Single cylinder misfire
One cylinder misfires part of the time. Its unburned charge carries oxygen past the upstream sensor, which reads lean, so the trims add fuel on that bank. The misfire counter names the cylinder and the downstream sensor sits rich.
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Misfire counts on one cylinder Must be present
Rule: At least 20 misfires per 1,000 revolutions on one cylinder in some cell.
The misfire counter names the cylinder. Unburned charge carries its oxygen past the upstream sensor, which then reads lean.
Downstream sensor sits rich in closed loop Must be present
Rule: Downstream sensor at least 0.78 V at idle and at cruise.
The module adds fuel for oxygen that was never burned, so the mixture is rich overall and the converter shows it downstream.
Only one bank is affected Supports it on a V engine
Rule: On a dual bank engine, the two banks' total trims differ by at least 5 points in some cell.
Only the bank with the misfiring cylinder is fooled.
Why the numbers move
When a cylinder fails to burn, its air and fuel go into the exhaust. The upstream sensor responds to the oxygen in that charge and reads lean, so closed loop adds fuel to the whole bank. The mixture is now rich overall, and the converter, which burns the leftovers, shows that on the downstream sensor.
A misfire is the classic trap for trim diagnosis: positive trims that look like a lean engine when the real fault is ignition, compression or a single injector. Read the misfire counters before trusting a positive trim.
The reference numbers
Computed by the model at build time: the fault at its clean reference size on a 3.5 l v6, dual bank, wideband air fuel sensors, with no noise. The fault sits on bank 2; bank 1 is healthy.
| Reading | Idle | 2,500 rpm, no load | Cruise | WOT snapshot |
|---|---|---|---|---|
| Total trim bank 1 (%) | 0.0 | 0.0 | 0.0 | open loop |
| Total trim bank 2 (%) | +8.0 | +4.0 | +6.0 | open loop |
| Airflow check (%) | 100 | 99 | 99 | 100 |
| MAF (g/s) | 4.59 | 12.72 | 19.10 | 146.81 |
| MAP (kPa) | 30.0 | 21.0 | 38.0 | 97.0 |
| Upstream B1S1 | lambda 1.00 | lambda 1.00 | lambda 1.00 | lambda 0.85 |
| Upstream B2S1 | lambda 1.00 | lambda 1.00 | lambda 1.00 | lambda 0.86 |
| Downstream B1S2 (V) | 0.66 | 0.66 | 0.66 | 0.90 |
| Downstream B2S2 (V) | 0.90 | 0.89 | 0.90 | 0.90 |
| ECT (C) | 90 | 90 | 90 | 90 |
| Misfire per 1,000 revs | cyl 4: 120 | cyl 4: 60 | cyl 4: 90 | cyl 4: 100 |
Easy to confuse with
- Upstream O2 or A/F sensor biased lean. Both put the trims up with the downstream sensor rich. The misfire counter splits them: a misfire names its cylinder, a biased sensor leaves the counter at zero.
- Exhaust leak upstream of the upstream sensor. Both fool the upstream sensor. Misfire counts and a rich downstream sensor point to the misfire; an exhaust leak shows neither.
On a real car
- Swap ignition components between cylinders and see whether the misfire follows.
- Run a compression or leak down test on the cylinder.
TrimCase is a training aid built on a simulated engine model. It is not a diagnostic instruction for any real vehicle. Follow the service information for the vehicle in front of you.
Where the model is simpler than an engine
The model uses a fixed misfire rate per cell. Real misfires are often worse under load and with heat.
The full model is on how the model works.
An example case
Case C-ES-4, clean tier. Open it in projection mode.
Case C-ES-4
- Engine
- 3.5 L V6, dual bank, wideband air fuel sensors
- Customer says
- Hesitation or stumble on acceleration
- Check engine light
- On
- Tier
- Single fault, clean
Warm engine, sea level, 100 kPa barometric pressure. Simulated data for training, not from a real vehicle.
Freeze frame
Captured when P0303 set.
- Codes
- P0303
- Captured at
- Idle
- Engine speed
- 645 rpm
- Vehicle speed
- 0 mph
- Calculated load
- 20.5 %
- MAP
- 29.8 kPa
- MAF
- 4.50 g/s
- ECT
- 90 C / 194 F
- IAT
- 38 C / 100 F
- Fuel system
- Closed loop
- STFT / LTFT bank 1
- +0.9 % / +6.7 %
- STFT / LTFT bank 2
- -0.1 % / 0.0 %
Live data by load cell
| PID | Idle | 2,500 rpm, no load | Cruise | WOT snapshot |
|---|---|---|---|---|
| Engine speed (rpm) | 645 | 2501 | 1792 | 4998 |
| Vehicle speed (mph) | 0 | 0 | 60 | 45 |
| MAP (kPa) | 29.8 | 20.8 | 37.8 | 97.3 |
| MAF (g/s) | 4.50 | 12.57 | 18.85 | 147.57 |
| Airflow check, MAF as % of speed density estimate (%) worked out | 100 | 99 | 99 | 100 |
| Calculated load (%) | 20.5 | 14.7 | 30.9 | 86.6 |
| IAT | 38 C / 100 F | 35 C / 95 F | 30 C / 86 F | 28 C / 82 F |
| ECT | 90 C / 194 F | 90 C / 194 F | 90 C / 194 F | 91 C / 196 F |
| Fuel system | Closed loop | Closed loop | Closed loop | Open loop |
| STFT bank 1 (%) | +0.9 | +0.2 | +0.4 | 0.0 |
| LTFT bank 1 (%) | +6.7 | +3.3 | +5.0 | +5.0 |
| Total trim bank 1 (%) worked out | +7.6 | +3.5 | +5.4 | +5.0 |
| STFT bank 2 (%) | -0.1 | -0.5 | -0.3 | 0.0 |
| LTFT bank 2 (%) | 0.0 | 0.0 | 0.0 | 0.0 |
| Total trim bank 2 (%) worked out | -0.1 | -0.5 | -0.3 | 0.0 |
| A/F sensor B1S1 | lambda 1.01 | lambda 1.00 | lambda 1.00 | lambda 0.86 |
| A/F sensor B2S1 | lambda 1.00 | lambda 1.00 | lambda 1.00 | lambda 0.85 |
| O2 sensor B1S2 (V) | 0.92 | 0.89 | 0.91 | 0.91 |
| O2 sensor B2S2 (V) | 0.64 | 0.66 | 0.66 | 0.90 |
| Injector pulse bank 1 (ms) | 4.64 | 3.37 | 6.56 | 20.35 |
| Injector pulse bank 2 (ms) | 4.35 | 3.27 | 6.23 | 19.40 |
| EVAP purge command (%) | 0 | 20 | 35 | 0 |
| Misfire count per 1,000 revs | cyl 3: 112 | cyl 3: 56 | cyl 3: 84 | cyl 3: 93 |
Rows marked worked out are arithmetic on the rows above them, done for you: total trim is STFT plus LTFT, and the airflow check divides the MAF by the speed density estimate on the model page.