Where is the air mass sensor(air fuel sensor) on 1999 toyota 4runner sr5 3.4 6cylinder. I see a pre cat sensor and post cat. Are there more

have throttle body checked also the MDK throttle motor,
make sure that the dealer has followed service bulletin 12 07 99 for procedure to replace the MDK throttle motor
also have them check for codes,here are some codes,
it could be an expensive job,have them check everything,it is most likely the throttle motor,,throttle body is 600 if they switch it,dont let them do it without checking throttle motor,
codes,
MS41, MS42 and MS43 engine DME bmw codes...
• 1 Ignition coil Cyl 2
• 2 Ignition coil Cyl 4
• 3 Ignition coil Cyl 6
• 5 Fuel injector Cyl 2
• 6 Fuel injector Cyl 1
• 8 Air Flow Meter (HFM)
• 10 Coolant Temperature Sensor
• 11 Tank Pressure Sensor (EVAP System) or Radiator Outlet Temp (MS43)
• 12 TPS or Plausibility - Maximum Coolant Temp (MS43)
• 13 Plausibility- Radiator Outlet Temp
• 14 Intake Air Temperature Sensor
• 15 Plausibility - Cut Out Time
• 16 AirCon Compressor - PWM Signal or Plausibility Intake Air Temp (MS43)
• 17 Plausibility - Engine Coolant Temp
• 18 EWS Signal or Camshaft Sensor (MS43)
• 19 Activation VANOS Inlet Valve or Exhaust Valve (MS43)
• 20 "CHECK ENGINE" Light Failure
• 21 VANOS -Electrical Fault or Activation VANOS Inlet Valve (MS43)
• 22 Fuel Injector Cyl 3
• 23 Fuel Injector Cyl 6
• 24 Fuel Injector Cyl 4
• 25 Lambda Sensor Heater -Bank 1
• 27 Idle Control Valve - Malfunction
• 29 Ignition Coil Cyl 1
• 30 Ignition Coil Cyl 3
• 31 Ignition Coil Cyl 5
• 33 Fuel Injector Cyl 5
• 35 Aux. Air Injection System Relay
• 36 DME Main Relay
• 37 DME Main Relay : Delay
• 38 Clutch Switch -Plausibility
• 39 Brake Light Switch or Brake Light Test Switch
• 40 Brake Light Switch or Pedal Value Signal
• 42 Multi Function Steering Wheel- Plausibility
• 43 Multi Function Steering Wheel: Button
• 45 Multi Function Steering Wheel: Port
• 47 Temp Sensor -Downstream of Pre-Cat or Torque Limitation Level 1 (MS43)
• 48 DME Control Unit -Self Test 1
• 49 DME Control Unit or Torque Monitoring Level 2 (MS43)
• 50 EVAP Control Valve or Response Monitoring Level 2 (MS43)
• 51 Shut-off Valve -Charcoal Filter or Request Control Unit Reset (MS43)
• 52 Solenoid Valve -Exhaust Flap
• 53 Idle Speed Actuator
• 55 Lambda Sensor Heater -Bank 2
• 56 Ignition Current Feedback Resistor - Open Circuit
• 57 Knock sensor -Bank 1
• 58 DME Control Unit -Self Test 2
• 59 Knock Sensor -Bank 2
• 61 Lambda Sensor Heater -Bank 2 Post Cat
• 62 Aux. Air Injection System -Switching Valve
• 63 DME Control Unit or Ambient Temp Signal via CAN (MS43)
• 64 Plausibility - Ambient Temperature
• 65 Camshaft Position Sensor (Inlet MS43)
• 66 DME Control Unit
• 67 DME Control Unit
• 68 Tank Venting Valve
• 69 Fuel Pump Relay
• 70 DME Control Unit
• 71 DME Control Unit
• 72 DME Control Unit
• 74 AirCon Compressor Relay
• 75 Lambda Sensor Voltage -Bank 1
• 76 Lambda Sensor Voltage -Bank 2
• 77 Lambda Sensor Voltage -Bank 1 Post Cat
• 78 Lambda Sensor Voltage -Bank 2 Post Cat
• 79 Lambda Sensor Heater -Bank 1 Post Cat
• 80 ABS/ASC interface
• 81 MSR Signal -Active too Long
• 82 ABS/ASC Interface -Advance Adjustment
• 83 Crankshaft Sensor
• 90 Exhaust Temperature Pre Cat Conv - Bank 1
• 91 Exhaust Temperature Pre Cat Conv - Bank 2
• 92 Exhaust Temperature Post Cat Conv -Bank 1
• 93 Exhaust Temperature Post Cat Conv -Bank 2
• 94 Auxiliary Air -Air Mass Flow Sensor
• 95 Auxiliary Air Valve or Auxiliary Air Hose Blocked
• 96 Auxiliary Air Pump - Function
• 97 Auxiliary Air -Flow Rate too Low
• 98 Auxiliary Air -Flow Rate too High
• 99 Auxiliary Air Valve Jammed Open
• 100 DME Control Unit -Self-Test Failed
• 103 VANOS Error -Inlet Camshaft
• 104 VANOS Error -Exhaust Camshaft
• 105 VANOS Error -Position Inlet Camshaft
• 106 VANOS Error -Position Exhaust Camshaft
• 109 Throttle Valve Plausibility
• 110 Pedal Sensor Value Potentiometer 1
• 111 Pedal Sensor Value Potentiometer 2
• 112 TPS Potentiometer 1
• 113 TPS Potentiometer 2
• 114 Throttle Valve Final Stage
• 115 Reference Voltage -Voltage Regulator 1 or Throttle Pedal Adaptation (MS43)
• 116 Reference Voltage -Voltage Regulator 2
• 117 Plausibility -Pedal Position Sensor 1/2
• 118 Plausibility -TPS 1/2 or TPS1/Airflow Plausibility (MS43)
• 119 Throttle Valve Sensor -Mechanical Error or TPS2/Airflow Plausibility (MS43)
• 120 Plausibility Pedal Sensor or TPS
• 122 Engine Oil Temperature
• 123 Map Cooling Thermostat Control
• 124 Activation DISA Solenoid
• 125 Activation Electric Fan
