P0138 on Hyundai vehicles
Post-Cat O2 Sensor High Voltage (Bank 1 Downstream) — what it means on a Hyundai, whether it's safe to drive, and the fixes by model and year.
P0138 on a Hyundai means post-cat o2 sensor high voltage (bank 1 downstream). Most vehicles stay drivable short-term, but diagnose it promptly. The most common cause is failed bank 1 downstream o2 sensor (stuck high) (typically $150–$450). Pick your Hyundai model below for its known failure patterns, recalls, and TSBs.
What does P0138 mean on a Hyundai?
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.
P0138 is a standardized OBD-II code, so its definition is the same on every Hyundai. What differs by model and engine is which cause is most likely — pick your Hyundai model below for a diagnosis tuned to its known failure patterns, recalls, and TSBs.
Is it safe to drive a Hyundai with P0138?
In most cases a Hyundai 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. Exact tolerance depends on the model and engine.
What causes P0138 on a Hyundai?
| 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 |
Repair costs are typical US ranges and vary by Hyundai model, model year, and labor rate. A diagnostic trouble code is a symptom, not a guaranteed failed part — confirm the root cause before replacing anything.
P0138 by Hyundai model
Select your Hyundai model and year for a P0138 diagnosis tuned to that vehicle's failure patterns, recalls, and TSBs.
Hyundai Elantra
1,515 NHTSA owner complaints · 13 recalls on record across the years covered below.
Hyundai Tucson
1,671 NHTSA owner complaints · 9 recalls on record across the years covered below.
Hyundai Santa Fe
1,450 NHTSA owner complaints · 12 recalls on record across the years covered below.
Complaint and recall counts are NHTSA totals for the model years covered on this site, not counts specific to P0138.
P0138 on other makes
Other codes on Hyundai vehicles
P0138 on a Hyundai: frequently asked questions
What does diagnostic trouble code P0138 mean on a Hyundai?
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. P0138 is a standardized OBD-II code, so the definition is the same on every Hyundai — which cause is most likely varies by model and engine.
What are the symptoms of P0138 on a Hyundai?
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 Hyundai?
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 Hyundai with P0138?
In most cases a Hyundai 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. Exact tolerance depends on the model and engine.