Negative fuel trim: the causes, and how to check each one
Negative fuel trim means the engine computer is taking fuel away because the upstream oxygen or air fuel sensor reports rich. The usual causes are extra fuel the computer did not meter (a leaking injector, fuel pressure too high, a purge valve stuck open with a loaded canister, fuel vapour coming from the crankcase), an airflow reading that is too high (a MAF over reporting), or a coolant sensor that reads cold, so the computer adds warm up fuel that the trims then have to remove. The shape of the trims across idle, 2,500 rpm and cruise, together with the MAF check and the coolant reading, tells them apart.
Negative trims are the sensor doing its job, so a new oxygen sensor rarely fixes them. Find where the extra fuel, or the missing air, comes from.
What negative means
Fuel trim is reported as a percentage of the fuel the computer meters from its air reading. Positive means it is adding fuel because the sensor says lean; negative means it is removing fuel because the sensor says rich. The standard scan data parameters run from minus 100 to plus 99.2 percent, and what matters for diagnosis is the total, short term plus long term, per bank and per load cell. A total of minus 10 means the engine is getting, or seems to be getting, about 10 percent more fuel than the air it has.
Two shapes, two groups of causes
Negative at idle and near zero at cruise means a fixed amount of extra fuel, which is a large share of the small idle fuel flow and a small share at cruise:
- A leaking injector: a drip while the injector is shut. The MAF agrees with the engine, and on a V engine only the bank with the injector moves. See the leaking injector fingerprint.
- An EVAP purge valve stuck open with a loaded canister: vapour the computer did not expect, carried in air the MAF never saw, so the MAF reads short at idle as well. Often worse just after a fill up. See the purge valve fingerprint.
Negative by about the same amount everywhere means the error scales with the fuel or the air itself:
- Fuel pressure too high: every injector passes more fuel for the same pulse, in every cell. The MAF agrees and the coolant reading is normal. See high fuel pressure.
- A MAF over reporting: the computer meters fuel for air that is not there, and the MAF reads above the speed density estimate in every cell. See MAF over reporting.
- A coolant sensor stuck cold: the computer adds warm up enrichment to a warm engine and the trims take it back out. The coolant reading is frozen at one low number. See ECT stuck cold.
Outside what the TrimCase model simulates, three more are worth knowing: engine oil diluted with fuel, whose vapour reaches the intake through the PCV system; a ruptured diaphragm in a vacuum referenced fuel pressure regulator, which lets fuel into the vacuum line; and an upstream sensor biased rich, which usually shows as a downstream sensor reading lean.
Checking negative trims in order
- Read the coolant temperature. A warm engine should sit near its regulating temperature. One frozen low number means the sensor, and its extra fuel explains flat negative trims.
- Find the shape. Add STFT and LTFT per bank at idle, 2,500 rpm with no load and cruise. Negative at idle only points to a fixed extra fuel source; flat negative points to pressure, the MAF or the coolant reading.
- Do the airflow check. Divide the MAF reading by what the engine should draw. Above the estimate everywhere means the MAF reads high; short at idle with negative trims means the purge stream.
- Command the purge valve closed. With a scan tool, where the vehicle supports it, close the purge valve and watch the trims. If they recover, the purge path is the cause.
- Test fuel pressure. Compare pressure with a gauge against the specification in the service information, and check how it holds after shut off.
- Test the injectors. A leak down, balance or flow test, as the service information calls for, finds the injector that drips.
Worked example: three flat negative trims that are not the same fault
Three faults on the same 2.0 L inline four, each at its reference size with no noise. The trims alone cannot separate them. The airflow check and the coolant reading can.
| Fault | Idle trim | Cruise trim | Airflow check | Coolant |
|---|---|---|---|---|
| Fuel pressure too high | -10.6 | -10.4 | 100% | 90 C |
| MAF over reporting | -9.1 | -9.0 | 110% | 90 C |
| ECT stuck cold | -8.5 | -8.5 | 100% | 38 C |
| Purge stuck open | -9.0 | -1.1 | 90% | 90 C |
The first three pull between -8.5 and -10.6 percent in every cell. The MAF reading 110 percent of the estimate names the MAF; a coolant reading of 38 C on a warm engine names the coolant sensor; with both normal, the fuel pressure is the one left, and a gauge confirms it. The purge valve is the odd one out: negative at idle only, with the MAF 90 percent of the estimate because the purge stream is air the MAF never measured.
Common mistakes
- Replacing the upstream sensor because it reads rich. It is usually reporting a real rich mixture.
- Condemning injectors before checking fuel pressure. High pressure makes every injector look like it over fuels.
- Missing a coolant sensor that reads cold on a warm engine. Always read the coolant temperature before the trims.
- Testing just after a fill up and blaming the injectors, when a stuck purge valve with a full canister is the source.
- Clearing codes and retesting straight away. On many vehicles that resets long term trim, which then needs a drive to relearn.
Practise it
- Work the daily drill, which mixes every family, including the fuel supply faults.
- Read long term vs short term fuel trim if the two trims still feel like one number.
- Follow the diagnostic order the drills reward.
Sources
Last reviewed . Standards and regulations are named so you can look them up; the service information for the vehicle always governs.
- SAE J1979, the OBD II diagnostic services standard, and its successor SAE J1979-2 for OBD on UDS. They define the fuel trim parameters (PIDs 06 to 09: short and long term trim for bank 1 and bank 2, scaled from minus 100 to plus 99.2 percent, where positive means fuel is being added) and the mass airflow parameter (PID 10, in grams per second).
- SAE J2012, which defines the standard trouble code meanings, including P0171 and P0174 (system too lean, bank 1 and bank 2) and P0172 and P0175 (system too rich, bank 1 and bank 2).
- The US EPA onboard diagnostics requirement for light duty vehicles, 40 CFR 86.1806, which requires a fuel system monitor and largely adopts California's OBD II regulation, 13 CCR 1968.2.
- Manufacturer service information for the vehicle in front of you. It sets the real trim limits, when a lean or rich code is allowed to set, which side is bank 1, and the confirmation tests. These vary by make, engine and model year, so no number on this page replaces them.
- The TrimCase engine model, which computes every number in the worked example on this page.
TrimCase is a training aid built on a simulated engine model. It is not a diagnostic instruction for any real vehicle.
Frequently asked questions
Is negative fuel trim bad?
A few percent either side of zero is normal. A total that stays beyond about minus 10 percent means the engine is running rich before the correction and the computer is working to hide it, and past the manufacturer's limit it sets a rich code.
Can a bad oxygen sensor cause negative fuel trim?
It can if it is biased rich, but that is less common than a real rich mixture. A biased upstream sensor usually shows up as a downstream sensor reading lean while the trims pull fuel.
What does a fuel trim of minus 10 percent mean?
The computer is delivering about 10 percent less fuel than its air reading calls for, because the sensor says the mixture would otherwise be rich. Where it happens, idle only or everywhere, says more than the number itself.
Why are my fuel trims negative only at idle?
A fixed amount of extra fuel is a big share of the small idle fuel flow and a small share at cruise. A leaking injector or a purge valve stuck open are the usual suspects, and the MAF check separates them.