• 126 Activation Tank Leak Pump Solenoid
• 127 Activation Pump Solenoid
• 128 DME/EWS Communication
• 129 CAN Signal SMG 1
• 130 CAN Signal ASC -Timeout
• 131 CAN Signal Instrument Cluster -Timeout
• 132 CAN Signal Instrument Cluster -Timeout
• 133 CAN Signal ASC -Timeout
• 134 SMG Intervention -Plausibility
• 135 Throttle Valve Re-Adaptation Required
• 136 Throttle Valve -Spring Test and Limp-home Position Failed
• 137 CAN Signal -Steering Angle Sensor
• 139 CAN Signal -Tank Level Sensor
• 140 Tank Leak Pump Solenoid - Reed Switch Open or Output Stage (MS43)
• 141 Tank Leak Pump Solenoid - Reed Switch Stuck Closed or Tank Level Sensor (MS43)
• 142 Tank Leak Pump Solenoid - Reed Switch Stuck Open or DMTL Module (MS43)
• 143 Tank Ventilation or Tank Leakage (MS43)
• 144 Fuel System - Large Leak Recognised
• 145 Fuel System - Small Leak Recognised
• 146 EVAP System Leak Detected (Small Leak) or Pedal Sensor Supply Voltage Pot 1 (MS43)
• 147 Pedal Position Sensor Potentiometer Supply Channel 1 (Pot 2 MS43)
• 149 Air Flow Sensor or Pedal Value Sensor Mismatch
• 150 Lambda Post Cat Bank 1 Max Limit
• 151 Lambda Post Cat Bank 2 Max Limit
• 152 Lambda Post Cat Bank 1 Min Limit
• 153 Lambda Pre Cat Bank 2 Max Limit
• 154 Lambda Pre Cat Bank 2 Min Limit
• 155 Lambda Pre Cat Bank 2 No Signal
• 156 Lambda Pre Cat Bank 1 No Signal
• 157 Lambda Post Cat Bank 1 Min Limit
• 159 Lambda Post Cat Bank 2 Max Limit
• 160 Lambda Post Cat Bank 2 (MS41) or Throttle Valve Stuck
• 161 Throttle Valve - Stuck
• 162 Throttle Valve -Control Deviation
• 168 Pedal Position Sensor Pot Supply 1 or MAP Cooling Thermostat Jammed (MS43)
• 169 Throttle Valve Output Stage Cut off after Fault
• 170 DME Control Unit -Self Test Failed
• 171 Plausibility - Throttle Valve
• 172 Pedal Sensor Potentiometer 1/2 Short Circuit
• 173 TPS Potentiometer 1/2 Short Circuit
• 174 Throttle Valve Potentiometer 1/2 Adaptation
• 175 Pedal Sensor 1 Adaptation
• 176 Pedal Sensor 2 Adaptation
• 186 Voltage Post Cat Bank 1
• 187 Voltage Post Cat Bank 2
• 188 Voltage Pre Cat Bank 1
• 189 Voltage Pre Cat Bank 2
• 190 EVAP -Reed Switch Open or Voltage Post Cat Bank 1 (MS43)
• 191 EVAP -Reed Switch Closed or Voltage Post Cat Bank 2 (MS43)
• 192 EVAP -Reed Switch Open
• 193 EVAP -Check Hoses
• 194 EVAP -Large Leak Detected
• 195 EVAP -Small Leak Detected
• 196 EVAP -Electrical Valve from LDP Pump or Barometric Pressure Sensor (MS43)
• 197 EVAP -Barometric Pressure Sensor
• 198 Cat Efficiency during Start -Bank 1
• 199 Cat Efficiency during Start -Bank 2
• 200 Lambda Regulation Bank 1 Pre Cat
• 201 Lambda Regulation Bank 2 Pre Cat
• 202 Lambda Regulation Bank 1 Post Cat
• 203 Lambda Regulation Bank 2 Post cat
• 204 Idle Control System -Idle speed not plausible
• 208 EWS -RPM Signal Error
• 209 EWS -Message Error
• 210 Ignition Feedback Resistor (ZSR)
• 211 Idle Speed Actuator -Mechanical
• 212 VANOS Bank 1 -Mechanical
• 214 Vehicle Speed Signal (VSS)
• 215 Lambda Sensor Bank 1 or ASC/MSR/EML -Interface not plausible
• 216 Lambda Sensor Bank 2 or EGS Position Signal
• 217 CAN bus error -EGS Signal not present
• 218 CAN module -Warning
• 219 CAN module -CAN Offline
• 220 Lambda Voltage Range Bank 1 Sensor 1
• 221 Lambda Voltage Range Bank 2 Sensor 1
• 222 Low Coolant Temperature or Lambda Sensor Control (MS43)
• 223 Lambda Sensor Switching Bank 1 Sensor 2
• 224 Lambda Sensor Switching Bank 2 Sensor 2
• 225 Cat Efficiency Bank 1
• 226 Cat Efficiency Bank 2
• 227 Mixture Deviation Bank 1
• 228 Mixture Deviation Bank 2
• 229 Lambda Sensor Switching Bank 1
• 230 Lambda Sensor Switching Bank 2
• 231 Lambda Sensor Switching Bank 1 Pre Cat
• 232 Lambda Sensor Switching Bank 2 Pre Cat
• 233 Catalytic Converter Overall Efficiency Bank 1
• 234 Catalytic Converter Overall Efficiency Bank 2
• 235 Lambda Heater Bank 1 Post Cat or Pre Cat Signal (MS43)
• 236 Lambda Heater Bank 2 Post Cat or Pre Cat Signal (MS43)
• 238 Misfire Cyl 1
• 239 Misfire Cyl 2
• 240 Misfire Cyl 3
• 241 Misfire Cyl 4
• 242 Misfire Cyl 5
• 243 Misfire Cyl 6
• 244 Crankshaft Interval Timing
• 245 Aux Air Injection System Bank 1
• 246 Aux Air Injection System Bank 2
• 247 Aux Air Injection System -Incorrect Flow Detected
• 248 Pre Cat Converter Efficiency -Bank 1
• 249 Pre Cat Converter Efficiency -Bank 2
• 250 Tank Venting Valve -Function
• 251 Tank Ventilation Diagnosis Error
• 252 Tank Ventilation System Vacuum
• 253 Activated Charcoal Filter Shut-off Valve Stuck Shut
• 254 Tank Ventilation System -Large Air Leak
• 255 Tank Ventilation System -Valve Stuck Open

