Is it safe to keep driving?
Generally safe to drive, with one exception: if the car is running rich enough to smell of fuel at the exhaust, you are loading the catalytic converter with unburnt fuel and risking an expensive failure. Get it looked at promptly rather than running it for weeks.
General guidance, not a diagnosis of your specific vehicle. If you are unsure, do not drive it and have it inspected.
A thermostat stuck open is the classic
If the engine never reaches its target temperature, the ECU holds a warm-up enrichment strategy far longer than it should and may never enter closed-loop fuel control properly. Fuel consumption rises noticeably, the heater blows lukewarm in cold weather, and the temperature gauge sits below its normal position. It is a cheap part that produces a large fuel bill.
Oxygen sensors that have gone lazy
An upstream oxygen sensor does not have to fail outright to cost you fuel. As it ages it switches more slowly, and the ECU — working from stale information — holds a richer mixture than necessary. A heater circuit fault has the same effect, because the sensor takes far longer to reach the temperature at which it reads accurately, and the engine stays in open loop meanwhile.
Running rich from the air side
The ECU calculates fuel from measured airflow. If the mass airflow sensor over-reports, the engine gets more fuel than it needs. Negative fuel trim is the signature: the ECU is actively subtracting fuel to compensate for a sensor that is exaggerating. Look at trim before replacing anything.
Things that are not the engine
Under-inflated tyres, a dragging brake caliper, a roof rack left on, and short cold trips in winter all cost real fuel. Rule them out before chasing sensors — a binding caliper also makes one wheel noticeably hot after a drive.
What to watch in live data
These are the readings that separate the likely causes. OBDAssistant streams them from an ELM327 adapter while the engine is running, which is what turns a list of possibilities into a diagnosis.
| Reading | What it tells you |
|---|---|
| Coolant temperature at steady cruise | Should reach and hold the manufacturer target. Sitting well below it points at the thermostat. |
| Short- and long-term fuel trim | Persistently negative trim means the ECU is removing fuel to correct for something over-reporting air. |
| Upstream O2 sensor switching | A healthy sensor crosses its midpoint briskly and repeatedly. Slow, lazy transitions cost fuel even without setting a code. |
| Fuel system status (open vs closed loop) | An engine that stays in open loop long after warm-up is running a fixed, rich map. |
Trouble codes that match this symptom
If the car has already been scanned, these are the codes most often behind it. Each page covers the definition, likely causes, diagnostic steps and repair information.
- P0128 Coolant Temperature Below Thermostat Regulating Temperature
- P0172 System Too Rich (Bank 1)
- P0175 System Too Rich (Bank 2)
- P0131 O2 Sensor Circuit Low Voltage (Bank 1, Sensor 1)
- P0135 O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)
- P0141 O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 2)
- P0101 Mass Air Flow (MAF) Circuit Range/Performance Problem
- P0102 Mass Air Flow (MAF) Circuit Low Input
- P0420 Catalyst System Efficiency Below Threshold (Bank 1)
Background reading
- Fuel Trim Mastery: OBD2 Diagnostic Guide
- O2 Sensor Volatility: Signal Cross-Count & Waveform Analysis
- OBD2 Emissions Readiness Monitors Explained
Questions
Can a failing catalytic converter reduce fuel economy?
Indirectly. A converter that is physically breaking up can restrict exhaust flow, which costs power and makes you use more throttle for the same progress. But a converter that is simply less efficient chemically does not itself burn more fuel — the fault that destroyed it usually does.
My economy dropped and there is no check engine light. Is that possible?
Yes, and it is common. Fuel trim can drift a long way, and an oxygen sensor can become slow, well before anything crosses the threshold that stores a code. Live data shows the problem long before the light does.
Does fuel brand or grade explain it?
Rarely to the degree people expect, unless you have moved to a fuel with a higher ethanol content, which carries less energy per litre and will show a genuine, modest drop. A sudden large change is a fault, not a fuel.