My Highlander has about 170,000 miles, and recently the P0171, P0174, P0037 codes have come up. I replaced the MAF sensor based on some advice and reset the codes, but the P0037 code returned and I imagine the others will as well.
I was wondering if these 3 codes together indicate some common problem?
P0171: System too lean bank 1
P0174: System too lean bank 2
P0037: H02S Heater Control Circuit Low Bank 1 Sensor 2
P0037 the heater circuit for that particular oxygen sensor . . . bank 2 downstream sensor, the one after the cat . . . is probably dead. Translation . . . you need to replace that sensor
As for those lean codes . . . are you sure the maf sensor didn’t take care of the problem?
Do you have a scanner which will show you live data?
If so, what are the short and long term fuel trims for both banks . . . both at idle, under a load, crusing, etc.?
The oxygen sensor code most likely has nothing to do with the other codes
There should be a spec in the service data for the resistance of the o2 sensor’s heater. That’s the first test I’d make for the p37 code . As posted above, don’t presume the two lean codes will return. Your MAF replacement may well have addressed that problem. Lean on both banks is usually caused by one of these
MAF needs cleaning or replacement
Air leak between air cleaner and throttle body, vacuum, intake manifold, or exhaust system