Check engine Code 21

[deemer]

Hi I am new to the forum

I have 1991 Lexus ES250 V6 160,000+ miles
Check engine light came on after some exhaust down pipe works most likely coincidence
Wisconsin car, O2 sensor (1 sensor only I believe) and TWC original still.
Tried to clear the check engine light by disconnecting the negative terminal of the battery and by pulling the 15A EFI fuse but the light will came back on shortly when engine rpm goes to 2000 rpm driving forward.
By the jumper method: the code is 21
=Oxygen sensor and heater circuit
I took the car into smog testing station and analysis is OK on CO, OK on HC but failed NOx at twice the level.
Smog testing station also print out A/F mixture lean.

Maybe someone with more experience can help me :
1. Do I have a flinge type sensor or screw type
2. The O2 sensor is on the exhaust manifold on the passenger side, blocked by many things. What is the best way to get to the part? Jack up the car/ drive it on ramps/ or jack up the car and remove the passenger front tire to get to the part.
Kind of cold on the ground now!
Thank you

[Doctor J]

Heated Oxygen Sensor
See Figure 1

1. Start the engine and bring it to normal operating temperature, then run the engine above 1200 rpm for two minutes.
2. Turn the ignition OFF disengage the H02S harness connector.
3. Connect an ohmmeter between terminals +B and HT of the sensor. Resistance should be 11-16 ohms at 68 degrees F (20 degrees C). If not, replace the sensor.

Fig. 1: Check the resistance of the heater portion of the sensor

1. Start the engine and bring it to normal operating temperature, then run the engine above 1200 rpm for two minutes.
2. Backprobe with a high impedance averaging voltmeter (set to the DC voltage scale) between the oxygen sensor (02S) signal wire and battery ground.
3. Verify that the 02S voltage fluctuates rapidly between 0.40-0.60 volts.
4. If the 02S voltage is stabilized at the middle of the specified range (approximately 0.45-0.55 volts) or if the 02S voltage fluctuates very slowly between the specified range (02S signal crosses 0.5 volts less than 5 times in ten seconds), the 02S may be faulty.
5. If the 02S voltage stabilizes at either end of the specified range, the ECM is probably not able to compensate for a mechanical problem such as a vacuum leak or a faulty fuel pressure regulator. These types of mechanical problems will cause the 02S to sense a constant lean or constant rich mixture. The mechanical problem will first have to be repaired and then the 02S test repeated.
6. Pull a vacuum hose located after the throttle plate. Voltage should drop to approximately 0.12 volts (while still fluctuating rapidly). This tests the ability of the 02S to detect a lean mixture condition. Reattach the vacuum hose.
7. Richen the mixture using a propane enrichment tool. Voltage should rise to approximately 0.90 volts (while still fluctuating rapidly). This tests the ability of the 02S to detect a rich mixture condition.
8. If the 02S voltage is above or below the specified range, the 02S and/or the O2S wiring may be faulty. Check the wiring for any breaks, repair as necessary and repeat the test.

If CO reading is more then .05% and HC is less then 50 ppm, the mixture is normal.
For NOx , check the EGR system

[deemer]

Thank you Doctor J

I will wait until the weekend to get to the part to try out.