As the title suggests, I was getting the too lean codes after successfully replacing my fuel rail pressure sensor. I couldn't find any vacuum leaks and the MAF appears to be reading properly. My OBD2 scanner showed the bank2 downstream O2 sensor was stuck at about 0.1 volts whereas the bank1 sensor was reading fine. However, I have no fault codes for the sensor itself, only the constant low voltage reading. Could it be the cat that has shat the bed? Hope it's just the sensor.

 

Check the condition of the accordion section of your plastic air duct between the airbox and the throttle body. Look for cracks in the plastic nooks and crannies, particularly on its underside....

Check for oil seepage around your two VVT solenoid O-ring seals. They get hard, brittle, and allow for both oil and air seepage....

Both of these issues can throw P0171 / P0174 OBDII codes....

 

How do you know? There are so many opportunities for air to leak. There are of course top offenders like Jon mentioned, but remember that a leak is a leak, and they all accumulate. Best by far is to do a smoke test. You can buy a smoke tester, or even make one with mineral oil in a jar, an old soldering iron and a bicycle pump.

FWIW, since you mention the fuel pressure sensor, our 4.2l cars have a pretty common/cheap MAF. Might be worth replacing.

You would get a more complete picture by reading the fuel trims. Easiest is to check at idle (highest vacuum and low air flow, letting leaks be most prominent). Codes are thrown at 25%.

You can always replace the downstream sensors, but they are midway down the converter, and not the easiest to remove. AFAIK, the trick of using the A/C drain holes for access does not apply to our later cars unfortunately.

Best of luck, keep us posted.

 

it will cause fine tuning problems and the cat monitor will never pass but the air fuel sensors are doing most of the work.

air fuel sensors maf and vacuum leaks are all big players.

 

Thanks for your suggestions, guys. I have inspected the air intake duct (it was the cause of lean fault codes 2 years ago) and seems good so far, but as was suggested, a smoke test is in order. Although the MAF is reading normally, it is probably worth replacing. I'll go ahead with the downstream 02 sensor replacement but get my garage workshop to do it.

 

Well, the good news is I found the culprit for my DTC’s. It was a MAF sensor that was still functioning but giving erroneous information to the ECU. Replaced it with a used Denso part for a Toyota (about $20) and all is fine.

 

what toyota did you use to match the sc maf

there’s a couple different maf curves out there. the one for the 4.0 fj cruiser 4runner and such works great on the NA engines.

 

Can't remember which Toyota is was for off the top of my head but I will look it up later. Probably it is not exactly suited for the SC variant but it's a stop gap measure until I get my car through the car inspection process here. At least the check engine light is off and fuel economy is great, but I will look around for a better fit later.