P2099: Bank 2 rich post-catalyst trim as an upstream lambda-offset diagnosis

P2099 carries the generic description post-catalyst fuel trim too rich on bank 2. GM's illustrated monitor evaluates an upper-range fault in the upstream sensor's learned lambda offset. The generic title and the internal control quantity describe different aspects of the same documented diagnostic.

A high offset alone is not the full sequence

The primary upper boundary is an offset above 0.03. Confirmation is qualified by adaptation and debounce conditions, with branches that reference faults set in the previous driving cycle or healed in the current one. The row's 0.1-second evaluation time therefore does not represent the complete learning and confirmation process.

The continuation includes qualified lambda control and post-fuel-cutoff conditions. Those operating states matter when deciding whether a later measurement is a valid retest.

Avoid substituting a convenient parameter

A scanner's fuel-trim percentage, rear-sensor voltage and learned lambda offset are not interchangeable quantities. Retain the actual bank 2 offset and adaptation status if available, together with the fault history that the control logic uses.

The code does not independently establish a defective catalyst or oxygen sensor. It reports that an adapted control quantity met the failure logic, leaving the physical cause to be isolated through the applicable procedure. A cleared code or temporarily normal observation outside eligible control conditions is not proof of recovery. The source supports investigating the adaptation result without inventing a universal part replacement or a complete numeric test sequence from a simplified label.

Source and scope

2025 GM 25OBDG07A Part 2 monitor tables only; model, engine and transmission applicability are not established by the unresolved application-matrix filename join. These are documented monitor criteria, not a universal repair procedure. GM PDF, physical page 257 · GM PDF, physical page 258.