P0038
PowertrainHO2S Heater Control Circuit High (Bank 1 Sensor 2)
For most people the only sign is the engine warning light coming on, and the car otherwise drives exactly as it did before. Behind that light, the ECU is seeing a higher-than-expected voltage on the heater control circuit for the rear oxygen sensor on bank 1, the one that sits after the cat. That sensor has a small built-in heater so it reaches working temperature quickly, and the ECU has decided something in that heater wiring or the sensor itself isn't behaving. It's an electrical fault first and foremost, not a sign your cat or engine is on the way out.
ⓘ Information only. This page provides general educational information about fault code P0038. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0038 mean?
P0038 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: HO2S Heater Control Circuit High (Bank 1 Sensor 2).
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light on, very often the only thing you'll notice
- • Fuel economy a touch worse than usual, though many drivers won't spot it
- • The rear sensor takes longer to come up to temperature, so emissions are slightly higher on a cold start
- • Occasionally a rough idle from cold that clears once the engine warms through
- • No drivability problems at all in a lot of cases, the car just throws the light
Possible causes
Causes commonly associated with P0038, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Failed rear oxygen sensor, the heater element inside has gone open or shorted. This is the single most common cause and the sensor is usually the part that ends up being replaced
- 2. Damaged or corroded connector at the sensor, common on UK cars because that plug sits low down and cops road salt and water all winter
- 3. Chafed or shorted heater wiring between the sensor and the loom, often where the cable runs near the exhaust heat shield
- 4. Blown fuse feeding the oxygen sensor heater circuit, cheap to check and worth ruling out early
- 5. Poor earth on the heater circuit causing odd voltage readings
- 6. ECM/PCM fault driving the circuit incorrectly, rare and only a suspect once everything else checks out
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Unplug the rear sensor on bank 1 and measure resistance across the two heater terminals. Cold, you want roughly 5 to 20 ohms. Open circuit or a silly high reading points straight at the sensor
- 2. Have a proper look at the connector and the last few inches of wiring. Green corrosion on the pins, melted insulation near the heat shield, or a plug full of water will all do it
- 3. Check the heater fuse with the key on. A blown fuse is a five minute fix and people skip it
- 4. Back-probe the connector and confirm you've got battery voltage on the supply side with the ignition on. No supply means the fault is upstream, not the sensor
- 5. Test the earth side of the heater circuit for continuity to a known good ground
- 6. If the sensor resistance, supply, earth and wiring all check out, you're into PCM territory, which is uncommon and worth a second opinion before spending money
Common questions about P0038
If I just clear the code, will it stay gone? +
If the heater element in the sensor has actually failed or there's a broken wire, the code will come straight back, usually within a drive cycle or two once the ECU runs its heater test again. Clearing it only sticks if the cause was something intermittent like a loose or damp connector that you've since cleaned and seated properly. So clearing it is a useful test rather than a fix. If P0038 returns, you've got a real electrical fault to find.
What's the harm in leaving it for a while? +
Mechanically very little in the short term. The car won't go into limp mode and the engine still runs normally because this is the rear sensor, which monitors the cat rather than controlling your fuelling. What you lose is the heater bringing that sensor up to temperature quickly, so the ECU runs slightly richer on cold starts and your emissions and economy take a small hit. Leave it long enough and the constant warning light just hides the next, more serious code when it appears.
How quickly do I need to sort this out? +
It's not an emergency. You can drive on it for a few weeks without doing damage to the engine or the cat. The two reasons to get on with it are an MOT, where an illuminated warning light at the time of test will commonly fail you, and the fact that the light masks anything else that crops up. If your MOT is months away and the car drives fine, book it in when convenient. Just don't let the light become permanent furniture.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →