P0138 on a 2017 Hyundai Santa Fe
Post-Cat O2 Sensor High Voltage (Bank 1 Downstream)
P0138 on a 2017 Hyundai Santa Fe indicates post-cat o2 sensor high voltage (bank 1 downstream). It usually stays drivable short-term but should be diagnosed promptly. The most common cause is failed bank 1 downstream o2 sensor (stuck high) (typically $150–$450). Confirm the root cause before replacing parts.
What does P0138 mean on a 2017 Hyundai Santa Fe?
P0138 is set when the downstream (post-catalyst) oxygen sensor on Bank 1 reports a voltage above the calibrated maximum (typically above 1.0 V) for an extended period. The downstream sensor normally sits at a relatively steady 0.6–0.8 V because the catalyst smooths the rich/lean oscillation it sees coming in. A reading stuck above 1.0 V is either a genuine very-rich condition reaching the catalyst, a shorted signal wire, or a failed sensor biased high.
This guide covers P0138 across the 2015-2019 Hyundai Santa Fe generation — the symptoms, causes, and diagnostic steps below apply to every model year from 2015 through 2019.
Is it safe to drive a 2017 Hyundai Santa Fe with P0138?
In most cases a 2017 Hyundai Santa Fe stays drivable for short trips with P0138 active, but diagnose and repair it promptly. This is a low-severity code — ignoring it can lead to further damage or a failed emissions test.
What are the symptoms of P0138 on a 2017 Hyundai Santa Fe?
- Check Engine Light is illuminated
- Possible mild reduction in fuel economy
- Often no drivability symptoms at all
- Vehicle may fail emissions / smog testing
- Sometimes accompanied by P0172 (rich Bank 1)
What causes P0138 on a 2017 Hyundai Santa Fe?
| Cause | Likelihood | Estimated repair (USD) |
|---|---|---|
| Failed Bank 1 downstream O2 sensor (stuck high) | Most common | $150–$450 |
| Shorted-to-power O2 signal wire (sensor reads max) | Common | $80–$350 |
| Genuine rich condition reaching the catalyst (leaking injector, FPR) Address the rich condition; the sensor is correctly reporting it. | Common | $150–$1,200 |
| Damaged sensor connector — corrosion biasing the signal | Common | $30–$200 |
| Internal sensor short due to contamination | Occasional | $150–$450 |
| Coolant or oil in the exhaust (head gasket / valve seals) | Rare | $1,200–$3,500 |
How to diagnose this on a 2017 Hyundai Santa Fe
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Determine whether the reading is real or sensor-reported
Read fuel trims at idle and 2500 RPM. If Bank 1 LTFT is around −10 % or worse (the ECM pulling fuel out), the engine is genuinely running rich and the downstream sensor is correctly reporting it. If LTFT is near zero, the engine is fine and the sensor or its wiring is the problem.
Tools: Scan tool with fuel trim PIDs
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Inspect for shorted signal wire
Disconnect the Bank 1 downstream O2 sensor. With the connector unplugged and the key on, scan-tool voltage on the signal wire should fall to bias voltage (0.4–0.5 V). If voltage stays high (above 1 V), the signal wire is shorted to power somewhere between the sensor and the PCM.
Tools: Scan tool, Multimeter
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Verify with a known-good sensor
The downstream sensor is essentially a slow O2 sensor that should not switch rapidly. If a known-good sensor (or the Bank 2 downstream sensor if equipped) reads correctly while the Bank 1 downstream sticks high, the Bank 1 sensor is bad.
Tools: Scan tool with sensor swap capability
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Inspect for fuel contamination of the catalyst
If a separate rich-running fault (P0172, stuck injector) has been present for months, fuel may have entered the catalyst and altered its oxygen-storage behavior — which the downstream sensor sees as a chronic rich signal. This requires the rich cause fixed AND likely catalyst replacement.
Tools: Diagnostic history review
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Check for exhaust system damage
Look for any damage to the exhaust between the catalyst and the downstream sensor that could let oil mist or coolant into the exhaust stream. Coolant entering the exhaust (head gasket, cracked head) creates rich-mimicking sensor readings.
Tools: Visual inspection, Coolant pressure tester
NHTSA complaints & recalls for the 2017 Hyundai Santa Fe
Owner-reported safety complaints and official recalls filed with the U.S. National Highway Traffic Safety Administration for the 2017 Hyundai Santa Fe. Use these to gauge how common a problem is on your specific vehicle before you start chasing Hyundai Santa Fe diagnostics.
