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Toyota Highlander Hybrid 2006: Codes P0171, P0174, P0037

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

Any obvious vacuum leaks?

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
  • Fuel pressure problem

Thanks guys. I will look into this.