O2 sensors: Upstream Air/Fuel Denso 234-9049 & Downstream Denso 234-4149

Denso is the OE Manufacturer for Toyota

Denso makes the OEM Toyota O2 sensors for the 2005-2010 Scion tC. You can save a little bit of money buying just the Denso sensors, but make sure you buy from a reputable source!

Symptoms of Bad O2 (Oxygen) Sensors

Check Engine Light Comes On
One of the most common signs of a failing O2 sensor is an illuminated check engine light. Faulty sensors often trigger codes related to air-fuel ratio or sensor response.
Poor Fuel Economy
O2 sensors help the ECU adjust the air-fuel mixture. When a sensor sends incorrect data, the engine may run too rich or too lean, causing unnecessary fuel consumption and a noticeable drop in MPG.
Rough Idle or Engine Hesitation
Incorrect air-fuel mixture feedback can lead to unstable idling, stumbling, or hesitation during light acceleration or cruising.
Engine Misfires or Surging
A failing O2 sensor can cause inconsistent combustion, resulting in misfires or a surging sensation while driving—especially at steady speeds.
Failed Emissions Test
Because O2 sensors directly affect emissions control, a bad sensor can cause elevated exhaust emissions, leading to a failed smog or emissions inspection.
Strong Fuel Smell or Black Exhaust Smoke
If the engine runs rich due to faulty sensor input, you may notice a raw fuel smell from the exhaust or dark smoke—both signs of incomplete combustion.
Catalytic Converter Damage
Prolonged driving with bad O2 sensors can send excess unburned fuel into the exhaust, overheating and damaging the catalytic converter—an expensive repair if ignored.

Places to buy Genuine Denso products:

Your local O’reilly or Autozone parts counter
RockAuto
- Upstream Denso 234-9049
- Downstream Denso 234-4149
DensoProducts.com
SELECT Amazon retailers
- Upstream Denso 234-9049
- Downstream Denso 234-4149

When shopping for replacement parts, it’s important to be cautious of fake or counterfeit Denso products, especially on large marketplaces like eBay or Amazon. Counterfeit Denso spark plugs, sensors, and ignition components are increasingly common and often look convincing at first glance, but they can suffer from poor build quality, incorrect specifications, and drastically shorter lifespans. Using these knockoff parts can lead to misfires, check engine lights, poor performance, or even engine damage—costing far more in the long run than the money saved upfront. To protect yourself, buy only from authorized sellers, verify packaging and part numbers carefully, and be wary of prices that seem too good to be true, as genuine Denso parts rarely sell at deep discounts.

Upstream Denso 234-9049 O2 sensor – also known as the air/fuel sensor

Here’s a link to the Denso 234-9049 Sensor from a known good distributor on Amazon. You can also purchase it from RockAuto.com or DensoProducts.com. I would avoid buying this from eBay. This is the sensor before the catalytic converter, located on the top of the exhaust manifold

Upstream air fuel o2 sensor original OEM vs Denso — Upstream air fuel o2 sensor original OEM (top) vs Denso (bottom)

Downstream Denso 234-4149 O2 sensor – the one after the Catalytic Converter

Here’s a link to the Denso 234-4149 sensor from a known good distributor on Amazon. You can also purchase it from RockAuto.com or DensoProducts.com. I would avoid buying this from eBay. This is the sensor after the catalytic converter, located on the S-pipe.

Denso 234-4149 replacement downstream O2 sensor OEM (top) Denso (bottom)

You can try a defouler

There are products that can bypass this entirely by spacing the oxygen sensor slightly out of the exhaust stream so it reads lower oxygen fluctuations, often preventing false check-engine lights from modified or high-flow exhaust systems. Note, these may not work at all, or you may not pass emissions with these installed.

Make sure to find a defouler that matches the O2 bung thread M18x1.5.

BLOX Racing 02 Diagnostic Plug Stainless Steel

J Style defouler w/ Restrictor Fittings for adjustable flow

Fastline Performance V3 J-Type Spark Plug / Sensor Defouler, 90-degree

Fastline Performance V2 Spark Plug / Sensor Defouler, Straight

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DIY maintenance, part numbers