2002 Toyota Camry 3.0L, Eng Cfg V6, Eng Des 1MZFE, USA/Canada
2000 Toyota Camry 2.2L, GAS, Eng Cfg L4, Eng Des 5SFE 2002 Toyota Corolla 1.8L, Vin R, USA/Canada
2000 Toyota Sienna 3.0L 2002 Toyota Sienna 3.0L
2000 Toyota Solara 2.2L, Eng Cfg L4, Eng Des 5SFE

Customer Concern: Trouble code P0420.
Tests/Procedures: 1. Monitor the Air Fuel Ratio (AFR) Sensor and rear oxygen sensor while driving the vehicle at approximately 55 MPH. Check to see if the sensors become lazy or stop cross-counting. Any time the vehicle has a code P0420, the oxygen sensors or air fuel ratio sensors should be replaced before the catalytic converter is replaced. A lazy sensor can cause a P0420 code.

2. Monitor the fuel trims for being normal at both idle and at 55 MPH. Fuel trims are always targeting 0 percent (plus or minus 10 percent is normal) anytime the throttle is held steady.

3. Check for any exhaust leaks. With the vehicle raised in the air and the engine running, have a helper block off the tail pipe. Listen for any small exhaust leaks.

4. Always replace any catalytic converter that is in front of the rear oxygen sensor. Replacing just parts of the system will not be sufficient to cure a code P0420. Replace the catalytic converter with the highest quality parts available.
Potential Causes: Malfunctioning Air Fuel Ratio (AFR) Sensor
Catalytic Converter
Malfunctioning Oxygen (02) Sensor

Diagnostic Codes: P0420

The TSB number is EGO47-05, no where does it say that the cats have to be changed. It is a matter of programming with the new PCM.
The owner of the car really needs to take it to a independant Toyota freindly shop.
yes. the cat codes do not come from the ecm problem. it looks at the front o2 sensors and rears and compares them to determine effiency of the cats. they are bad.
was reading the tsb. if you have less than 90k you are covered under warranty.
what they did was widen the parameters for setting the engine light for the 420 code. if the code still existed after the ecm replacement, then the cats are bad.

my advice is to have a other shop check for a trouble code wich will possibly show a faulty o2 sensor / air flow sensor problem . these manage air and fuel ratios for mileage and performance. also try to find a trusting repair shop to deal with other than the dealer and use them for future needs !
Didn't find what you were looking for?