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Discussion Starter · #1 · (Edited)
2003 v6 Highlander. I've been eyeing this forum for a while now. I thought it might be wise for more opinions here!

After going through a pricey tune-up, I didn't want to spend an additional $700 to get by CEL issue resolved. ($700 was the estimate I was quoted at a local mechanic, assuming I'm using OEM sold by Toyota) The shop's front desk said it might not be worth a fix, especially if the car takes on just 4000-5000 miles a year. They did not do a full diagnostics test on whether the oxygen sensors are really faulty or not, since I came in just for a safety check.

But anyway. With the gas price expected to rise, I was wondering if this might be worth a try. If the highlander's MPG (right now I have 13.8 for 60% city driving) could go up by 4 after chancing the sensors out, I will be quite happy. At the moment, I have P0171/P0174 (lean bank 1, 2) and P0135/P0136 (O2 heater bank 1, sensor 1 and O2 sensor circuit bank 1, sensor 2) codes on.

So here is my question:
If you were in my shoes, would you spend $350-700 to get a/f sensor and oxygen sensor changed?
How much improvement in MPG am I looking at?
Would you recommend changing the bank 2 a/f sensor out as well?
 

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How many miles? What condition is it in? An AWD is probably worth fixing.
Do you have any P0300 to P0306 codes pending? What are the misfire counts?
You will probably not pick up 4MPG. I would be surprised at more than 2 MPG.
I would:
1) Clean the MAF or and TB and replace air filter. If the MAF has more than 150K on it, consider replacement.
2) Use a scanner to read freeze frame data
3) With engine at normal temperature, graph fuel trims at idle, 1500 rpm, and 2500 rpm, holding each rpm until the long term fuel trims are steady - up to 60 seconds.
4) Monitor misfire counters
This should be enough data to get an idea if it is a vacuum leak.
5) Set up the scanner to read AF and HO2S voltages. Do 3 to 5 throttle snaps to see sensor responses.
6) Ohm out the B1S1 heater. See if it meets spec.

It has been shown on the forum that rear HO2 sensor can influence lambda on the 3.0 liter. I would probably start there if there were no other indications.
If it is idling poorly you could try cleaning the IACV
 

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Discussion Starter · #3 ·
How many miles? What condition is it in? An AWD is probably worth fixing.
Do you have any P0300 to P0306 codes pending? What are the misfire counts?
You will probably not pick up 4MPG. I would be surprised at more than 2 MPG.
I would:
1) Clean the MAF or and TB and replace air filter. If the MAF has more than 150K on it, consider replacement.
2) Use a scanner to read freeze frame data
3) With engine at normal temperature, graph fuel trims at idle, 1500 rpm, and 2500 rpm, holding each rpm until the long term fuel trims are steady - up to 60 seconds.
4) Monitor misfire counters
This should be enough data to get an idea if it is a vacuum leak.
5) Set up the scanner to read AF and HO2S voltages. Do 3 to 5 throttle snaps to see sensor responses.
6) Ohm out the B1S1 heater. See if it meets spec.

It has been shown on the forum that rear HO2 sensor can influence lambda on the 3.0 liter. I would probably start there if there were no other indications.
If it is idling poorly you could try cleaning the IACV
Thank you for the step-by-step advice!
It has 180,000 miles on it. I'm not knowledgeable enough to evaluate its condition, maybe "fair" (but it never had rough idling, no). Does have new battery, spark plugs, water pump, timing/serpentine belts. The air filter was changed about 10,000 miles ago, MAF was never replaced. No misfire codes stored at the moment -- the visit to the mechanic was very recent and they probably cleared it out. But I will keep my eyes out for more misfire codes.

What is HO2 sensor and lambda? I should probably read up on that.
 

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HO2S is heated oxygen sensor. On the 3.0 it is Bank1 Sensor 2, downstream of the manifold cats.
Lambda is the ratio of at air/fuel mixture to the stoichiometric value. Lambda = 1 means the mixture is at the stoichiometric value. Less than 1 is rich. Greater than 1 is lean. The upstream sensors (Bank 1 Sensor 1 and Bank 2 Sensor 1) are air/fuel ratio sensors rather than simple oxygen sensors on the Highlanders, even though the OBD code descriptions may reference them as O2 sensors.
Above, Instead of "read sensor voltages" I should have said "graph sensor voltages" to make it easier to follow the changes.

Another point in fixing it - significantly lean running can cause the pistons and valves to overheat and cause real damage. Most techs I know would suggest to fix it.
You can find common causes with a simple search. Here are a few:
AF sensors (B1S1, B2S1)
HO2 sensor (B1S2) impacting the mixture control on the 3.0 - this one is complicated.
MAF
Clogged air filter
Vacuum or Air Tube leak, including the EVAP system
Clogged injectors
Low fuel pressure (not too common on HL's)
Clogged fuel screen / filter
Less likely but possible -
PCV inadequate
cam timing.

I would use Denso sensors if I planned to keep it.
 

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Discussion Starter · #5 ·
I can report back that after putting Denso 234-9009 Air/Fuel Ratio sensor in, all the CEL are gone for good. I'm glad I did not have to get the downstream oxygen sensor replaced. The VSC light remains on, which I found surprising because I thought when CEL is cleared the VSC light will be cleared too.
I'm curious to see how much MPG I've gained when I fill up my fuel tank next time. Anyway, it's good to see the CEL gone.
 

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I can report back that after putting Denso 234-9009 Air/Fuel Ratio sensor in, all the CEL are gone for good. I'm glad I did not have to get the downstream oxygen sensor replaced. The VSC light remains on, which I found surprising because I thought when CEL is cleared the VSC light will be cleared too.
I'm curious to see how much MPG I've gained when I fill up my fuel tank next time. Anyway, it's good to see the CEL gone.
Good to hear that the CEL is fixed.
The VSC will be on if the system detects a sensor value out of spec. The CEL will come on primarily if there is a parameter out of spec which affects emissions.
First thing is to use your scanner to see if the monitors ran.
If they did, then try to access the ABS/VSC/TRAC module with your scanner, which generally may be found under enhanced networks, view networks, etc in the vehicle or settings menus.
If not, see if your scanner is compatible with the OBD Fusion app. That may give you access to the other modules.
If only the VSC light is on (not TPMS, ABS, or TRAC), it may be as simple as re-zero of the steering angle sensor.
If the light is on, there is almost always a code in the specific module. Generic scanners can not all access the proprietary modules like ABS.
 

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Discussion Starter · #7 ·
Good to hear that the CEL is fixed.
The VSC will be on if the system detects a sensor value out of spec. The CEL will come on primarily if there is a parameter out of spec which affects emissions.
First thing is to use your scanner to see if the monitors ran.
If they did, then try to access the ABS/VSC/TRAC module with your scanner, which generally may be found under enhanced networks, view networks, etc in the vehicle or settings menus.
If not, see if your scanner is compatible with the OBD Fusion app. That may give you access to the other modules.
If only the VSC light is on (not TPMS, ABS, or TRAC), it may be as simple as re-zero of the steering angle sensor.
If the light is on, there is almost always a code in the specific module. Generic scanners can not all access the proprietary modules like ABS.
Thank you C R for the response! VSC light is the only light on. I went through the previous owner's old service records again, and there is a note from Firestone mechanic's engine computer analysis stating that VSC light is probably linked with CEL and wouldn't go off even after 'zero-point calibration to reset vsc'. After looking at that, and the fact that I only have access to a generic scanner right now, I think I might leave this piece of the puzzle alone until my next oil change.
 

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If you are interested I find that the OBDLINK MX+ is a good scanner for these first generation Highlanders.
 

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Discussion Starter · #9 · (Edited)
An update on what happened with the Highlander. Although the CEL went off after an upstream oxygen sensor was replaced, few weeks later it came on again for downstream oxygen sensor. Since the parts was relatively cheap, I just went ahead to the mechanic to get that CEL off while also getting engine oil, filter, and air filter replaced. After I arrived home I got CEL light on -- P0330, which is Knock sensor 2 circuit (bank 2).
Anyway, the Highlander has two knock sensors from what I searched here. The vehicle itself doesn't feel different when I drive. Do I have to replace the bad knock sensor, since the one on bank 1 is fine? If this is something that I have to do in order to drive it safely for 2-3 more years -- if so what are the parts that you recommend?
 

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Knock sensors seem to go on your year and earlier highlanders. There is a good write up here on Toyota nation on how to wire both sensors up to just one working sensor. Look that up if you want to try and buy some time or save money. Otherwise replace the knock sensor and harness. ( I believe the wiring there can also cause issues)
 
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