Buy Toyota Sequoia Air / Fuel Ratio Sensors

The air-fuel ratio sensor-also called an AFR sensor, wide-band oxygen sensor, lambda sensor, or simply O₂ sensor-is the exhaust probe that helps your Toyota Sequoia's computer mix gasoline and air just right. It reacts faster and over a wider range than older narrow-band sensors, so the engine can burn cleaner, save fuel, and protect the catalytic converter. (DENSO) In the Sequoia, one AFR sensor sits in each exhaust manifold (upstream), while standard O₂ sensors live farther downstream. (JustAnswer) Keeping these probes healthy avoids check-engine lights, rough idle, and failed emissions tests.

The AFR sensor constantly "tastes" the exhaust and sends a precise voltage signal (about 3.3 V at the perfect 14.7 : 1 mix) so the engine control module can fine-tune injection and spark. (DENSO) That fine control trims unburned fuel, lowers tail-pipe pollution, and can boost miles-per-gallon.

Inside the probe is a heated zirconia element that pumps oxygen ions; the current needed to hold its output steady tells the computer exactly how much oxygen sits in the exhaust. Unlike an older "switching" sensor that only flips rich or lean around 0.45 V, the wide-band AFR sensor delivers a smooth, linear reading covering rich to lean. (Wikipedia)

• Bank 1 Sensor 1 - upstream, on the manifold for cylinder 1 side (driver side on most Sequoias).
• Bank 2 Sensor 1 - upstream, passenger-side manifold.
  Both units thread directly into the cast manifold just inches from the exhaust ports. (JustAnswer)

• Check-engine lamp with codes P0138, P0158, P2195 or P2197. (Vehicle Service Pros, YourMechanic)
• Rough idle or stalling at stop lights. (Innova)
• Poor gas mileage and black exhaust smoke.
• Strong fuel smell on cold starts.

A failing sensor may send a false lean signal, causing the ECM to dump extra fuel and foul the catalytic converter. (Vehicle Service Pros)

• P0138 / P0158 - High voltage from downstream O₂; often traced to a slow upstream AFR sensor.
• P2195 / P2197 - Upstream AFR signal stuck lean. (YourMechanic)
  The factory bulletin advises checking harness pins A1A+ and A1A- for shorts, then replacing only the faulty bank's AFR sensor. (Vehicle Service Pros)

1. Disconnect battery to protect the ECU.
2. Unplug the AFR connector and spray penetrant on the threads.
3. Use an O₂-sensor socket and breaker bar to remove. A short burst of heat or penetrating oil helps on rusty manifolds. (Tacoma World)
4. Thread in the new sensor by hand, then torque to spec (see next section).
5. Clear stored codes and road-test.

A how-to video shows the process is nearly identical on the Sequoia's cousin, the Tundra. (YouTube)

Toyota lists 25 N·m (18 ft-lb) for the AFR sensor on late-model trucks and SUVs. (NHTSA) Forum techs advise "snug plus a bit," making sure the crush washer seats but threads aren't over-stressed. (Tacoma World) Always add a dab of high-temp anti-seize to the stainless-steel threads (avoid the tip).

The Sequoia has used AFR sensors since its 2001 launch. First-generation (2001-2007) V8s share part numbers with same-year Tundra trucks, while second-generation (2008-2022) 5.7 L 3UR-FE engines switched to updated six-wire Denso designs. Models built from 2023 onward at Toyota's San Antonio plant continue with wide-band sensors to meet stricter emissions. (Wikipedia, Wikipedia)

• Run quality fuel; lead or silicone additives can poison the element and force the mixture rich. (EPA Nepis)
• Fix exhaust leaks ahead of the sensor-a leak lets outside air fool the reading. (Vehicle Service Pros)
• Replace at 100 k mi intervals or when response time slows (cross-counts drop). (Wikipedia)
• Keep the engine tuned; misfires send raw fuel that overheats the sensor heater.

A lazy AFR sensor can trim fuel use by up to 10 %, raising CO and NOx levels above federal limits. EPA studies show poisoned sensors push mixtures rich, spiking emissions and wasting fuel. (EPA Nepis) Timely replacement helps the Sequoia stay within Clean Air Act standards and pass state inspections.

• Downstream Heated O₂ Sensors verify catalytic-converter efficiency.
• Mass Air Flow Sensor feeds intake-air data that pairs with AFR feedback.
• Catalytic Converter relies on accurate AFR for proper light-off and long life.

Q: Is an air-fuel ratio sensor the same as an oxygen sensor?
A: Both live in the exhaust, but the AFR sensor is "wide-band." It measures exact fuel mix, while a classic O₂ sensor only flips rich/lean. (DENSO)

Q: Can I drive with a bad AFR sensor?
A: The truck will run, but you risk poor mileage, engine damage, and failed emissions. Fix it soon. (AutoZone.com)

Q: Which bank is which on a Sequoia?
A: Bank 1 is the driver-side (cylinder 1), bank 2 is passenger-side. Sensor 1 is always upstream. (JustAnswer)

Q: What scan-tool data shows a dying sensor?
A: Look for voltage stuck high (>3.7 V) or low (<2.9 V) on the AFR PID, slow response, or codes P0138/P2195. (Vehicle Service Pros, YourMechanic)

Q: Do I need to change both sensors at once?
A: Only replace the failing bank's unit unless mileage is high; matching response on both sides helps long-term balance. (Vehicle Service Pros)

Q: What size wrench fits the sensor?
A: A 22 mm (7/8-inch) O₂-sensor socket with a side cut-out for the wire lead. (YouTube)

Q: What torque should I use?
A: Tighten to 25 N·m (18 ft-lb) or just past snug with the crush washer seated. (NHTSA, Tacoma World)

Q: Will a faulty AFR sensor damage my catalytic converter?
A: Yes-running rich overheats the catalyst; running lean can raise NOx. Fixing the sensor protects the cat and your wallet. (DENSO)

For deeper reading, see the Toyota Sequoia specs, EPA's guidance on On-Board Diagnostics, and the technical breakdown of AFR technology on Wikipedia.