- ENGINE 618
- POWER TRAIN 149
- UNKNOWN OR OTHER 83
- ELECTRICAL SYSTEM 64
- SERVICE BRAKES 59
6 active recalls
- TIRES:PRESSURE MONITORING AND REGULATING SYSTEMS Mar 2017
Hyundai Motor America (Hyundai) is recalling certain 2017 Santa Fe Sport vehicles. The affected vehicles have a tire pressure monitoring system (TPMS) that may not have been set in the correct mode during vehicle assembly, and therefore will not provide an appropriate warning in…
NHTSA campaign 17V142000 - LATCHES/LOCKS/LINKAGES:HOOD:LATCH Jun 2017
Hyundai Motor America (Hyundai) is recalling certain 2013-2017 Santa Fe and Santa Fe Sport vehicles. In the affected vehicles, the secondary hood latch actuating cable may corrode and bind, causing the secondary hood latch to remain in the unlatched position when the hood is clo…
NHTSA campaign 17V358000 - ENGINE AND ENGINE COOLING:ENGINE Sep 2017
Hyundai Motor America (Hyundai) is recalling certain 2017 Santa Fe vehicles equipped with 3.3L engines. The crankshaft assemblies may have been produced with surface irregularities in the crankshaft pin, causing engine bearing wear.…
NHTSA campaign 17V578000 - ELECTRICAL SYSTEM:WIRING Nov 2016
Hyundai Motor America (Hyundai) is recalling certain model year 2016-2017 Tucson vehicles manufactured May 19, 2015, to November 14, 2016, and 2017 Santa Fe vehicles manufactured November 28, 2015, to November 14, 2016. The affected vehicles may be equipped with an accessory tra…
NHTSA campaign 16V842000
How do I fix P0138 on a 2017 Hyundai Santa Fe?
- Replace the Bank 1 downstream O2 sensor with an OEM part
- Repair shorted-to-power O2 signal wire
- Address underlying rich condition (P0172) first if present
- Clean or replace corroded O2 sensor connector
- Repair coolant or oil entry into the exhaust (root cause)
About the 2015-2019 Hyundai Santa Fe
The 2015-2019 Hyundai Santa Fe was commonly sold with the following powertrains: 2.5L I4, 2.5L Turbo I4, 1.6L Hybrid I4. Common trims include SE, SEL, XRT, Limited, Calligraphy.
What the downstream O2 sensor actually does
Unlike the upstream sensor, which feeds closed-loop fuel control, the downstream sensor’s main job is to monitor catalyst efficiency. It also provides a slow secondary fuel-trim correction. A properly functioning catalyst smooths the rich/lean oscillation coming from the engine — so the downstream voltage normally hovers steadily around 0.6–0.8 V instead of switching.
P0138 means that smoothing is gone and the post-cat reading is biased high — either the cat is gone, the engine is running rich, or the sensor itself is misreading.
P0138 with no other rich codes
This pattern usually points at the sensor or its wiring rather than a fuel system problem. A genuine rich condition severe enough to peg the downstream sensor at >1 V would also set P0172. P0138 alone with normal fuel trims is the textbook “downstream sensor has failed” pattern.
When P0138 reflects a real fuel problem
If P0138 sets alongside P0172 (system too rich), P0171 (lean) with heavy intake leak, or recent fuel-injector work, the sensor is likely correct and the fueling is at fault. Replacing the sensor without fixing the upstream condition will result in P0138 returning within days.
P0138 on a 2017 Hyundai Santa Fe: frequently asked questions
What does diagnostic trouble code P0138 mean on a 2017 Hyundai Santa Fe?
P0138 is set when the downstream (post-catalyst) oxygen sensor on Bank 1 reports a voltage above the calibrated maximum (typically above 1.0 V) for an extended period. The downstream sensor normally sits at a relatively steady 0.6–0.8 V because the catalyst smooths the rich/lean oscillation it sees coming in. A reading stuck above 1.0 V is either a genuine very-rich condition reaching the catalyst, a shorted signal wire, or a failed sensor biased high.
What are the symptoms of P0138 on a 2017 Hyundai Santa Fe?
Check Engine Light is illuminated. Possible mild reduction in fuel economy. Often no drivability symptoms at all. Vehicle may fail emissions / smog testing. Sometimes accompanied by P0172 (rich Bank 1)
What causes P0138 on a 2017 Hyundai Santa Fe?
Failed Bank 1 downstream O2 sensor (stuck high) (most-common). Shorted-to-power O2 signal wire (sensor reads max) (common). Genuine rich condition reaching the catalyst (leaking injector, FPR) (common). Damaged sensor connector — corrosion biasing the signal (common). Internal sensor short due to contamination (occasional). Coolant or oil in the exhaust (head gasket / valve seals) (rare)
Is it safe to drive a 2017 Hyundai Santa Fe with P0138?
In most cases a 2017 Hyundai Santa Fe stays drivable for short trips with P0138 active, but it should be diagnosed and repaired promptly — this is a low-severity code. Ignoring it can lead to further damage or a failed emissions test.