Are you sure it's 02 sensors? Just because code says 02 sensors doesn't necessarily mean they're bad. Bad 02 sensor code can also indicate bad cat. converter. If 02 sensors read exact same reading before and after converter they will trigger OBC to "sensor fault" - they should be reading 2 different readings. (Pre CAT/POST cat) I'd suggest replacing cat converter before replacing sensors.

have you had the air mass sensor checked?

Change your oxygen sensors - they're responsible for feedback that the computer uses to determine the air/fuel mixture. They usually are good for about 60k miles and then start to go downhill. When they get to a point that the computer won't trust them, it ignores them and runs on preprogrammed fuel maps that run excessively rich (too much fuel) to safeguard the engine. You've probably seen a decrease in fuel mileage as well. And the O2 sensors have to be up around 900 degrees before they'll work - the car ignores them until then. That's why you run fine after a cold start but run rough once the car is warmed up.

The O2 sensors you want to change are in the exhaust system between the engine and catalytic converters (the "pre-cat" sensors). There are O2 sensors after the cats as well, but those are used comparatively to determine how well the cats are functioning. The primaries are the ones used for fuel mixture. If it's been this way for a few weeks or so, I'd put new spark plugs in as well - rich fuel conditions can foul them and contribute to the rough running.

Code P0420 is a code for Catalyst efficiency. The computer is seeing the post Catalytic converter o2 sensor having too much activity. The Cat. should be cleaning the exaust of Hc, co, and Nox. The post cat o2 will read the o2 content in the exhaust and if it moves up and down too much or to high and low, the computer interperates that as the cat not ridding the exhaust of it's pollutants. This indicates that the converter is unable to preform properly. The problem could be a bad cat, o2 sensor (Pre- and/or post cat)or o2 sensor wires, or an air fuel mixture problem that has caused the cat to fail. Many other engine operating system failures can cause the converter to not work properly.

Possible bad fuel injector

Clean mass air flow sensor good, make sure air cleaner is in good shape, then retry. If your using an oil soaked air filter system like K&N, it can cause that also.

mass air flow sensor located ontop your airfilter or a clogged injector the mMAF sensor can be adjusted if ya know what your doin and take apart gently counter clockwise for more fuel /clockwise for more air

Replace the coil pack.

Replace the coil pack..