I don't live in a state that has emission laws, but that code is emission related. It should be a simple fix, since it is an older car I would go in order of highest probability that either the line is broken, the solenoid needs to be replaced, or a break/corrosion in the wiring. There could be a couple of other things but unlikely.
SOURCE: What is the cost of a 90000 mile service for a lexus RX 300?
$700
timing belt
idler pulley
water pump
timing belt tensioner
valve cam gasket
coolant flush
spark plug
oil change
pcv valve
tranny oil change
differential fluid cahnge
drive belt
upper/lower radiator hose
brake system flush
air filter
good luck,i hope this help pls dont forget to rate me thanks
SOURCE: HEATER NOT WORKING IN 2001 LEXUS RX 300
you most likely have a bad airmix servo. this happens alot on these. they are expensive to replace too.
SOURCE: my car came up with
Tests/Procedures:
1. With the sensor disconnected and the key on, check the voltage at the wire harness side of the connector. The Black wire should have 12 volts the Orange wire and the White wire should be approximately 3 volts.
2. Reconnect the sensor connector and check the voltage on the Black wire on start up. It should remain battery voltage. The Orange wire and the White wire should vary slightly in voltage from 3-3.3 volts but the sensors will not change much.
3. Install a new sensor.
Potential Causes:
Defective Air Fuel (A/F) Ratio Sensor
Tech Tips:
This is a common issue with the air fuel ratio sensors. The firewall side will fail and the technician will install a new sensor and then a few days later a code for the other sensor will appear. Always replace these sensors as a pair.
SOURCE: The engine light and VSC
your o2 sensor is bad. vsc will go on for any small DTC displayed on the MIL (cel)
P0037 LEXUS
P0037 LEXUS - Oxygen Sensor Heater Control Circuit Low Bank 1 Sensor 2
Symptoms
- Engine Light ON (or Service Engine Soon Warning Light)
Possible causes
- Heated Oxygen Sensor (H2OS) Bank 1 Sensor 2 circuit open shorted to ground
- Heated Oxygen Sensor (H2OS) Bank 1 Sensor 2 circuit poor electrical connection
- Faulty Heated Oxygen Sensor (H2OS) Bank 1 Sensor 2 circuit
- Heated Oxygen Sensor (H2OS) Bank 1 Sensor 2 circuit fuse
- Faulty Engine Control Module (ECM)
When is the code detected?
The P0037 is set when the ECM detects an excessively low voltage through the heated oxygen sensor heater circuit
P0037 LEXUS Description
Oxygen Sensors (O2S) or Heated Oxygen Sensors (HO2S) need to reach a minimum operating temperature of 750 degrees F to produce an accurate voltage signal. The faster the heated oxygen sensor reaches that temperature the faster the sensor will start sending an accurate signal to the Engine Control Module (ECM).
In order to achieve the require temperature, a heater element is included inside the heated oxygen sensor. The ECM controls the heated oxygen sensor heater element based on signals from the engine coolant temperature and engine load. The ECMcontrols the heater element circuit by allowing current flow to ground.
The ECM monitors the voltage signal received through the heater element circuit and determines the state of the circuit by comparing the voltage detected with the factory specifications. 
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